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K-T extinctions: the end of dinosaurs?

2007-06-27T06:11:00.000-07:00

Since the early decades of the 19th century, it had been known that
different groups of organisms dominate different periods of Earth
history. One of the more notable groups was the dinosaurs, and
there was a steady reinforcement from palaeontological surveys of
the idea that none were to be found in rocks younger than the end of
the Cretaceous period (approximately 65 Ma). In fact, it came to be
recognized that the very end of the Cretaceous Period, leading into
the Tertiary Period (now universally referred to as the K-T
boundary) marked a major time of change. Many species became
extinct and were replaced in the Early Tertiary by a diversity of new
forms: the K-T boundary therefore seemed to represent a major
punctuation in life and consequently a mass-extinction event. The
types of species that became extinct at this time included the fabled
dinosaurs on land, of which there were many different varieties by
Late Cretaceous times; a multiplicity of sea creatures, ranging from
giant marine reptiles (mosasaurs, plesiosaurs, and ichthyosaurs), to
the hugely abundant ammonites, as well as a great range of chalky
planktonic organisms; while in the air the flying reptiles
(pterosaurs) and enantiornithine birds disappeared forever.

Clearly it was necessary to try to understand what might have
caused such a dramatic loss of life. The flip side of this general
question was just as important: why did some creatures survive?
After all, modern birds survived, so did mammals, and so did lizards
and snakes, crocodiles and tortoises, fish and a whole host of other
sea creatures. Was it just luck? Up until 1980, most of the theories
that had been put forward to explain the K-T extinctions and
survivals ranged from the sublime to the ridiculous.

One of the more persistent of the pre-1980 theories revolved around
detailed studies of the ecological make-up of the time zones closest
to the K-T boundary. The consensus suggested that there was a shift
to progressively more seasonal/variable climatic conditions at the
end of the Cretaceous Period. This was mirrored in the decline of
those animals and plants less able to cope with more stressful
climatic conditions. This was linked, rather inconclusively, to
tectonic changes towards the close of the Cretaceous Period; these
included marked sea-level rises and greatly increased continental
provinciality. The general impression was that the world was slowly
changing in character, and this eventually culminated in a dramatic
faunal and floral turnover. Clearly such explanations require a
longer timescale for the extinction event to take place, but the
Achilles heel was that this did not adequately account for the
simultaneous changes seen in marine communities. In the absence
of better-quality data, arguments waxed and waned with no obvious
resolution.

In 1980, this field of investigation was completely revolutionized
by, of all people, an astronomer, Luis Alvarez. His son Walter, a
palaeobiologist, had been studying changes in plankton diversity at
the K-T boundary. It seemed logical to assume that the interval
between the Late Cretaceous and Early Tertiary might simply
represent a longish period of ‘missing’ time – a genuine gap in
the continuity of the fossil record. To assist Walter in his studies
concerning the changes in planktonic communities at this critical
time in Earth history, Luis suggested that he could measure the
amount of cosmic dust that was accumulating in boundary
sediments in order to be able to provide an estimate of the extent
of this presumed geological gap. Their results shocked the
palaeontological and geological world. They found that the
boundary layer, which was represented by a thin band of clay,
contained enormous quantities of cosmic debris that could only be
explained by the impact and subsequent vaporization of a gigantic
meteorite. They calculated that this meteorite would have needed to
be at least 10 kilometres in diameter. Considering the effect of the
impact of such a giant meteorite, they further proposed that the
huge debris cloud generated (containing water vapour and dust
particles) after the impact would have shrouded the Earth
completely for a significant period of time, perhaps several
months or even a year or two. Shrouding the Earth in this way
would have shut down photosynthesis of land plants and planktonic
organisms, and triggered the simultaneous collapse of terrestrial
and aquatic ecosystems. At a stroke, the Alvarezes and their
colleagues seemed to have found a unifying explanation for
the K-T event.

As with all good theories, the impact hypothesis generated an
impressive volume of research. Throughout the 1980s, more and
more teams of researchers were able to identify cosmic debris and
violent impact-related signals in K-T boundary sediments from
the four corners of the globe. By the late 1980s, the attention of a
number of workers was drawn to the Caribbean area. Reports
showed that on some of the Caribbean islands, such as Haiti,
deposits of sediments at the K-T boundary not only showed the
impact signal, but immediately above this an enormous thickness
of breccia (broken masses of rock that had been thrown together).
This, as well as the greater thicknesses of the meteorite debris
layer and its chemical signature, prompted the suggestion that
the meteorite had impacted somewhere in the shallow sea in this
area. In 1991, the announcement was made that researchers had
identified a large subterranean meteorite impact crater, which they
called Chicxulub, on the Yucatán Peninsula of Mexico. The crater
itself had been covered by 65 million years of sediment, and had
only been visualized by studying seismic echoes of the Earth’s crust
(rather like the principle of underground radar). The crater
appeared to be approximately 200 kilometres across and coincided
with the K-T boundary layer, so Alvarez’s theory was vindicated in a
most remarkable way.

From the early 1990s onwards, study of the K-T event shifted
away from the causes, which then seemed to have been established,
to attempting to link the extinctions at this time to a single
catastrophic event. The parallels to the nuclear winter debate are
fairly clear. Advances in computer modelling, combined with
knowledge of the likely chemical composition of the ‘target’ rocks
(shallow sea deposits) and their behaviour under high-pressure
shock, have shed light on the early phases of the impact and its
environmental effects. At Yucatán, the meteorite would have
impacted on a sea floor that was naturally rich in water, carbonate,
and sulphate; this would have propelled as much as 200 gigatons
each of sulphur dioxide and water vapour into the stratosphere.
Impact models based on the geometry of the crater itself suggest
that the impact was oblique and from the south-east. This trajectory
would have concentrated the expelled gases towards North
America. The fossil record certainly suggests that floral extinctions
were particularly severe in this area, but more work elsewhere is
needed before this pattern can be verified. Alvarez and others’ work
on the effects of the impact suggested that dust and clouds would
have plunged the world into a freezing blackout. However,
computer modelling of atmospheric conditions now suggests that
within a few months light levels and temperatures would have
begun to rebound because of the thermal inertia of the oceans, and
the steady fall-out of particulate matter from the atmosphere.
Unfortunately, however, things would have become no better for
some considerable time because the sulphur dioxide and water in
the atmosphere would have combined to produce sulphuric acid
aerosols, and these would have severely reduced the amount of
sunlight reaching the Earth’s surface for between 5 and 10 years.
These aerosols would have had the combined effects of cooling the
Earth to near freezing and drenching the surface in acid rain.

Clearly these estimates are based only on computer models,
which may be subject to error. However, even if only partly true,
the general scope of the combination of environmental effects
following the impact would have been genuinely devastating,
and may well account for many aspects of the terrestrial and
marine extinctions that mark the end of the Cretaceous Period.
In a sense, the wonder is that anything survived these apocalyptic
conditions at all.

Ancient biomolecules and tissues

2007-06-27T06:10:00.000-07:00

I cannot finish this chapter without mentioning the Jurassic Park
scenario: discovering dinosaur DNA, using modern biotechnology
to reconstitute that DNA, and using this to bring the dinosaur
back to life.

There have been sporadic reports of finding fragments of dinosaur
DNA in the scientific literature over the past decade, and then using
PCR (polymerase chain reaction) biotechnology to amplify the
fragments so that they can be studied more easily. Unfortunately,
for those who wish to believe in the Hollywood-style scenario,
absolutely none of these reports have been verified, and in truth it is
exceedingly unlikely that any genuine dinosaur DNA will ever be
isolated from dinosaur bone. It is simply the case that DNA is a long
and complex biomolecule which degrades over time in the absence
of the metabolic machinery that will maintain and repair it, as
occurs in living cells. The chances of any such material surviving
unaltered for over 65 million years while it is buried in the ground
(and subject there to all the contamination risks presented by
micro-organisms and other biological and chemical sources, and
ground water) are effectively zero.

All reports of dino-DNA to date have proved to be records of
contaminants. In fact the only reliable fossil DNA that has been
identified is far more recent, and even these discoveries have
been made possible because of unusual preservational conditions.
For example, brown bear fossils whose remains are dated back to
about 60,000 years have yielded short strings of mitochondrial
DNA – but these fossils had been frozen in permafrost since the
animals died, providing the best chance of reducing the rate of
degradation of these molecules. Dinosaur remains are of course
1,000 times more ancient than those of arctic brown bears.
Although it might be possible to identify some dinosaur-like genes
in the DNA of living birds, regenerating a dinosaur is beyond the
bounds of science.

One final, but extremely interesting, set of observations concerns
the analysis of the appearance and chemical composition of the
interior of some tyrannosaur bones from Montana. Mary Schweitzer
and colleagues from North Carolina State University were given
access to some remarkably well preserved T. rex bones collected by
Jack Horner (the real-life model for ‘Dr Alan Grant’ in the film
Jurassic Park). Detailed examination of the skeletal remains
suggested that there had been minimal alteration of the internal
structure of the long bones; indeed, so unaltered were they that the
individual bones of the tyrannosaur had a density that was consistent
with that of modern bones that had simply been left to dry.
Schweitzer was looking for ancient biomolecules, or at least the
remnant chemical signals that they might have left behind. Having
extracted material from the interior of the bones, this was powdered
and subjected to a broad range of physical, chemical, and biological
analyses. The idea behind this approach was not only to have the
best chance of ‘catching’ some trace, but also to have a range of
semi-independent support for the signal, if it emerged. The burden
really is upon the researcher to find some positive proof of the
presence of such biomolecules; the time elapsed since death and
burial, and the overwhelming probability that any remnant of such
molecules has been completely destroyed or flushed away, seem to
be overwhelming. Nuclear magnetic resonance and electron spin
resonance revealed the presence of molecular residues resembling
haemoglobin (the primary chemical constituent of red blood cells);
spectroscopic analysis and HPLC (high performance liquid
chromatography) generated data that was also consistent with the
presence of remnants of the haeme structure. Finally, the dinosaur
bone tissues were flushed with solvents to extract any remaining
protein fragments; this extract was then injected into laboratory
rats to see if it would raise an immune response – and it did! The
antiserum created by the rats reacted positively with purified avian
and mammalian haemoglobins. From this set of analyses, it seems
very probable that chemical remnants of dinosaurian haemoglobin
compounds were preserved in these T. rex tissues.

Even more tantalizingly, when thin sections of portions of bone
were examined microscopically, small, rounded microstructures
could be identified in the vascular channels (blood vessels) within
the bone. These microstructures were analysed and found to be
notably iron-rich compared to the surrounding tissues (iron being
a principal constituent of the haeme molecule). Also the size and
general appearance was remarkably reminiscent of avian nucleated
blood cells. Although these structures are not actual blood cells,
they certainly seem to be the chemically altered ‘ghosts’ of the
originals. Quite how these structures have survived in this state
for 65 Ma is a considerable puzzle.

Schweitzer and her co-workers have also been able to identify
(using immunological techniques similar to the one mentioned
above) biomolecular remnants of the ‘tough’ proteins known as
collagen (a major constituent of natural bone, as well as ligaments
and tendons) and keratin (the material that forms scales, feathers,
hair, and claws).

Although these results have been treated with considerable
scepticism by the research community at large – and rightly so, for
the reasons elaborated above – nevertheless, the range of scientific
methodologies employed to support their conclusions, and the
exemplary caution with which these observations were announced,
represent a model of clarity and application of scientific
methodologies in this field of palaeobiology.

Dinosaur mechanics: how Allosaurus fed

2007-06-27T06:09:00.000-07:00

Computed tomography has clearly proved to be a very valuable aid
to palaeobiological investigations because it has this ability to see
inside objects in an almost magical way. Some technologically
innovative ways of using CT imaging have been developed by Emily
Rayfield and colleagues, at the University of Cambridge. Using CT
images, sophisticated computer software, and a great deal of
biological and palaeobiological information, it has proved possible
to investigate how dinosaurs may have functioned as living
creatures.
As with the case of Tyrannosaurus, we know in very general terms
that Allosaurus (Figure 31) was a predatory creature and probably
fed on a range of prey living in Late Jurassic times. Sometimes
tooth marks or scratches may be found on fossil bones and these
can be quite literally lined up against the teeth in the jaws of an
allosaur as a form of ‘proof’ of the guilty party. But what does such
evidence tell us? The answer is: not as much as we might like. We
cannot be sure if the tooth marks were left by a scavenger feeding
off an already dead animal, or whether the animal that left the
tooth marks was the real killer; equally, we cannot tell what style of
predator an allosaur might have been: did it run down its prey
after a long chase, or did it lurk and pounce? Did it have a
devastating bone-crushing bite, or was it more of a cut and
slasher?

Rayfield was able to obtain CT scan data created from an
exceptionally well-preserved skull of the Late Jurassic theropod
Allosaurus. High-resolution scans of the skull were used to create
a very detailed three-dimensional image of the entire skull.
However, rather than simply creating a beautiful hologram-like
representation of the skull, Rayfield converted the image data into a
three-dimensional ‘mesh’. The mesh consisted of a series of point
coordinates (rather like the coordinates on a topographic map),
each point was linked to its immediate neighbours by short
‘elements’. This created what in engineering terms is known as
a finite element map of the entire skull (Figure 38): nothing quite
as complicated as this had ever been attempted before.

Having mapped the virtual skull of this dinosaur, it was then
necessary to work out how powerful its jaw muscles were in life.
Using clay, Rayfield was able to quite literally model the jaw muscles
of this dinosaur. Once she had done this, she was able to calculate
from their dimensions – their length, girth, and angle of attachment
to the jaw bones – the amount of force that they could generate.
To ensure that these calculations were as realistic as possible,
two sets of force estimates were generated: one based on the
view that dinosaurs like this one had a rather crocodile-like
(ectotherm) physiology, the other assumed an avian/mammalian
(endotherm) physiology.
Using these sets of data, it was then possible to superimpose these
forces on the finite element model of the Allosaurus skull and
quite literally ‘test’ how the skull would respond to maximum bite
forces, and how these would be distributed within the skull. The
experiments were intended to probe the construction and shape of
the skull, and the way it responded to stresses associated with
feeding.
What emerged was fascinating. The skull was extraordinarily strong
(despite all the large holes over its surface that might be thought to
have weakened it significantly). In fact, the holes proved to be an
important part of the strength of the skull. When the virtual skull
was tested until it began to ‘yield’ (that is to say, it was subjected to
forces that were beginning to fracture its bones), it was found to
be capable of withstanding up to 24 times the force that the jaw
muscles could exert when they were biting as hard as ‘allosaurianly’
possible.

Investigating hadrosaurian crests

2007-06-27T06:07:00.002-07:00

One obvious use of CT scanning can be demonstrated by referring
to the extravagant range of crests seen on some hadrosaurian
ornithopods. These dinosaurs were very abundant in Late
Cretaceous times and have remarkably similarly shaped bodies;
they only really differ in the shape of their headgear, but the reason
for this difference has been a long-standing puzzle. When the first
‘hooded’ dinosaur was described in 1914, it was considered likely
that these were simply interesting decorative features. However, in
1920 it was discovered that these ‘hoods’, or crests, were composed
of thin sheaths of bone that enclosed tubular cavities or chambers of
considerable complexity.

Theories to explain the purpose of these crests abounded from the
1920s onwards. The very earliest claimed that the crest provided an
attachment area for ligaments running from the shoulders to the
neck that supported the large and heavy head. From then on,
ideas ranged from their use as weapons; that they carried highly
developed organs of smell; that they were sexually specific (males
had crests and females did not); and, the most far-sighted, that the
chambers might have served as resonators, as seen in modern birds.
During the 1940s, there was a preference for aquatic theories: that
they formed an air-lock to prevent water flooding the lungs when
these animals fed on underwater weeds.

Most of the more outlandish suggestions have been abandoned,
either because physically impossible or they do not accord with the
known anatomy. What has emerged is that the crests probably
performed a number of interrelated functions of a mainly social/
sexual type. They probably provided a visual social recognition
system for individual species; and, in addition, some elaboration of
the crests undoubtedly served a sexual display purpose. A small
number of hadrosaur crests were sufficiently robust to have been
used either in flank or head-butting activities as part of pre-mating
rituals or male–male rivalry competitions. Finally, the chambers
and tubular areas associated with the crests or facial structure are
thought to have functioned as resonators. Again, this presumed
vocal ability (found today in birds and crocodiles) can be linked to
aspects of social behaviour in these dinosaurs.

One of the greatest problems associated with the resonator theory
was gaining direct access to skull material that would allow detailed
reconstruction of the air passages within the crest, without breaking
open prized and carefully excavated specimens. CT techniques
made such internal investigations feasible. For example, some
new material of the very distinctively crested hadrosaur
Parasaurolophus tubicen was collected from Late Cretaceous
sediments in New Mexico. The skull was reasonably complete,
well preserved, and included a long, curved crest. It was CT
scanned along the length of the crest, then the scans were digitally
processed so that the space inside the crest, rather than the crest
itself, could be imaged. The rendered version of the interior cavity
revealed an extraordinary degree of complexity. Several parallel,
narrow tubes looped tightly within the crest, creating the equivalent
of a cluster of trombones! There is now little doubt that the crest
cavities in animals like Parasaurolophus were capable of acting as
resonators as part of their vocal system.

Soft tissues: hearts of stone?

In the late 1990s, a new partial skeleton of a medium-sized
ornithopod was discovered in Late Cretaceous sandstones in South
Dakota. Part of the skeleton was eroded away, but what remained
was extraordinarily well preserved, with evidence of some of the
soft tissues, such as cartilage, which are normally lost during
fossilization, still visible. During initial preparation of the specimen,
a large ferruginous (iron-rich) nodule was discovered in the centre
of the chest. Intrigued by this structure, the researchers obtained
permission to CT scan a major part of the skeleton using a large
veterinary hospital scanner. The results from these scans were
intriguing.

The ferruginous nodule appeared to have distinctive anatomical
features, and there appeared to be associated nearby structures.
The researchers interpreted these as indicating that the heart
and some associated blood vessels had been preserved within the
nodule. The nodule appeared to show two chambers (interpreted
by the researchers as representing the original ventricles of the
heart); a little above these was a curved, tube-like structure that
they interpret as an aorta (one of the main arteries leaving the
heart). On this basis, they went on to suggest that this showed that
dinosaurs of this type had a very bird-like, fully divided heart, which
supported the increasing conviction that dinosaurs were generally
highly active, aerobic animals.

As early as 1842, and the extraordinarily prophetic speculations of
Richard Owen, it had been supposed that dinosaurs, crocodiles, and
birds had a relatively efficient four-chambered (i.e. fully divided)
heart. On that basis, this discovery is not so startling. What is
astonishing is the thought that the general shape of the soft tissues
of the heart of this particular dinosaur might have been preserved
through some freak circumstance of fossilization.
Soft tissue preservation is known to occur under some exceptional
conditions in the fossil record; these generally comprise a mixture
of very fine sediments (muds and clays) that are capable of
preserving the impressions of soft tissues. Also, soft tissues, or
rather their chemically replaced remnants, can be preserved by
chemical precipitation, usually in the absence of oxygen. Neither of
these conditions apply to the ornithopod skeleton described above.
The specimen was found in coarse sandstone, and under conditions
that would have been oxygen-rich, so from a simple geochemical
perspective, conditions would appear to be very unlikely to preserve
soft tissues of any type.

Not surprisingly, the observations made by the researchers have
been challenged. Ironstone nodules are commonly reported in these
deposits and are frequently found associated with dinosaur bones.
The sedimentary conditions, the chemical environment in which
the structures might have been preserved, and the interpretation of
all the supposedly heart-like features have been contested. At
present, the status of this specimen is therefore uncertain, but
whatever else is claimed, if these features are simply those of an
ironstone nodule, then it is extraordinary that they are so heart-like.

Dinosaur research: the scanning revolution

2007-06-27T06:07:00.001-07:00

The steady improvement in technological resources, as well as their
potential to be used to answer palaeobiological questions, has
manifested in a number of distinct areas in recent years. A few
of these will be examined in the following section; they are not
without their limitations and pitfalls, but in some instances
questions may now be asked that could not have been dreamt
of 10 years ago.

One of the most anguished dilemmas faced by palaeobiologists is
the desire to explore as much of any new fossil as possible, but at the
same time to minimize the damage caused to the specimen by such
action. The discovery of the potential for X-rays to create images on
photographic film of the interior of the body has been of enormous
importance to medical science. The more recent revolution in
medical imaging through the development of CT (computed
tomography) and MRI (magnetic resonance imaging) techniques
that are linked directly to powerful data-processing computers has
resulted in the ability to create three-dimensional images that allow
researchers to see inside objects such as the human body or other
complex structures that would only normally be possible after
major exploratory surgery.

The potential to use CT scanning to see inside fossils was rapidly
appreciated. One of the leaders in the field is Tim Rowe, with his
team based at the University of Texas in Austin. He has managed to
set up one of the finest fossil-dedicated, high-resolution CT
scanning systems and, as we shall see below, has put it to some
extremely interesting uses.

Coprolites

2007-06-27T06:06:00.000-07:00

Another slightly less romantic branch of palaeobiological
investigation focuses on the dung of animals such as dinosaurs.
This material is refered to as coprolites (copros means dung, lithos
means stone), and their study has a surprisingly long and relatively
illustrious history. The recognition of the importance of preserved
dung dates back to the work of William Buckland of Oxford
University (the man who described the first dinosaur,
Megalosaurus). A pioneering geologist from the first half of the
19th century, Buckland spent considerable time collecting and
studying rocks and fossils from his native area around Lyme Regis
in Dorset, including fossil marine reptiles. Alongside these,
Buckland noted large numbers of distinctive pebbles that often had
a faint spiral shape. On closer inspection, breaking them open and
looking at polished sections, Buckland was able to identify shiny
fish scales, bones, and the sharp hooks of belemnite (a cephalopod
mollusc) tentacles in great concentrations. He concluded that these
stones were most probably the lithified excreta of the predatory
reptiles found in the same rocks. Clearly, though at first sight
somewhat distasteful, the study of coprolites had the potential to
reveal evidence concerning the diet of the once-living creature that
would not otherwise be obtainable.

As was the case with footprints, the question ‘who did this?’, though
obviously amusing, can present significant problems. Occasionally,
coprolites, or indeed gut contents, have been preserved inside the
bodies of some fossil vertebrates (notably fish); however, it has been
difficult to connect coprolite fossils to specific dinosaurs or even
groups of dinosaurs. Karen Chin of the US Geological Survey has
devoted herself to the study of coprolites and has had singular
difficulty in reliably identifying dinosaur coprolites – until quite
recently.

In 1998, Chin and colleagues were able to report the discovery of
what they referred to in the title of their article as ‘A king-sized
theropod coprolite’. The specimen in question was discovered in
Maastrichtian (latest Cretaceous) sediments in Saskatchewan and
comprised a rather nobbly lump of material, over 40 centimetres
long, that had a volume of approximately 2.5 litres. Immediately
around and inside the specimen were broken fragments of bone,
and a finer, sand-like powder of bone material was present
throughout the mass. Chemical analysis of the specimen confirmed
that it had very high levels of calcium and phosphorous, confirming
a high concentration of bone material. Histological thin sections of
the fragments further confirmed the cellular structure of bone and
that the most likely prey items that had been digested were
dinosaurian;as suspected, this specimen was most likely a large
carnivore’s coprolite. Surveying the fauna known from the rocks in
this area, the only creature that was large enough to have been able
to pass a coprolite of these dimensions was the large theropod
Tyrannosaurus rex (‘king’ of the dinosaurs). Examination of the
bone fragments preserved in the coprolite showed that this animal
had been able to pulverize the bones of its prey in its mouth, and
that the most likely prey was a juvenile ceratopian ornithischian
(from the structure of the bone in the histological sections). The fact
that not all the bone had been digested in this coprolite indicated
that the material had moved through the gut with considerable
speed, which could be used by some as evidence that T. rex was
perhaps a hungry endotherm.

Dinosaur ichnology

2007-06-27T06:04:00.000-07:00

Some aspects of dinosaur research have an almost sleuth-like
quality to them, perhaps none more so than ichnology – the study
of footprints.

There is no branch of detective science which is so important and so
much neglected as the art of tracing footsteps.
(Conan Doyle, The Study in Scarlet, 1891)
The study of dinosaur footprints has a surprisingly long history.
Some of the first to be collected and exhibited were found in
1802 in Massachusetts by the young Pliny Moody while
ploughing a field. These and other large three-toed prints were
eventually illustrated and described by Edward Hitchcock in 1836
as the tracks left by gigantic birds; some can still be seen in the
Pratt Museum of Amherst College. From the mid-19th century
onwards, tracks were discovered at fairly regular intervals in
various parts of the world. With the development of an
understanding of the anatomy of dinosaurs, and most particularly
the shape of their feet, it was realized that the large ‘bird-like’
three-toed prints that were found in Mesozoic rocks belonged to
dinosaurs rather than giant birds. Such tracks, though of local
interest, were rarely regarded as of great scientific value.
However, in recent years, largely prompted by the work of Martin
Lockley of the University of Colorado at Denver, it has begun to be
appreciated more widely that tracks may provide a great deal of
information.

First, and most obviously, preserved tracks record the activities of
living dinosaurs. Individual prints also record the overall shape of
the foot and the number of toes, which can often help to narrow
down the likely trackmaker, especially if dinosaur skeletons have
been discovered in similarly aged rocks nearby. While individual
prints may be intrinsically interesting, a series of tracks provides a
record of how the creature was actually moving. They reveal the
orientation of the feet as they contact the ground, the length of the
stride, the width of the track (how closely the right and left feet
were spaced); from this evidence, it is possible to reconstruct how
the legs moved in a mechanical sense. Furthermore, taking
observations using data from a wide range of living animals it has
also proved possible to calculate the speeds at which animals
leaving tracks were moving. These estimates are arrived at by
simply measuring the size of the prints and length of each stride and
making an estimate of the length of the leg. Although the latter
might seem at first sight difficult to estimate with great accuracy,
the actual size of the footprints has proved to be a remarkably good
guide (judging by living animals), and in some instances foot and
leg bones or skeletons of dinosaurs that lived at the time the tracks
were made are known.

The study of tracks can also reveal information about dinosaur
behaviour. On rare occasions, multiple tracks of dinosaurs have
been discovered. One famous example, recorded in the Paluxy River
at Glen Rose in Texas, was revealed by a famous dinosaur footprint
explorer named Roland T. Bird. Two parallel tracks were found at
this site, one made by a huge brontosaur and the other by a large
carnivorous dinosaur. The tracks seemed to show the big carnivore
tracks converging on the brontosaur. At the intersection of the
tracks, one print is missing, and Bird suspected that this indicated
the point of attack. However, Lockley was able to show from maps
of the track site that the brontosaurs (there were several) continued
walking beyond the supposed point of attack; and, even though the
large theropod was following the brontosaur (some of its prints
overlap those of the brontosaur), there is no sign of a ‘scuffle’. Very
probably this predator was simply tracking potential prey animals
by following at a safe distance. More convincing were some tracks
observed by Bird at Davenport Ranch, also in Texas. Here he was
able to log the tracks of 23 brontosaur-like sauropods walking in the
same direction at the same time (Figure 35). This suggested very
strongly that some dinosaurs moved around in herds. Herding or
gregarious behaviour is impossible to deduce from skeletons, but
tracks provide direct evidence.

Increased interest in dinosaur tracks in recent years has brought to
light a number of potentially interesting avenues of research.
Dinosaur tracks have sometimes been found in areas that have not
yielded skeletal remains of dinosaurs, so tracks can help to fill in
particular gaps in the known fossil record of dinosaurs. Interesting
geological concepts have also emerged from a consideration of
dinosaur track properties. Some of the large sauropodomorph
dinosaurs (the brontosaurs referred to above) may have weighed as
much as 20–40 tonnes in life. These animals would have exerted
enormous forces on the ground when they walked. On soft
substrate, the pressure from the feet of such dinosaurs would have
distorted the earth at a depth of a metre or more beneath the
surface – creating a series of ‘underprints’ formed as echoes of the
original footprint on the surface. The spectre of ‘underprints’ means
that some dinosaur tracks might be considerably over-represented
in the fossil record if a single print can be replicated through
numerous ‘underprints’.

If herds of such enormous creatures trampled over areas, as they
certainly did at Davenport Ranch, then they also had the capacity to
greatly disturb the earth beneath – pounding it up and destroying
its normal sedimentary structure. This relatively recently
recognized phenomenon has been named ‘dinoturbation’.
‘Dinoturbation’ might be a geological phenomenon, but it hints at
another distinctly biological effect linked to dinosaur activities that
may or may not be measurable over time. That is the potential
evolutionary and ecological impact of dinosaurs on terrestrial
communities at large. Great herds of multitonne dinosaurs moving
across a landscape had the potential to utterly devastate the local
ecology. We are aware that elephants today are capable of causing
considerable damage to the African savannah because of the way
that they can tear up and knock down mature trees. What might a
herd of 40-tonne brontosaurs have done? And did this type of
destructive activity have an effect upon the other animals and plants
living at the time; can we identify or measure such impacts in the
long term, and were they important in the evolutionary history of
the Mesozoic?

Birds from dinosaurs: an evolutionary commentary

2007-06-27T06:03:00.000-07:00

The implications of these new discoveries are truly fascinating. It
has already been argued, with logic and some force, that small
theropod dinosaurs were highly active, fast-moving, and
biologically ‘sophisticated’ animals. On this basis, they seemed
reasonable candidates as potential endotherms; in a sense, our
inferences about their way of life suggested that they had most to
benefit from being endothermic. The Liaoning discoveries confirm
that many of these highly active, bird-like dinosaurs were small
animals. This is a crucial point, as small size puts greatest
physiological stress on endotherms because a large percentage of
internally generated body heat can be lost through the skin surface;
so small, active endotherms would be expected to insulate their
bodies to reduce heat loss. Small theropod dinosaurs, therefore,
evolved insulation to prevent heat loss because they were
endotherms – not because they ‘wanted’ to become birds!
Liaoning discoveries indicate that various types of insulatory
covering developed, most probably by subtle modifications to the
growth patterns of normal skin scales; these ranged from hair-like
filaments to full-blown feathers. It may well be that genuinely
bird-like flight feathers did not evolve for the purposes of flight,
but had a far more prosaic origin. Several of the ‘dinobirds’ from
Liaoning seem to have tufts of feathers on the end of the tail (rather
like a geisha’s fan) and fringes of feathers along the arms, on the
head, or running down the spine. Clearly preservational biases may
also play a part in how and on which parts of the body these may
be preserved. But for the present, it seems at least possible that
feathers evolved as structures linked to the behaviour of these
animals: providing recognition signals, perhaps, as in living birds,
or being used as part of their mating rituals, long before any
genuine flight function had developed.

In this context, gliding and flight, rather than being the sine qua
non of avian origins, become later, ‘add-on’ benefits. Obviously,
feathers have the potential for aerodynamic uses; just as with
modern birds, the ability to jump and flutter may well have
embellished ‘dinobird’ mating displays. For example, in the case of
the small creature Microraptor, a combination of fringes of feathers
along the arms, legs, and tail would have provided it with the ability
to launch itself into the air from branches or equivalent vantage
points. From just this sort of starting point, gliding and true
flapping flight seem a comparatively short ‘step’ indeed.

We should not, however, get too carried away with the scenario
outlined above. Although the Liaoning discoveries are indeed
incredibly important, offering, as they do, a richly detailed window
on dinosaurian and avian evolution in the Cretaceous, they do not
necessarily provide all the answers. One crucial point that must be
remembered is that the quarries of Liaoning are Early Cretaceous in
age, and their fossils are therefore considerably younger (by some
30 Ma at least) than the earliest well-preserved feathered dinosaur
with highly developed and complex wings, Archaeopteryx.
Whatever the path that led to the evolution of the first flying
dinosaurs, and ultimately to birds, it was emphatically not via the
extraordinary feathered dinosaurs from Liaoning. What we see at
Liaoning is a snap shot of the evolutionary diversification of avian
theropods (and some true birds), not the origin of birds: bird
origins are still shrouded by sediments of Middle or possibly even
Early Jurassic age – before Archaeopteryx ever fluttered to Earth.
Everything that we know to date points to a very close relationship
between theropod dinosaurs and early birds, but those crucial Early
or Middle Jurassic theropods that were ancestral to Archaeopteryx
are yet to be discovered. It is to be hoped that in future years some
spectacular discoveries will be made that fill in this part of the
story.

Chapter 5 concluded with the view that dinosaurs lived at a time in
Earth history that favoured large-bodied, highly active creatures
that were able to maintain a stable, high body temperature without
most of the costs of being genuinely endothermic. The ‘dinobirds’
from Liaoning suggest that this view is wrong – small, insulated
theropods simply had to be endothermic and their close
relationship to birds, which we know are endothermic, simply
reinforces the point.

My response to this is: well, yes and no. There is now little doubt
that bird-like theropod dinosaurs were endotherms in a true sense.
However, I do think that the arguments suggesting that the
majority of more traditional dinosaurs were inertial homeotherms
(their large body size enabled stable internal temperature) still hold.
There is some evidence in support of my view to be found among
living endotherms. Elephants, for example, have a much lower
metabolic rate than mice – for exactly these reasons. Mice are small,
lose heat rapidly to the environment, and have to maintain a high
metabolic rate to replenish the heat loss. Elephants are large
(generally dinosaur-sized) and have a stable internal body
temperature due to their size, not just because they are
endothermic. Indeed, being a large endotherm is, in part at least, a
physiological challenge. For example, elephants suffer problems if
they move around too quickly: their postural and leg muscles create
a great deal of extra chemical heat, and they need to use their large,
‘flappy’ ears to help them to radiate heat rapidly to prevent fatal
overheating.

Dinosaurs were on the whole super-large and their bodies would
have been capable of maintaining a constant internal temperature;
extrapolating from the elephant, it would not have been in
dinosaurs’ interests to be genuine endotherms, in a world that
was in any case very warm. Having evolved physiologically as
mass-homeotherms (having a stable internal body temperature
that was made possible by large body size), the only group of
dinosaurs that bucked the general dinosaurian trend toward large
size and evolved into a small-bodied group were the
dromaeosaurian theropods.

It is clear, from their anatomy alone, that dromaeosaurians were
highly active and would have benefited from homeothermy, and
their relatively large brains would have demanded a constant supply
of oxygen and nutrients. Paradoxically, homeothermy cannot be
maintained at small body size without an insulatory covering
because of the unsustainable heat loss through the skin. The choice
was stark and simple: small theropods had to either abandon their
high-activity lifestyle and become conventionally reptilian, or boost
internal heat production and become properly endothermic,
avoiding heat loss by developing skin insulation. So, I propose that
it is not a case of ‘all or nothing’; most dinosaurs were basically
mass-homeotherms that were able to sustain high activity levels
without the full costs of mammalian or avian styles of endothermy;
however, the small, and in particular the dromaeosaurian,
theropods (and their descendants, the true birds) were obliged to
develop full-blown endothermy and the associated insulatory
covering.

Dromaeosaurian theropods

2007-06-27T06:00:00.000-07:00

These bird-like dinosaurs exhibit a number of interesting
anatomical changes to the basic theropod body plan. Some changes
are quite subtle, but others are less so.
One notable feature is the ‘thinning’ of the tail: the tail becomes
very narrow and stiffened by bundles of long, thin bones, the only
flexible part being close to the hips (Figure 16, top). As argued
earlier, this thin, pole-like tail may well have been valuable as a
dynamic stabilizer to assist with the capture of fast-moving and
elusive prey. However, this type of tail dramatically changed the
pose of these animals because it was no longer a heavy, muscular
cantilever for the front half of the body. If no other changes had
been made to its posture, such a dinosaur would have been
unbalanced and constantly pitch forward on to its nose!
To compensate for the loss of the heavy tail, the bodies of these
theropods were subtly altered: the pubic bone, which marks the
rearmost part of the gut and normally points forward and
downward from each hip socket in theropods, was rotated
backwards so that it lay parallel to the ischium (the other lower
hip bone). Because of this change in orientation, the gut and
associated organs could be swung backwards to lie beneath the
hips. This change shifted the weight of the body backwards, and
compensated for the loss of the heavy counterbalancing tail. This
layout of hip bones, with the pubis rotated backward, is seen in
living and fossil birds as well as maniraptoran theropods.
Another equally subtle way of compensating for the loss of the
counterbalancing tail would be to shorten the chest in front of the
hips, and this is also seen in these bird-like theropods. The chest
also shows signs of being stiffened, and this probably reflects the
predatory habits of these animals. The long arms and three-clawed
hands were important for catching and subduing their prey and
needed to be very powerful. The chest region was no doubt
strengthened to help securely anchor the arms and shoulders to
withstand the large forces associated with grappling and subduing
prey. Birds also have a short, and greatly stiffened, chest region to
withstand the forces associated with anchoring the powerful flight
muscles.

At the front of the chest, between the shoulder joints, there is a
V-shaped bone (which is in fact the fused clavicles, or collar bones –
Figure 17) that acts as a spring-like spacer separating the shoulders,
it also helped to anchor the shoulders in place while these animals
were wrestling their prey. Birds also exhibit fused collar bones; they
form the elongate ‘wish bone’, or furcula, that similarly acts as a
mechanical spring that separates the shoulder joints during
flapping flight.

The joints between the bones of the arm and hand were also
modified so that they could be swung outward and downward with
considerable speed and force to strike at prey in what has been
called a ‘raking’ action. When not in use, the arms could be folded
neatly against the body. The leverage for this system was also of
considerable advantage to these creatures, because the arm muscles
that powered this mechanism were located close to the chest and
operated long tendons that ran down the arm to the hand (rather
than having muscles positioned further out along the arm); this
remote control system kept the weight of the body closer to the hips
and helped to minimize the delicate problem of balance in these
theropods. The arm-striking and arm-folding mechanism is closely
similar to that employed by birds when opening and closing their
wings during and after flight.

Archaeopteryx
The early bird-like fossil Archaeopteryx (Figure 16, bottom) exhibits
many maniraptoran theropod features: the tail is a long and very
thin set of vertebrae that anchored the tail feathers on either side;
the hip bones are arranged with the pubis pointing backward and
downward; at the front of the chest there is a boomerang-like
furcula; the jaws are lined with small, spiky teeth, rather than a
more typical bird-like horny beak; the arms are long, jointed so that
they can be extended and folded just as in theropods, and the hands
are equipped with three sharply clawed fingers that in their
arrangement and proportions are identical to those seen in
maniraptoran theropods.

Chinese wonders
During the 1990s, explorations in quarries in Liaoning Province
in north-eastern China began to yield some extraordinary, and
extraordinarily well preserved, fossils of Early Cretaceous age. At
first, these comprised beautifully preserved early birds such as
Confuciusornis, and the skeletons included impressions of feathers,
34. Restoration of the living Archaeopteryx
beaks, and claws. Then in 1996, a complete skeleton of a small
theropod dinosaur, very similar in anatomy and proportions to
the well known theropod Compsognathus (Figure 14), was
described by Ji Qiang and Ji Shu’an. They named the dinosaur
Sinosauropteryx. This dinosaur was remarkable because there was
a fringe of filamentous structures along its backbone and across its
body, suggesting some sort of covering to the skin that was akin to
the ‘pile’ on a roughly made carpet; there was also evidence of soft
tissues in the eye socket and in the region of the gut. It was clear
that some small theropods had some type of body covering. These
discoveries led to concerted efforts to find more such fossils at
Liaoning; they began to appear with increasing regularity and
ushered in some truly breathtaking revelations.
Shortly after Sinosauropteryx was discovered, another skeleton
was revealed. This animal, named Protoarchaeopteryx, was the
first to show the presence of true bird-like feathers attached to its
tail and along the sides of its body, and its anatomy was much
more similar to that of dromaeosaurians than Sinosauropteryx.
Another discovery revealed an animal that was extremely similar
to Velociraptor, but this time named Sinornithosaurus (again,
apparently covered in a ‘pile’ of short filaments). Newer discoveries
have included Caudipteryx, a large (turkey-sized), rather
short-armed creature noted for a pronounced tuft of tail feathers
and shorter fringes of feathers along its arms; smaller, heavily
feathered dromaeosaurians; and in the spring of 2003 a quite
remarkable ‘four-winged’ dromaeosaurian, Microraptor, was
unveiled to the world. This latter creature was small and classically
dromaeosaur-like, with the typically long, narrow tail, bird-like
pelvis, long, grasping arms, and sharp rows of teeth lining its
jaws. The tail was fringed by primary feathers and its body
covered in downy ones. However, what was singularly impressive
was the preservation along the arms of flight feathers forming
Archaeopteryx-like wings and, very unexpectedly, similar wing-like
fringes of feathers attached to the lower parts of the legs – hence the
name ‘four-wing’.

Legs, heads, hearts, and lungs of Dinasaurs

2007-06-26T20:26:00.000-07:00

Dinosaurs place their feet vertically beneath the body on straight,
pillar-like legs. The only living creatures that also adopt this
posture are birds and mammals; all the rest ‘sprawl’ with their
legs directed sideways from the body. Many dinosaurs were also
slender-limbed and apparently built for moving quickly; this
line of argument reflects the fact that Nature does not tend to do
things unnecessarily. If an animal is built as if it could run fast, it
probably did so; it might therefore seem reasonable to expect such a
creature to have an energetic ‘motor’, or endothermic physiology,
to allow it to move quickly. We do, however, need to be careful,
because it is also the case that ectotherms can move very quickly
indeed – crocodiles and Komodo dragons can outrun and catch
unwary humans! The crucial thing is that crocodiles and Komodo
dragons cannot sustain fast running – their muscles build up a large
oxygen debt very quickly and the animals then have to rest so their
muscles can recover. Endotherms, by contrast, can move quickly for
much longer periods of time because their high-pressure blood
system and efficient lungs replenish the oxygen in their muscles
very quickly.

A further refinement of this argument is the suggestion that the
ability to walk bipedally is linked exclusively to endothermy; many
mammals, all birds, and many dinosaurs are bipedal. This
argument relates not only to posture, but also to how that posture is
maintained. A quadruped has the advantage of considerable
stability when it walks. A biped is inherently unstable, and to walk
successfully a sophisticated system of sensors monitoring balance,
as well as a rapid coordinating system (the brain and central
nervous system), and rapid-response muscles to correct and
maintain balance, are essential.

The brain is central to this whole dynamic ‘problem’ and must
have a constant capacity to work quickly and efficiently. This
implies that the body is able to provide constant supplies of oxygen,
food, and heat to allow the chemistry of the brain to work optimally
all the time. The prerequisite for this type of stability is a ‘steady’
endothermic physiology. Ectotherms periodically shut down their
activity levels, when cold, for example, and reduce the supply of
nutrients to the brain, which is consequentially less sophisticated
and closely integrated to overall body functions.

Another posture-related observation can be linked to the efficiency
of the heart and its potential to sustain high activity levels. Many
birds, mammals, and dinosaurs adopt an upright body posture in
which the head is normally held at levels appreciably higher than
the position of the heart. This difference in head-heart level has
important hydrostatic consequences. Because the head is above the
heart, it has to be capable of pumping blood at high pressure ‘up’
to the brain. But the blood that is pumped at the same time with
each heartbeat from the heart to the lungs must circulate at low
pressure, otherwise it would burst the delicate capillaries that line
the lungs. To permit this pressure difference, the heart in mammals
and birds is physically divided down the middle, so that the left
side of the heart (the systemic, or head and body, circuit) can
run at a higher pressure than the right side (the pulmonary, or
lung, circuit).

All living reptiles carry their head at roughly the same level as their
heart. Their hearts are not divided down the middle like those of
mammals and birds because there is no need to differentiate
between the systemic and pulmonary circuits. Curiously, the
reptilian heart and circulation offers advantages for these creatures;
they can shunt blood around the body in ways that mammals
cannot. For example, ectotherms spend a lot of time basking in the
sun to warm their bodies. While basking, they can preferentially
shunt blood to the skin, where it can be used to absorb heat (rather
like the water in solar panel central heating pipes). The major
disadvantage of this system is that the blood cannot be circulated
under high pressure – a feature that is essential in any animal that
is behaving very actively and must bring food and oxygen to its
hard-working muscles.

The implication from all these considerations is that dinosaurs,
because of their posture, had a high-pressure blood circulation
system that was compatible with high and sustained activity levels
that are only found in living endotherms. This more comprehensive
and elaborate set of considerations resoundingly supports Richard
Owen’s provocative speculation.

Intimately associated with the efficiency of the heart and
circulatory system must be the ability to supply sufficient oxygen
to muscles to allow high levels of aerobic activity. In some groups of
dinosaurs, notably the theropods and the giant sauropodomorphs,
there are some tantalizing anatomical hints concerning lung
structure and function. In both these groups of saurischian
dinosaurs (but not the ornithischians), there are traces of distinct
pouches or cavities (called pleurocoels) in the sides of the vertebrae
of the backbone. In isolation, these might not have attracted
particular attention; however, living birds show similar features
that equate with the presence of extensive air sacs. Air sacs are
part of a bellows-like mechanism that permits birds to breathe
with remarkable efficiency. It is highly probable that saurischian
dinosaurs had bird-like, and therefore extremely efficient,
lungs.

This observation certainly supports the contention that some
dinosaurs (theropods and sauropodomorphs) had the ability to
maintain high aerobic activity levels. However, it also highlights
the fact that all dinosaurs (saurischians and ornithischians)
should not be presumed to have been the same in all aspects of
their physiology, because ornithischians show no trace of an
air-sac system.

Dinosaurs and warm blood

2007-06-26T20:24:00.002-07:00

A number of areas of research on dinosaurs have attracted attention
far beyond the realm of those who take a purely academic interest in
these creatures. This common interest appears to arise because
dinosaurs capture the public imagination in a way that few other
subjects do. The following chapters focus on these topics in order
to illustrate the extraordinary variety of approaches and types of
information that are used in our attempts to unravel the mystery of
dinosaurs and their biology.

Dinosaurs: hot-, cold-, or luke-warm-blooded?
As we have seen in Chapter 1, Richard Owen, at the time of his
invention of the word ‘dinosaur’, speculated about the physiology
of dinosaurs. Extracting meaning from the rather long-winded final
sentence of his scientific report:
The Dinosaurs . . . may be concluded to have . . . [a] superior
adaptation to terrestrial life . . . approaching that which now
characterizes the warm-blooded Vertebrata. [i.e. living mammals
and birds]
(Owen 1842: 204)
Although the ‘mammaloid’ reconstructions of dinosaurs that he
created for the Crystal Palace Park clearly echo his sentiments, the
biological implications he was hinting at were never grasped by
other workers at the time. In a sense, Owen’s visionary approach
was tempered by rational Aristotelian logic: dinosaurs were
structurally reptilian, it therefore followed that they had scaly
skins, laid shelled eggs, and, like all other known reptiles, were
‘cold-blooded’ (ectothermic).
In a similar vein to Owen, Thomas Huxley proposed, almost
50 years later, that birds and dinosaurs should be considered close
relatives because of the anatomical similarities that could be
demonstrated between living birds, the earliest known fossil bird
Archaeopteryx, and the newly discovered small theropod
Compsognathus. He concluded that:
. . . it is by no means difficult to imagine a creature completely
intermediate between Dromaeus [an emu] and Compsognathus [a
dinosaur] . . . and the hypothesis that the . . . class Aves has its root
in the Dinosaurian reptiles; . . .
(Huxley 1868: 365)
If Huxley was correct, it should have been possible to ask:
were dinosaurs then conventionally reptilian (physiologically)
or were they closer to the ‘warm-blooded’ (endothermic) birds?
There appeared to be no obvious way of answering such
questions.

Despite such intellectual ‘nudges’, it was close to a century after
Huxley’s paper that palaeontologists began to search with greater
determination for data that might have a bearing on this central
question. The spur to renewed interest in the topic finds an echo in
the adoption of the broader and more integrated agenda for the
interpretation of the fossil record: the rise of palaeobiology, as
outlined in Chapter 2. We saw there how some wide-ranging
observations were strung together by Robert Bakker into a case for
endothermy in dinosaurs. Let’s now consider these and other
arguments in greater detail.

New approaches: dinosaurs as climatic proxies?
Attempts were being made to investigate the degree to which fossils
could be used to reconstruct climates in the ancient world. It is
widely recognized that endotherms (basically mammals and birds)
are not particularly good indicators of climate because they are
found everywhere, from equatorial to polar regions. Their
endothermic physiology (and clever use of body insulation) allows
them to operate more or less independently of prevailing climatic
conditions. By contrast, ectotherms, such as lizards, snakes, and
crocodiles, are reliant on ambient climatic conditions, and as a
result they tend to be found mainly in warmer climatic zones.
Using this approach to examine the geographic distribution of
obvious ectotherms and endotherms in the fossil record proved
useful, but then threw up several interesting questions. For
example, what about the immediate evolutionary ancestors of
endothermic mammals in Permian and Triassic times? Were they
also able to control their internal body temperatures? If they did,
how would it have affected their geographic distribution? And more
pointedly in this context, dinosaurs seemed to have a wide
geographic spread, so did this mean that they were capable of
controlling their body temperature rather like endotherms?
Patterns in the fossil record
The foundation of Bakker’s approach to endothermy in dinosaurs
was the pattern in the succession of animal types in the early
Mesozoic. During the time leading up to the end of the Triassic
Period synapsid reptiles were by far the most abundant and diverse
animals on land.

Right at the close of the Triassic and the beginning of the Jurassic
Period (205 Ma) the very first true mammals appeared on Earth
and were represented by small, shrew-like creatures. In complete
contrast, the latter part of the Triassic Period also marks the
appearance of the first dinosaurs (225 Ma), and across the
Triassic/Jurassic divide the dinosaurs become widespread, very
diverse, and clearly dominant members of the land fauna. This
ecological balance – rare, small, very probably nocturnal mammals
and abundant, large, and increasingly diverse dinosaurs – was then
maintained for the next 160 million years, until the close of the
Cretaceous Period (65 Ma).

As animals living in the present day, we are comfortable with the
notion that mammals are, along with birds, the most conspicuous
and diverse of land-living vertebrates. Mammals are self-evidently
fast-moving, intelligent, generally highly adaptable creatures, and
much of this present-day ‘success’ we attribute to their physiological
status: their high basal metabolic rate, which permits the
maintenance of a high and constant body temperature, complex
body chemistry, comparatively large brains, and consequently high
activity levels, and their status as endotherms. In contrast, we
generally observe that reptiles are considerably less diverse and
quite sharply climatically restricted; this is largely explained by the
fact that they have a much lower metabolic rate, rely on external
sources of heat to keep the body warm and therefore chemically
active, and have much lower and more intermittent levels of
activity: the ectothermic condition.

These, admittedly very general, observations permit us to have
expectations that can be superimposed on the fossil record. All
things being equal, we would predict that the first appearance
of true mammals at the Triassic/Jurassic boundary, in a world
otherwise dominated by reptiles, would spark the former’s rapid
evolutionary rise and diversification at the expense of the latter.
So the fossil record of mammals would be expected to show a rapid
rise in abundance and diversity in Early Jurassic times, until they
completely dominated the ecosystems of the Mesozoic Era.
However, the fossil record reveals exactly the opposite pattern: the
(reptilian) dinosaurs rose to dominance in the Late Triassic
(220 Ma) and the mammals only began to increase in size and
diversity after the dinosaurs had become extinct at the end of the
Cretaceous period (65 Ma).

Bakker’s explanation for this counterintuitive set of events was that
dinosaurs could have succeeded, evolutionarily, in the face of true
mammals only if they too had endotherm-like high basal metabolic
rates and could be as active and resourceful as contemporary
mammals. Dinosaurs quite simply had to be active endotherms – it
was to Bakker a self-evident truth. While the pattern revealed by the
fossil record was indeed clear, the scientific proof necessary to
support his ‘truth’ needed to be assembled and tested.

Dinosaurs: a global perspective

2007-06-26T20:24:00.001-07:00

In more recent times, this approach has been applied much more
broadly and in a much more ambitious way. Paul Upchurch of
University College London and Craig Hunn at Cambridge hoped to
explore the entire family tree of the Dinosauria for evidence of
similarities in patterns of stratigraphic ranges and cladistic
patterns by looking at large numbers of dinosaurs. These were
compared to the currently established distributions of the
continents at intervals through the entire Mesozoic Era. An
attempt was being made to find out whether an overall signal did
emerge that was suggestive of a tectonic influence on the
evolutionary history of all dinosaurs.

Despite the inevitable ‘noise’ in the system resulting largely from
the incompleteness of the fossil record of dinosaurs, it was
heartening to note that statistically significant coincident patterns
emerged within the Middle Jurassic, the Late Jurassic, and the
Early Cretaceous intervals. This indicates that tectonic events do, as
expected, play some role in determining where and when particular
groups of dinosaurs flourished. What is more, this effect has also
been preserved in the stratigraphic and geographic distributions of
other fossil organisms, so the evolutionary history of great swathes
of organisms was effected by tectonic events and the imprint is still
with us today. In a way, this is not new. I need only point to the
unusual distribution of marsupial mammals (found only in the
Americas and Australasia today), and the fact that distinct areas of
the modern world have their own characteristic fauna and flora.
What this new research suggests is that we may well be able to trace
the historical reasons for these distributions far more accurately
than we had supposed possible.

Ornithopod evolution

2007-06-26T20:23:00.000-07:00

The earliest work in this field of research, carried out in 1984,
concerned a group of dinosaurs that are quite closely related to
the familiar Iguanodon. Generally, these types of dinosaur are
known as ornithopods (‘bird feet’ – this comes from a passing,
trivial resemblance in the structure of the feet of these dinosaurs
to those of modern birds). Comparing in some detail the anatomy
of a number of the then known ornithopods, a cladogram was
constructed. To convert this into a genuine phylogeny it was
necessary to chart on to the cladogam the known distribution of this
group through time and their geographic distributions.

Some surprising patterns in the history of these ornithopod
dinosaurs emerged from this analysis. First it seemed to
demonstrate that the forms most closely related to Iguanodon
(that is to say, members of the group known as iguanodonts) and
their closest relatives (members of the hadrosaur family) probably
originated as a result of continental separation during Late Jurassic
times. The ancestral population from which both groups may have
evolved became subdivided by a seaway at this time. Following this
isolation, one population evolved into the hadrosaurs in Asia, while
iguanodonts evolved elsewhere. These two groups appear to have
evolved distinct from one another through the Late Jurassic and
Early Cretaceous period. However, during the latter half of the
Cretaceous, Asia became reconnected to the rest of the northern
hemisphere continents and its hadrosaurs were apparently able to
spread across the northern hemisphere pretty much unhindered
and replaced iguanodonts wherever they came into contact.
While the pattern of replacement of iguanodonts by hadrosaurs in
Late Cretaceous times appeared to be reasonably uniform, there
were one or two puzzling anomalies that needed to be investigated.
There were reports, written at the turn of the 20th century, of
iguanodonts from Europe (primarily France and Romania) in
rocks of very latest Cretaceous age. From the analysis above, these
would not have been expected to have survived into Late Cretaceous
times because everywhere else the pattern was one of hadrosaurs
replacing iguanodonts. In the early 1990s, the best-preserved
material came from Transylvania, a region of Romania. However,
the phylogenetic analysis prompted expeditions to reinvestigate
these discoveries. Fresh study proved that this dinosaur was not a
close relative of Iguanodon, but represented an unusually
long-lasting (relict) member of a more primitive group of
ornithopods. An entirely new name was created for this dinosaur:
Zalmoxes. So, one of the outcomes of the preliminary analysis was a
great deal of new information about an old, but apparently not so
well understood, dinosaur.

A report published in the 1950s suggested that a very Iguanodonlike
dinosaur lived in Mongolia in Early Cretaceous times. This
tantalizing report also needed to be investigated further to check
whether its anomalous geographic range – in Asia in Early
Cretaceous times – was real or, as in the Romanian example,
another case of mistaken identity. The material, though
fragmentary, was stored in the Russian Palaeontological Museum
in Moscow, and had to be re-examined. What emerged was again
not as expected. This time the earlier reports proved correct, the
genus Iguanodon itself seemed to be present in Mongolia in Early
Cretaceous times, and the pieces recovered were indistinguishable
from the very well known European Iguanodon.
This second discovery did not fit at all comfortably with the
evolutionary and geographic hypothesis that had been created in
the 1984 analysis. Indeed, in more recent years a suite of very
interesting Iguanodon-like ornithopods have emerged in Asia,
as well as North America, in what can best be described as
‘middle’ Cretaceous times. Much of this very recent, and steadily
accumulating, evidence suggests that the original evolutionary and
geographic model had a number of fundamental flaws that
continued investigation and new discoveries were able to expose.

Dinosaur systematics and ancient biogeography

2007-06-26T20:21:00.000-07:00

This type of research can have interesting, if slightly unexpected,
spin-offs. One spin-off that will be considered here links
phylogenetics with the geographic history of the Earth. The Earth
may in fact have exerted a profound influence on the overall pattern
of life.

Unravelling the genealogy of dinosaurs
The geological timescale of the Earth was pieced together through
painstaking analysis of the relative ages of sequences of rocks
exposed at various places on Earth. One important component
that assisted this process was the evidence of the fossils that they
contained: if rocks from different places contained fossils of exactly
the same type, then it could be assumed with reasonable confidence
that the rocks were of the same relative age.

In broadly similar fashion, evidence of the similarity of fossils from
different parts of the world began to suggest that the continents
might not have been as fixed in their positions as they appear to be
today. For example, it had been noted that rocks and the fossils that
they contained seemed to be remarkably similar on either side of
the southern Atlantic Ocean. A small aquatic reptile Mesosaurus
was known to exist in remarkably similar-looking Permian rocks in
Brazil and in South Africa. As long ago as 1620, Francis Bacon had
pointed out that the coastlines of the Americas and Europe and
Africa seemed remarkably similar, (see Figure 32d) to the extent
that it seemed as if they could have fitted together as a pair of
gigantic jigsaw pieces. On the basis of evidence from fossils, rocks,
and general shape correspondence, Alfred Wegener, a German
meteorologist, suggested in 1912 that at times in the past the
continents of the Earth must have occupied different positions to
the ones they are in today, with, for example, the Americas and
Eur-Africa nestled together in the Permian Period. Because he was
not a trained geologist, Wegener’s views were ignored, or dismissed
as irrelevant and unprovable speculations. For all its self-evident
persuasiveness, Wegener’s theory lacked a mechanism: common
sense dictated that it was impossible to move things the size of
continents across the solid surface of the Earth.

However, common sense proved to be deceptive. In the 1950s
and 1960s, a series of observations accumulated that supported
Wegener’s views. Firstly, very detailed models of all the major
continents showed that they did indeed fit together remarkably
neatly and with a correspondence that could not be accounted for
by chance. Secondly, major geological features on separate
continents became continuous when continents were reassembled
jigsaw-like. And finally, palaeomagnetic evidence demonstrated the
phenomenon of sea-floor spreading – that the ocean floors were
moving like huge conveyor belts carrying the continents – and the
historical remnants of magnetism in continental rocks confirmed
that the continents had moved over time. The ‘motor’ that was
driving this motion was in effect the heat at the core and the fluidity
of rocks in the mantle layer inside the Earth. The theory of plate
tectonics that accounts for the movement of continents over the
surface of the Earth over time is now well established and
corroborated.

From a dinosaur evolutionary perspective, the implications of
plate tectonics are extremely interesting. Reconstructions of past
configurations of the continents, largely based on palaeomagnetics
and detailed stratigraphy, indicate that at the time of their origin all
the continents were lying clustered together in a single gigantic
landmass, known as Pangaea (‘all Earth’) (Figure 32a). Dinosaurs at
this time were quite literally capable of walking all over the Earth,
and in reflection of this it appears to be the case that the fossil
remains of rather similar types (theropods and prosauropods) have
been found on nearly all continents.

During subsequent Periods, the Jurassic (Figure 32b) and
Cretaceous (Figure 32c), it is evident that the supercontinent began
to fragment as the immensely powerful tectonic conveyor belts
imperceptibly, but remorselessly, wrenched Pangaea apart. The end
product of this process at the close of the Cretaceous was a world
that, though still different geographically (note particularly the
position of India in Figure 32c), has some very familiar-looking
continents.

The earliest dinosaurs seem to have been able to disperse across
much of Pangaea, judging by their fossils. However, during the
Jurassic and subsequent Cretaceous Periods it was clearly the case that the unified supercontinent became gradually subdivided by
intervening seaways as continent-sized fragments gradually drifted
apart.
An inevitable biological consequence of this intrinsic (Earth-bound)
process of continental sundering is that the once cosmopolitan
population of dinosaurs became progressively subdivided and
isolated. The phenomenon of isolation is one of the keystones of
organismal evolution – once isolated, populations of organisms
tend to undergo evolutionary change in response to local changes to
their immediate environment. In this instance, although we are
dealing with comparatively huge (continent-sized) areas, each of
the continental fragments carried its own population of dinosaurs
(and associated fauna and flora); each of which, with the passing
time, had the opportunity to evolve independently in response to
local changes in environment, stimulated by, for example,
progressive changes in latitude, longitude, adjacent oceanic
currents, and prevailing atmospheric conditions.

Logic dictates that it must clearly have been the case that tectonic
events during the Mesozoic affected the scope and overall pattern of
32(d). The continents as they are today. Close the Atlantic Ocean and
the Americas fit neatly against West Africa.
the evolutionary history of dinosaurs. Indeed, it seems perfectly
reasonable to suppose that the progressive fragmentation of
ancestral populations over time must have done much to accelerate
the diversification of the group as a whole. Just as we can
represent the phylogeny of dinosaurs using cladograms, we could
also represent the geographic history of the Earth through
the Mesozoic Era as a series of branching events as
continental areas separated from the ‘ancestral’ Pangaean Earth.
Of course, this general approach is a simplification of true
Earth history because, on occasion, continental fragments have
coalesced, welding together previously isolated populations.
But at least as a first approximation, this provides a fertile area
for investigating some of the larger-scale events in Earth
history.

If this model of the natural history of dinosaurs were generally true,
we might expect to be able to detect some evidence in its support by
probing the details of the fossil record of dinosaur species, and the
tectonic models of continental distribution through the Mesozoic.
This type of approach has been developed in recent years to probe
for coincident patterns in the evolutionary history of dinosaurs and
whether their evolutionary history is echoed in their geographic
distribution.

Ornithischian dinosaurs

2007-06-26T20:19:00.001-07:00

All ornithischians are thought to have been herbivorous and, rather
like modern-day mammals, they seem to be far more diverse, and
numerous, than their potential predators.
Thyreophorans (Figure 28) are a major group of ornithischians that are characterized by bearing bony plates in their body wall, clubs or
spikes adorning their tails, and for having an almost exclusively
quadrupedal method of locomotion. These types of dinosaur
include the stegosaurs, named after the iconic Stegosaurus (well
known for its tiny head, the rows of large bony plates on its back,
and its spiky tail (Figure 31)); and the heavily armoured
ankylosaurs including such creatures as Euoplocephalus. The latter
was a huge tank-like animal that was so heavily armour-plated that
even its eyelids were reinforced by bony shutters and its tail was
terminated in a huge, bony club that it presumably used to skittle
potential predators.

Cerapodans (Figure 28) were very different to thyreophorans. These
were typically lightly built, unarmoured bipeds, although a few did
revert to quadrupedal methods of locomotion. Ornithopods were
one major group of cerapodans. Many of these dinosaurs were
medium-sized (2–5 metres long) and quite abundant (probably
filling the ecological niches occupied by antelopes, deer, sheep, and
goats today). These animals, such as Hypsilophodon, were balanced
at the hip ( just like theropods), had slender legs for fast running,
grasping hands, and, most importantly, teeth, jaws, and cheeks
adapted for a diet of plants. Throughout the reign of the dinosaurs,
small to medium-sized ornithopods were quite abundant, but
through the Mesozoic a significant number of larger types evolved;
these are known as iguanodontians (because they include animals
such as Iguanodon). Most important of all the iguanodontians were
the extraordinarily numerous duck-billed, or hadrosaurian,
dinosaurs of the Late Cretaceous of North America and Asia. Some
(but not all) of these dinosaurs did indeed have rather duck-shaped
snouts, and others had a wide range of quite extravagant, hollowcrested
headgear (see Chapter 7); this headgear may well have been
used for social signalling, and more particularly for making loud,
honking sounds. Marginocephalians were the other major
cerapodan group and appeared in Cretaceous times. These included
the extraordinary pachycephalosaurs (‘thick-headed dinosaurs’);
they had bodies that were very similar in general appearance to the
ornithopods, but their heads were very odd-looking. The majority
had a high dome of bone on the top, which looked vaguely similar to
the headgear of hadrosaurians, except for the fact that
pachycephalosaur headgear was made of solid bone. It has been
suggested that these creatures were the ‘headbangers’ of the
Cretaceous world – perhaps using head clashing in similar fashion
to that seen among some cloven-hooved animals today.
Finally, there were the ceratopians, a group of dinosaurs that
included the fabled Protoceratops referred to in the Introduction,
as well as the well-known Triceratops (‘three-horned face’). All
had a singular narrow beak at the tip of the jaws and tended to
have a ruff-like collar of bone at the back edge of the skull. While
some of these dinosaurs, particularly the early ones, maintained
a bipedal way of life, a considerable number grew greatly in
body size, with an enlarged head, which was adorned with a
huge frill-like collar and large eyebrow and nose horns. Their
great bulk and heavy head led them to adopt a four-footed
stance, and their similarity to modern-day rhinoceros has
not gone unnoticed. Clearly, as this all too brief survey shows,
dinosaurs were many and varied, judging by the discoveries
made over the past 200 years. But even though to date
about 900 genera of dinosaurs are known, this is only a tiny
fraction of the dinosaurs that lived during the 160 million
years of their reign during the Mesozoic Era. Many of these
will, unfortunately, never be known: their fossils were never
preserved. Others will be discovered by intrepid dinosaur hunters
in years to come.

Saurischian dinosaurs

2007-06-26T20:16:00.000-07:00

Saurischians include two major groups. Sauropodomorpha are
mainly large-bodied creatures with pillar-like legs, extraordinarily
91
Unravelling the genealogy of dinosaurs
long tails, long necks ending in small heads, and jaws lined with
simple, peg-shaped teeth, indicating a mainly herbivorous diet.
These include such giants as members of the diplodocoid,
brachiosauroid (Figure 31), and titanosaurian groups. Theropoda
are markedly different to their sauropodomorph relatives. They are
almost entirely agile, bipedal, and predominantly meat-eating
dinosaurs (Figures 30, 31). A long, muscular tail counterbalances the
front of the body at the hip, leaving the arms and hands free to be
used to grab their prey; their heads also tend to be rather large, and
their jaws lined with sharp, knife-like teeth. These types of dinosaur
range from small and rather delicate creatures similar to
Compsognathus, which are commonly referred to as coelurosaurs,
through to such enormous creatures such as the legendary
Tyrannosaurus, while other equally large and fearsome-looking
theropods include Giganotosaurus, Allosaurus, Baryonyx, and
Spinosaurus. Although some of these dinosaurs may be well known,
the group as a whole is proving to be extraordinarily diverse, and in
some cases quite bizarre. Newly discovered therizinosaurs, for
example, appear to have been huge, lumbering creatures with long,
scythe-like claws on their hands, enormous bellies, and ridiculously
small heads whose jaws were lined with teeth that are far more
reminiscent of plant-eaters than conventional meat-eaters. Yet
other theropods known as ornithomimians and oviraptorians were
lightly built, rather ostrich-like creatures that were entirely
toothless (and therefore beaked just like living birds). However, the
source of greatest interest among this entire group of dinosaurs is
the subgroup known as dromaeosaurians.
Dromaeosaurians include such renowned creatures as Velociraptor
and Deinonychus, and a host of similar but less famous creatures
that have been discovered recently. Their particular interest lies in
the fact that their skeletal anatomy is closely similar to that of living
birds; indeed, the similarities are so great that they are thought to
be direct bird ancestors. Dramatic new discoveries, at sites in
Liaoning Province, China, that exhibit truly exceptional
preservational conditions, of dromaeosaurian theropods reveal a body covering made of either keratinous filaments (like a coarse
form of hair) or in some cases genuinely bird-like feathers, which
emphasizes their similarity to modern birds.

Genealogy of dinosaurs

2007-06-26T20:14:00.000-07:00

Up to this point, our focus has been largely, if not exclusively,
tuned to exploring aspects of the anatomy, biology, and way of
life of the dinosaur Iguanodon. It must be obvious that Iguanodon
was just one dinosaur that fitted into far larger tableaux of life
in the Mesozoic Era. One of the important tasks that falls to
palaeontologists is to try to discover the genealogy, or evolutionary
history, of the species that they study. To put dinosaurs as a whole
into some sort of perspective, it will be necessary to outline the
techniques used to do this, and our current understanding of
dinosaurian evolutionary history.

One feature of the fossil record is that it offers the tantalizing
possibility of tracing the genealogy of organisms not just over a
few human generations (which is the ambit of modern genealogists)
but over thousands, or millions, of generations, across the
immensity of geological time. The primary means by which such
research is carried out at present is the technique known as
phylogenetic systematics. The premise of this technique is really
quite simple. It accepts that organisms are subject to the general
processes of Darwinian evolution. This does not require anything
more profound than the assumption that organisms that are more
closely related, in a genealogical sense, tend to physically resemble
each other more closely than they do more distantly related
creatures. To try to investigate the degree of relatedness of creatures
(in this particular case fossil creatures), palaeosystematists are most
interested in identifying as wide a range of anatomical features as
are preserved in the hard parts of their fossils. Unfortunately, a
great deal of really important biological information has simply
rotted and been lost during the process of fossilization of any
skeleton, so, being pragmatic about things, we simply have to make
the most of what is left. Until quite recently, the reconstruction of
phylogenies had relied on hard-part anatomical features of animals
alone; however, technological innovations have now made it
possible to compile data, based on the biochemical and molecular
structure of living organisms, that can add significant and new
information to the process.

What the dinosaur systematist has to do is compile lengthy lists of
anatomical characteristics, with the intention of identifying those
that are phylogenetically important, or contain an evolutionary
signal. The task is intended to produce a workable hierarchy of
relationship, based on groupings of ever more closely related
animals.

The analysis also identifies features that are unique to a particular
fossil species; these are important because they establish the special
characteristics that, for example, distinguish Iguanodon from all
other dinosaurs. This probably sounds blindingly obvious but,
in truth, fossil creatures are often based on a small number of
bones or teeth. If other partial remains are discovered in rocks
elsewhere from the original, but of very similar age, it can be quite a
challenge to prove convincingly whether the new remains belong to,
say, Iguanodon, or perhaps a new and previously undiscovered
creature.

Beyond the features that identify Iguanodon as unique, there is also
a need to identify anatomical features that it shares with other
equally distinct, but quite closely related animals. You might say
that these were the equivalent of its anatomical ‘family’. The more
general the characters that ‘family’ groups of dinosaurs share, the
more this allows them to be grouped into ever larger and more
inclusive categories of dinosaurs that gradually piece together an
overall pattern of relationships for them all.
The real question is: how is this overall pattern of relationships
achieved? For a very long time, the general method that was used
might be described simply as ‘I know best’. It was quite literally the
view of self-styled experts, who had spent much time studying a
particular group of organisms and then summarized the overall
patterns of similarity for their group; their methods for doing this
might vary considerably, but in the end their preferred pattern of
The case of Baryonyx
The Early Cretaceous rocks of south-east England have been
intensely investigated by fossil hunters (starting with Gideon
Mantell) and geologists (notably William Smith) for well
over 200 years. Iguanodon bones are very common, as are
the remains of a limited range of other dinosaurs, such as
‘Megalosaurus’, Hylaeosaurus, Polacanthus, Pelorosaurus,
Valdosaurus, and Hypsilophodon. Given the intensity of such
work, it would be thought highly unlikely that anything new
would ever be discovered. However, in 1983 the amateur collector
William Walker discovered a large claw bone in a clay
pit in Surrey that led to the excavation of an 8-metre-long
predatory dinosaur that was entirely new to science. It was
named Baryonyx walkeri in honour of its discoverer, and
has pride of place on exhibition at the Natural History
Museum in London.

The moral of this story is that nothing should be taken for
granted; the fossil record is likely to be full of surprises.
relationship was little more than just that: their preference,
rather than a rigorous, scientifically debated solution. While this
method worked reasonably well for restricted groups of organisms,
it proved far more difficult to properly debate the validity of one
interpretation compared with another because the arguments,
when boiled down to their essentials, were circular, relying on one
person’s belief over another’s.

This underlying problem was brought into sharp focus when
groups of organisms were very large in number and varied in
many subtle ways. Good examples are groups of insects, or some
of the bewildering varieties of bony fish. If the general scientific
community was happy to accept the authority of one scientist for a
period of time then all was apparently fine. However, if experts
could not agree, the end result was frustratingly circular debates.
Over the past four decades, a new methodology has gradually been
adopted that has proved far more valuable scientifically. It does not
necessarily give the correct answers, but it is at least open to
scientific scrutiny and real debate. This technique is now widely
known as cladistics (phylogenetic systematics). The name is treated
with a fair degree of trepidation by some, but this is largely because
there have been some very fierce arguments about how cladistics is
done in practice and what the overall significance of the results
might be in an evolutionary context. Fortunately, we do not need to
consider much of this debate because the principles are actually
surprisingly simple and clear-cut.

A cladogram is a branching tree diagram that links together
all the species that are being investigated at the time. To create
one, the researcher needs to compile a table (data matrix)
containing a column listing the species under consideration
and against this a compilation of the features (anatomical,
biochemical, etc.) that each species exhibits. Each species is
then ‘scored’ in relation to whether it does (1) or does not (0)
possess each character, or in some instances if the decision is
uncertain this can be signified as a (?). The resulting matrix
of data (these can be very large) is then analysed using a number
of proprietary computer programs, whose role is to assess the
distribution of 1s and 0s and generate a set of statistics that
determines the most parsimonious distribution of the data
that are shared by the various species. The resulting cladogram
forms the starting point for a considerable amount of further
investigation that is aimed at determining and understanding
the degree to which there are common patterns or overall
similarities, and the extent to which the data might be misleading
or erroneous.

The cladogram that results from this type of analysis represents
no more than a working hypothesis of the relationships of the
animals that are being investigated. Each of the branches on
the tree mark points at which it is possible to define a group
of species that are all connected by their sharing a number of
characteristic features. And using this information it is possible
to construct what is, in effect, a sort of genealogy or phylogeny
representing a model of the evolutionary history of the group
as a whole. For example, if the known geological times of
occurrence of each of the species are plotted on to this pattern,
it becomes possible to indicate the overall history of the group,
and also the probable time at which various of the species may
have originated. In this way, the cladogram, rather than simply
representing a convenient spatial arrangement of species, begins
to resemble a real genealogy. Obviously, each such phylogeny
created in this way is only as good as the data available, and
the data and how it is scored can change with the discovery
of new, better, or more complete fossils, and also as new
methods of analysis are developed or older ones are
improved upon.

Iguanodon and dietary

2007-06-26T20:11:00.000-07:00

The first recognizable fossils of Iguanodon were teeth, whose telltale
features showed that it was a herbivorous animal; they were
chisel-shaped to be able to slice and crush plants in the mouth
before they were swallowed.

The need to cut and crush plant food hints at some important
considerations concerning the diets of extinct creatures and some of
the clues that their skeletons may contain.

Carnivores have a diet largely comprising meat. From a biochemical
and nutritional perspective, a diet of meat is one of the simplest
and most obvious of options for any creature. Most of the other
creatures in the world are made of roughly similar chemicals as the
carnivores that eat them. Their flesh is therefore a ready and rapidly
assimilated source of food, provided the prey can be caught, sliced
into chunks in the mouth using simple knife-like teeth (or even
swallowed whole), and then quickly digested in the stomach.
Iguanodon’s brain
The structure of the brain cavity shows large olfactory
lobes at the front, suggesting that Iguanodon had a welldeveloped
sense of smell. Large optic nerves passed through
the braincase in the direction of the big eye sockets, apparently
confirming that these animals had good vision. The
large cerebral lobes indicate a well coordinated and active
animal. The inner ear cast shows the looped semicircular
canals that provided the animal’s sense of balance, and a
finger-like structure that was part of its hearing system.
Beneath the brain cavity hangs a pod-like structure that
housed the pituitary gland, which was responsible for regulating
its hormone functions. Down either side of the cast
are seen a series of large tubes, which represent the passages
through the original braincase wall (chipped away
here of course) for the twelve cranial nerves. Other smaller
pipes and tubes passing through the braincase wall are also
preserved, and these hint at the distribution of a set of blood
vessels that carried blood into the floor of the brain from the
heart (via the carotid artery) and, of course, drained the
blood away from the brain through the large lateral head
veins that lead back down the neck.

This whole process has the potential to be relatively quick and
biochemically very efficient in that little is likely to be wasted.
Herbivores face a rather more challenging problem. Plants are
neither particularly nutritious nor readily assimilable when
compared to animal flesh. Plants are primarily built from large
quantities of cellulose, a material that gives them strength and
rigidity. The crucial, and extremely awkward, point about this
unique chemical, so far as animals are concerned, is that it is
completely indigestible: there is simply nothing in the armoury of
chemicals in our guts that can actually dissolve cellulose. As a result,
the cellulose portion of plants passes straight through animals’ guts
as what we call roughage. So, how do herbivores survive on what
appears to be such an unpromising diet?
Plant-eaters have successfully adapted to this diet because they
exhibit a number of characteristic features. They have a good set
of teeth with hard-wearing, durable, complex, and rough grinding
surfaces, and powerful jaws and muscles that can be used to grind
up plant tissues between the teeth to release the nutritionally
usable ‘cell sap’ that is enclosed within plant cell walls. Herbivores
eat large quantities of plant food in order to be able to extract
sufficient nutrients from such comparatively nutrient-poor
material. As a result, herbivores tend to have barrel-shaped bodies
that accommodate large and complicated guts, which are
necessary to store the large volumes of plants that they have to eat
and allow sufficient time for digestion to take place. Herbivores’
large guts house dense populations of microbes that live within
special chambers or pouches in the gut wall; our appendix is a
tiny vestige of such a chamber, and hints at herbivory in our
primate ancestry. This symbiosis allows herbivorous animals to
provide a warm, sheltered environment and constant supplies of
food for the microbes; in their turn, the microbes have the ability
to synthesize cellulase, an enzyme that digests cellulose and
converts it into sugars that can then be absorbed by the host
animal.

By most standards, Iguanodon (11 metres long and weighing about
3–4 tonnes) was a large herbivorous animal, and would have
consumed plants in large quantities. Given this background
information, questions about precisely how Iguanodon fed and
assimilated its food can be explored in detail.
One persistent theory concerning its method of feeding was its
suggested use of a long tongue to pull vegetation into the mouth.
This began with Gideon Mantell, who described one of the first,
nearly complete lower jaws of Iguanodon. The new fossil included
some tell-tale teeth, so the ownership could not be doubted, and
it had a toothless, spout-shaped front end. Mantell speculated
that the spout shape allowed a long tongue to slide in and out of
the mouth, rather like a giraffe’s does. Mantell could not have
known that the tip of the newly discovered lower jaw was
incomplete and was capped by a predentary bone that filled in
the ‘spout’.

Careful re-examination of the lower jaws of a number of Iguanodon
skulls from Bernissart failed to reveal Dollo’s predentary tunnel.
The predentary has a sharp upper edge that supported a turtle-like
horny beak. The predentary, and its beak, bit against the similarly
toothless beak-covered premaxillae at the tip of the upper jaw, and
this arrangement allowed these dinosaurs to very effectively crop
the plants upon which they were feeding. The advantage of the
horny beak was that it would have grown continuously (unlike
teeth, which gradually wear away) no matter how tough and
abrasive the plants that were being cropped. The ceratobranchial
bones still require some explanation. In this instance, they would
have been used to anchor the muscles that moved the tongue
around the mouth to reposition the food as it was being chewed
and for pushing the food back into the throat when it was
ready to be swallowed. This is exactly the same role that is
performed by the ceratobranchial bones in the floor of the
human mouth.

How Iguanodon chewed its food
Apart from the horny beak that was able to nip off plants at the
front end of the mouth, the sides of the jaws are lined with a
formidable, nearly parallel array of chisel-like teeth that form
irregularly edged blades (Figure 26). Each working tooth slots
neatly against its neighbours in a rank-and-file arrangement, and
beneath the working teeth are replacement crowns that will slot
into place as the working teeth are worn away, forming what is in
effect a ‘magazine’, or battery, of teeth. This continuous replacement
pattern is normal for reptiles in general. What is unusual, even by
reptile standards, is that the working and replacement teeth are
held together in an ever-growing magazine as if they were all
contributing to one giant, grindstone-like tooth. Wear between
opposing (upper and lower) magazines maintains a grinding
surface throughout the life of the dinosaur. Rather than having
permanent, hard-wearing grinders (as we do), this could be
described as a disposable model that relies on constant replacement
of individually simpler teeth.

Opposing edges of each cutting blade of teeth have characteristics
that ensure efficiency in their cutting action. The inner surfaces
of the lower teeth are coated in a thick layer of extremely hard
enamel, while the remainder of the tooth is made of softer,
bone-like dentine. In contrast, the upper teeth have the reverse
arrangement: the outer edge being coated in thick enamel and
the remainder of the tooth is composed of dentine. When the
jaws are closed, these opposing blades slide past each other: the
hard, enamelled leading edge of the lower jaw magazine meets
the enamelled cutting edge of the upper teeth in a cutting/
shearing action rather like the blades of a pair of scissors (Figure
27). Once the enamelled edges have passed one another, the
enamel edges (unlike scissor blades) then cut against the less
resistant dentine parts of opposing magazines in a tearing
and grinding action, which is ideal for crushing up tough
plant fibres.

The geometry of the grinding surfaces of the upper and lower
‘magazines’ is particularly interesting. The worn surfaces are
oblique, the lower surfaces face outward and upward, while the
upper teeth have worn surfaces that face inward and downward.
This pattern has interesting consequences. In conventional reptiles,
the closure of the lower jaw is brought about by a simple hinge
effect, with the jaws on either side of the mouth closing
simultaneously in what is called an isognathic bite. If this type of
bite is proposed for Iguanodon, then it is immediately obvious that
the two sets of teeth on either side of the mouth would simply
become permanently wedged together: the lower jaws jamming
inside the upper ones. This means it is impossible to imagine
how the angled wear surfaces could ever have developed in the
first place.

For the angled wear surfaces to have developed, there would
have had to be some ability of the jaws to move sideways as they
closed. This type of movement is achieved in living herbivorous
mammals through the development of an anisognathic jaw closure
mechanism. This relies on the fact that the lower jaws are naturally
narrower than the upper jaws. Special muscles, arranged in a sling
on either side of each jaw bone, are capable to controlling the
position of the jaw very precisely so that the teeth on one side meet
one another and then the lower set is forcibly slid inwards so that
the teeth grind against one another. We humans employ this type
of jaw mechanism, especially when eating tough foods, but it is far
more exaggerated in some classically herbivorous mammals such
as cows, sheep, and goats, where the swing of the jaw is very
obvious.

The whole mammalian type of jaw mechanism is dependent upon
very complex jaw muscles, a complex nervous control system, and a
specially constructed set of skull bones to withstand the stresses
associated with this chewing method. By contrast, more
conventional reptiles. of which Iguanodon was one, do not have
an anisognathic jaw arrangement, lack the complex muscular
arrangements that allow the lower jaw to be very precisely
positioned (whether they had the nervous system to control
such movements is largely irrelevant), and their skulls are not
specially reinforced to withstand the lateral forces acting on the
skull bones.

More on Iguanodon

2007-06-26T20:07:00.000-07:00

A ‘twist’ in the tail
Re-examining the skeletal evidence from first principles, the
anatomy of the skeletons from Bernissart reveal some disconcerting
features. One of the most obvious concerns the massive tail of
Iguanodon. The well-known reconstruction shows the animal
(Figure 12) propped, in true kangaroo style, using its tail and hind
legs tripod-like. To adopt this posture, the tail curves upward to the
hip. In sharp contrast, all the documentary and fossil evidence
points to this animal normally holding its tail essentially straight or
somewhat downwardly curved. This is clearly seen in the specimens
arranged on banks of plaster in the museum, and in the wonderful
pencil sketches made of their skeletons before they were exhibited
(Figure 20). It could of course be argued that this shape was simply
an artefact of preservation, but this explanation is definitely not
plausible here. The backbone was in effect ‘trussed’ on either side by
a trellis-like arrangement of long bony tendons that held the
backbone quite deliberately straight; these can be seen in Figure 20.
As a result, the heavy, muscled tail served as an enormous cantilever
to balance the weight of the front part of the body at the hips. The
truth is that the upward sweep of the tail seen in Dollo’s
reconstructions would have been physically impossible for these
animals in life. Careful examination of the skeleton revealed that
the tail was deliberately broken in several places to achieve the
upward bend – a case perhaps of Louis Dollo making the skeleton
fit his personal ideas a little over-zealously.
This discovery disturbs the pose of the remainder of the skeleton. If
the tail is straightened so that it can adopt a more ‘natural’ shape,
then the tilt of the body changes dramatically, with the backbone
becoming more horizontal and balanced at the hip. As a result the
chest is lower, bringing the arms and hands closer to the ground
and raising questions about their likely function.
Hands or feet?

The hand of Iguanodon has become part of dinosaurian folklore
for one obvious reason. The conical thumb-spike was originally
identified as a rhinoceros-like horn on the nose of the Iguanodon
(Figure 9) and was immortalized in the giant concrete models
erected at London’s Crystal Palace (Figure 2, Chapter 1). It was not
until Dollo provided the first definitive reconstruction of Iguanodon
in 1882 that it was proved to everyone’s satisfaction that this bone
was indeed a part of the dinosaur’s hand. However, the hand (and
the entire forelimb) of this dinosaur held a few more surprises.
The thumb, or first finger, comprises a large, conical, claw-bearing
bone that sticks up at right angles to the rest of the hand and can be
moved very little (Figure 21A). The second, third, and fourth fingers
are very differently arranged: three long bones (metacarpals) form
the palm of the hand and are bound tightly together by strong
ligaments; the fingers are jointed to the ends of these metacarpals
and are short, stubby, and end in flattened and blunt hooves. When
these bones were manipulated, to see what their true range of
movement was likely to be, it was found that the fingers splayed
outwards (away from each other) and certainly could not flex to
form a fist and perform simple grasping functions, as might have
been expected. This distinctive arrangement looks similar to that
seen in the feet of this animal: the three central toes of each foot are
similarly shaped and jointed, splay apart, and end in flattened
hooves. The fifth finger is different from all the others: it is quite
separate from the previous four and set at a wide angle from the
remainder of the hand; it is also long and has a wide range of
movement at each joint, and was presumably unusually flexible.
This re-examination led me to dramatically revise earlier ideas and
conclude that the hand is one of the most peculiar seen in the entire
animal kingdom. The thumb was without doubt an impressive,
stiletto-like weapon of defence (Figure 21B); the three central
fingers were clearly adapted to bear weight (rather than for
grasping things as hands usually do); and the fifth finger was
sufficiently long and flexible to act as a truly finger-like grasping
(prehensile) organ (Figure 21A).

The idea that the hand could act as a foot for walking upon, or at
least supporting some of the body weight, was revolutionary – but
was it true? This prompted further research on the arm and
shoulder for additional evidence that might confirm such a radical
reinterpretation.

First of all, the wrist proved to be interesting. The bones of the wrist
are welded together to form a bony block, instead of being a row of
smooth, rounded bones that could slide past one another in order to
allow that hand to swivel against the forearm. All the individual
wrist bones have been welded together by bony cement, and are
further strengthened around the outside by strands of bony
ligament. These features obviously combined to lock the wrist
firmly against the hand and forearm bones and resist the forces
acting through them during weight-bearing, as would be necessary
if the hands were truly acting as feet.

The remainder of the arm bones are extremely stoutly built, again
primarily for strength during weight support, rather than for
allowing flexibility as is more normal with genuine arms. The
stiffness of the forearm has important consequences for the way in
which the hand would have been placed on the ground – the fingers
would have pointed outward and the palms inward – an unusual
consequence of converting a hand into a foot. The pose of the hand,
in this rather awkward manner, has been confirmed by examination
of the shape of forefoot prints left by this dinosaur.

Size and sex
The Bernissart discoveries are notable for comprising two
types of Iguanodon. One (Iguanodon bernissartensis – quite
literally ‘the Iguanodon that lived in Bernissart’) is large and
robustly built, and represented by more than 35 skeletons; the other
(Iguanodon atherfieldensis, formerly called I. mantelli – literally
‘Mantell’s Iguanodon’) is smaller and more delicately built
(approximately 6 metres in length) and represented by only
two skeletons.

These specimens were regarded as distinct species until they
were reassessed in the 1920s, by Francis Baron Nopcsa, a nobleman
from Transylvania and a palaeontologist. The discovery of two
quite similar types of dinosaur that evidently lived in the same
place, at the same time, prompted him to ask the simple and yet
obvious question: are they males and females of the same species?
Nopcsa attempted to determine sexual differences in a number of
fossil species. In the case of the Iguanodon from Bernissart he
concluded that the smaller and rarer species was the male and the
larger and more numerous species was the female. He observed,
perfectly reasonably, that it is often the case that female reptiles
are larger than males. The biological reason for this is that
females often have to grow large numbers of thick-shelled
eggs; these drain considerable resources from the body before
they are laid.

While this seems quite a reasonable supposition, it is in fact
very difficult to prove scientifically. Apart from size, which is
surprisingly variable among reptiles as a whole and not nearly
as consistent a feature as Nopcsa would have had us believe,
the features used to distinguish the sexes among living reptiles
are most commonly found in the soft anatomy of the sex organs
themselves, coloration of the skin, or behaviour. This is particularly
unfortunate because only very rarely do fossils ever preserve such
features.

The most valuable evidence would be the discovery of soft
anatomical fossils of the sexual organs of Iguanodon – unfortunately,
this is an extremely unlikely event. And, since we can never know
their true biology and behaviour, we have to be a little cautious
and also realistic. For the present, it is safer to record the differences
(we may have our own suspicions, perhaps), but simply leave it
at that.

A careful study of the more abundant large Iguanodon from
Bernissart revealed that a few were smaller than the average.
Measuring the proportions of each of these skeletons revealed
an unexpected growth change. Smaller, presumably immature
specimens had shorter arms than would have been expected.
The comparatively short-armed juveniles may well have
been more adept bipedal runners, but as large adult size and
stature was achieved they may have become progressively
more accustomed to moving around on all fours. This also
fits with the observation of an intersternal ossification in
only larger, presumably adult, individuals, which spent more
of their time on all fours compared to smaller, younger
individuals.

Soft tissues
Soft tissues of fossil creatures are preserved only rarely, and under
exceptional preservational conditions, so palaeontologists have
developed techniques to decipher clues concerning this type of
biology of dinosaurs both directly and indirectly.
Louis Dollo reported small patches of skin impression on parts
of the skeletons of Iguanodon. A number of the skeletons from
Bernissart are shown in a classic ‘death pose’ with the powerful neck
muscles contracted, during rigor mortis, pulling the neck into a
sharp curve and turning the head upward and backward. That this
pose has been maintained during the time between death and
eventual burial implies that the carcass of the animal had stiffened
and dried out. Under such conditions, its tough, parchment-like
skin would have formed a rigid surface against which the finegrained
muds would have moulded themselves during burial.
Provided that the entombing sediment compacted sufficiently to
retain their shape, prior to the inevitable rotting and disappearance
of the dinosaur’s organic tissues, then (as with simple clay moulds)
an impression of the texture of the skin surface would have been
preserved in the sediment.

New light on Iguanodon

2007-06-26T20:04:00.001-07:00

The resurgence in palaeobiology in the 1960s, and the new insights
into dinosaurs prompted by John Ostrom’s important work,
provided a spur to reinvestigate some of the earliest discoveries.
Louis Dollo’s description of the incredible discoveries of Iguanodon
at Bernissart created the image of a giant (5 metres tall, 11 metres
long) kangaroo-like creature. It had:
powerful back legs and a massive tail that helped it to balance . . . [and]
was a plant eater . . . it grasped bunches of leaves with its long tongue,
then pulled them into its mouth to be clipped off with the beak.
The picture of Iguanodon was of an animal that was the dinosaur
equivalent of a ‘tree browser’, represented in the recent past by the
giant South American ground sloths and today by giraffes. Dollo
himself referred to Iguanodon as a ‘girafe reptilienne’. Rather
surprisingly, nearly every aspect of this vision of Iguanodon is
incorrect or seriously misleading.

Bernissart: a ravine where Iguanodon perished?
Some of the earliest work at Bernissart focused on the extraordinary
circumstances of the original discovery. The dinosaurs had been
unearthed in a coal mine at depths of between 356 and 322 metres
below the surface (Figure 18). This was unexpected, as the coal
seams being excavated were known to be Palaeozoic in age and
dinosaurs are of course unknown in rocks of such antiquity.
However, the Iguanodon skeletons were not found in the coal seams
themselves, but in a pocket of shale of Cretaceous age that cut
across the more ancient coal-bearing rocks. Mining geologists had a
commercial interest in discovering the extent of these clays, and the
degree to which they might affect coal extraction, so they began
mapping the area.

Cross sections of the mine, created during these geological
investigations, suggested that the horizontal layers of Palaeozoic
rocks (with their valuable coal seams) were occasionally cut through
very steeply by beds of Mesozoic shale (finely laminated clays). The
cross sections gave the first impression of steep-sided ravines cut
into the ancient rocks, and formed the basis for a graphic and rather
appealing notion that the Bernissart dinosaurs represented a herd
that had tumbled to their deaths (Figure 18). Dollo, himself no
geologist, was more inclined to the idea that these dinosaurs had
lived, and died, in a narrow gorge. However, the more dramatic
story had the greater impact, and was further embellished by
suggestions that they had been stampeded into the ravine by huge
predatory dinosaurs (megalosaurs), or by some freak event such as a
forest fire. This was not entirely wishful thinking: extremely rare
fragments of a large predatory dinosaur were discovered within
the Iguanodon-bearing beds; and charcoal-like lumps of coal
were recovered from some of the rubble-like deposits found in the
region between the coal-bearing rocks and the dinosaur-bearing
shaly beds.

The discoveries at Bernissart presented a huge logistic challenge
in the 1870s and early 1880s. Complete skeletons of dinosaurs
measuring up to 11 metres in length had been discovered at
the bottom of a deep mine; they were the focus of worldwide
interest at the time, but how were they to be excavated and
studied? A cooperative venture was arranged between the Belgian
government, funding the scientists and technicians of the Royal
Natural History Museum in Brussels, and the miners and engineers
at the colliery in Bernissart. Each skeleton was carefully exposed
and its position in the mine recorded systematically on plan
diagrams. Every skeleton was divided into manageable blocks
approximately 1 metre square. Each block, protected by a jacket of
plaster of Paris, was carefully numbered and recorded on plan
drawings (Figure 19) before being lifted and transported to Brussels.
Back in Brussels, the blocks were reassembled from the records,
rather like a gigantic jigsaw puzzle. The plaster was painstakingly
removed to reveal the bones of each skeleton. At this point an artist,
Gustave Lavalette, specially commissioned for the project, drew the
skeleton in its death pose before any further preparation or
extraction was undertaken (Figure 20). Some skeletons were
completely extracted from the shale and mounted to create a magnificent display that can be seen to this day at the (renamed)
Royal Institute of Natural Sciences, in Parc Léopold, Brussels. Other
skeletons were cleared of the shale matrix on one side only and
arranged in their burial pose on wooden scaffolding supporting vast
banks of plaster. This display mimics their entombed positions
when they were first discovered in the mine at Bernissart.
The original plans of each excavation, and some crude geological
sections and sketches of the discoveries, are preserved in the
archives of the Royal Institute in Brussels. This information has
been ‘mined’, this time for clues concerning the geological nature of
the dinosaur burial site.

The geology of the coal-mining area of the Mons Basin, in which
lies the village of Bernissart, had been the subject of study before
dinosaurs were ever discovered. A major review in 1870 pointed out that the coal-bearing strata of the Mons Basin were pock-marked by
‘cran’ (naturally formed subterranean pits). Each ‘cran’ was of
limited extent and filled with shales. It was concluded that
these had formed by the dissolution of Palaeozoic rocks deep
underground. The roofs of such caverns collapse periodically under
the sheer weight of the overlying rocks, so the spaces become filled
with whatever lies above: in this case soft clays or shales. The
collapse of such sediments had been recorded locally in the Mons
area as rather alarming, earthquake-like shocks. By amazing
coincidence, a minor ‘earthquake’ of this type took place while the
dinosaurs were being excavated in August 1878 at Bernissart. Minor
collapses in the galleries were noted, as well as flooding, but the
miners and scientists were soon able to resume their work once
the flood water had been pumped out.
Despite all the local geological knowledge, it is very curious that the
scientists from the Museum in Brussels incorrectly interpreted the
geological nature of the ‘cran’ at Bernissart. The mining engineers
produced crude geological sections from the tunnels that yielded
the dinosaurs. These showed that immediately beyond the
coal-bearing seams there was a section of 10–11 metres of breccia
(broken beds containing irregular blocks of limestone and coal
mixed with silt and clay, the ‘collapsed coal-bearing rocks’ of Figure
18) before entering steeply dipping, but more regularly stratified,
shales that yielded the fossils. Toward the middle of the ‘cran’ the
clay beds were horizontally bedded, and as the tunnel approached
the opposite side of the ‘cran’ the beds once again became steeply
tilted in the opposite direction before passing again into a
brecciated region and finally re-entering the coal-bearing deposits.
The symmetry of the geology across the ‘cran’ is exactly what would
be expected if overlying sediments had slumped into a large cavity.
The sediments in which the dinosaurs are embedded also directly
contradict the ravine or river-valley interpretations. Finely stratified
shales containing the fossils are normally deposited in low-energy,
relatively shallow-water environments, probably equivalent to a
large lake or lagoon. There is simply no evidence for catastrophic
deaths caused by herds of animals plunging into a ravine. In fact,
the dinosaur skeletons were found in separate layers of sediment
(along with fish, crocodiles, turtles, thousands of leaf impressions,
and even rare insect fragments), proving that they definitely did not
all die at the same time and therefore could never have been part of
a single herd of animals.
Study of the orientation of the fossil skeletons within the mine
suggests that dinosaur carcasses were washed into the burial
area on separate occasions and from different directions. It was
as if the direction of flow of the river that carried their carcasses
had changed from time to time, exactly as happens in large,
slow-moving river systems today.
So, as early as the 1870s, it was clearly understood that there
were neither ‘ravines’ nor ‘river valleys’ in which the dinosaurs
at Bernissart might have perished. It is fascinating how the
dramatic discovery of dinosaurs at Bernissart seems to have
demanded an equally dramatic explanation for their deaths,
and that such fantasies were uncritically adopted even though
they flew in the face of the scientific evidence available at
the time.

The image of Iguanodon as a gigantic kangaroo-style creature has
become iconic because of the generous distribution of full-sized
skeletal casts to many museums around the world. But does the
evidence for this restoration survive further scrutiny?

Ostrom and Archaeopteryx: the earliest bird

2007-06-26T20:03:00.001-07:00

Having described Deinonychus, Ostrom continued to investigate
the biological properties of dinosaurs. In the early 1970s a trifling
discovery in a museum in Germany was to bring him right back to
the centre of some heated discussions. While examining collections
of flying reptiles, Ostrom noticed one specimen, collected from a
quarry in Bavaria, that did not belong to a pterosaur, or flying
reptile, as its label suggested. It was a section of a leg including the
thigh, knee-joint, and shin. Its detailed anatomical shape reminded
Ostrom of that of Deinonychus. On closer inspection, he could also
make out the faintest impressions of feathers! This was clearly an
unrecognized specimen of the fabled early bird Archaeopteryx
(Figure 13). Excited by his new discovery, and naturally puzzled by
its apparent similarity to Deinonychus, Ostrom began carefully
restudying all the known Archaeopteryx specimens.
The more Ostrom studied Archaeopteryx, the more convinced he
became of the extent of the anatomical similarity between this
creature and his much larger predatory dinosaur Deinonychus
(Figure 16). This led him to reassess the monumental and then
authoritative work on bird origins that had been written by
ornithologist and anatomist Gerhard Heilmann in 1926. The sheer
number of anatomical similarities between carnivorous theropod
dinosaurs and early birds drove Ostrom to question Heilmann’s
conclusion in that work that the similarities could only have been due
to evolutionary convergence. Armed with more recent discoveries of dinosaurs around the world,
Ostrom was able to show that a number of dinosaurs did actually
possess small clavicles, removing at a stroke Heilmann’s big
stumbling block to a dinosaurian ancestry for birds. Encouraged
by this discovery and his own detailed observations on theropods
and Archaeopteryx, Ostrom launched a comprehensive assault on
Heilmann’s theory in a series of articles in the early 1970s. This led
to the gradual acceptance of a theropod dinosaur ancestry of birds
by the great majority of palaeontologists, and would no doubt have
pleased the far-sighted Huxley and deeply irritated Owen.
The close anatomical, and therefore biological, similarity between
theropods and the earliest birds added fuel to the controversy
concerning the metabolic status of dinosaurs. Birds are highly
active, endothermic creatures; perhaps the theropod dinosaurs
might also have possessed an elevated metabolism. The once clear
dividing line between feathered birds, with their distinctive
anatomy and biology which merited them being separated off from
all other vertebrates as a discrete class, the Aves, and other more
typical members of the class Reptilia (of which the dinosaurs were
just one extinct group) became worryingly blurred. The extent of
this blurred line has become even more pronounced in recent years

Deducing the biology and natural history of Deinonychus

2007-06-26T20:00:00.000-07:00

Looking at Deinonychus using this type of ‘forensic’ perspective,
what do these features tell us about the animal and its way of life?
The jaws and teeth (sharp, with curved and serrated edges) confirm
that this was a predator capable of slicing up and swallowing its
prey. The eyes were large, pointed forward, and would have offered
a degree of stereoscopic vision, which would be ideal for judging
distance accurately: very useful for catching fast-moving prey, as
well as for monitoring athletic movements in three-dimensional
space. This serves, in part at least, to explain the relatively large
brain (implied from its large braincase): the optic lobes would need
to be large to process lots of complex visual information so that the
animal could respond quickly, and the motor areas of the brain
would need to be large and elaborate to process the higher-brain
commands and then coordinate the rapid muscular responses of
the body.

The need for an elaborate brain is further emphasized by
considering the light stature and slender proportions of its legs,
which are similar to those of modern, fast-moving animals and
suggest that Deinonychus was a sprinter. The narrowness of each
foot ( just two walking toes, rather than the more stable, and more
usual, ‘tripod’ effect of three) suggests that its sense of balance must
have been particularly well developed; this is further supported by
the fact that this animal was bipedal, and clearly able to walk while
balanced on two feet alone (a feat that, as toddlers prove daily,
needs to be learned and perfected through feedback between the
brain and musculoskeletal system).

Linked to this issue of balance and coordination, the ‘terrible claw’
on each foot was clearly an offensive weapon, evidence of the
animal’s predatory lifestyle. But how, exactly, would it have been
used? Two possibilities spring to mind: either it was capable of
slashing at its prey with one foot at a time, as some large
ground-dwelling birds such as ostriches and cassowaries do today
(this implies that it could have balanced on one foot from time to
time); alternatively, it may have attacked its prey using a two-footed
kick, by jumping on its prey or by grasping its prey in its arms and
giving a murderous double-kick – this latter style of fighting is
employed by kangaroos when fighting rivals. We are unlikely to
be able to decide which of these speculations might be nearest
the truth.

The long arms and sharply clawed hands would be effective
grapples for holding and ripping its prey in either of these
prey-capture scenarios and the curious raking motion made
possible by the wrist joints enhances their raptorial abilities
considerably. In addition, the long, whip-like tail may well have
served as a cantilever – the equivalent of a tightrope walker’s pole to
aid balance when slashing with one foot – or it could have served
as a dynamic stabilizer, which would prove useful when chasing
fast-moving prey that were capable of changing direction very
quickly or when leaping on prey.

While this is not an exhaustive analysis of Deinonychus as a
living creature, it does provide an outline of some of the reasoning
that led Ostrom to conclude that Deinonychus was an athletic,
surprisingly well-coordinated, and probably intelligent predatory
dinosaur. Why should the discovery of this creature be regarded as
so important to the field of dinosaur palaeobiology? To answer that
question, it is necessary to take a broader view of the dinosaurs as
a whole.

The traditional view of dinosaurs
Throughout the earlier part of the 20th century, it was widely (and
perfectly reasonably) assumed that dinosaurs were a group of
extinct reptiles. Admittedly, some were dramatically large or rather
outlandish-looking compared to modern reptiles, but they were

crucially still reptiles. Richard Owen (and Georges Cuvier before
him) had confirmed that dinosaurs were anatomically most similar
to living reptiles, creatures such as lizards and crocodiles. On
this basis it was inferred, logically, that most of their biological
attributes would have been similar, if not identical, to those of these
living reptiles: they laid shelled eggs, had scaly skins, and had a
‘cold-blooded’, or ectothermic, physiology.

To help demonstrate that this view was correct, Roy Chapman
Andrews had discovered that Mongolian dinosaurs laid shelled
eggs, and Louis Dollo (among others) had identified impressions of
their scaly skins; so their overall physiology would be expected to
resemble that of living reptiles. This combination of attributes
created an entirely unexceptional view of dinosaurs: they were
large, scaly, but crucially slow-witted and sluggish creatures.
Their habits were assumed to be similar to those of lizards, snakes,
and crocodiles, which most biologists had only ever seen in zoos.
The only puzzle was that dinosaurs were mostly built on a far
grander scale compared to even the very biggest of known
crocodiles.

There were many depictions of dinosaurs in popular books, and
scientific ones, wallowing in swamps, or squatting as if barely able
to support their gargantuan bodies. Some particularly memorable
examples, such as O. C. Marsh’s Stegosaurus and Brontosaurus,
reinforced these conceptions. Both had enormous bodies and
the tiniest of brains (even Marsh remarked in disbelief at the
‘walnut-sized’ brain cavity of his Stegosaurus). So lacking in
brainpower was Stegosaurus that it was deemed necessary to
invent a ‘second brain’, in its hip region, to act as a sort of back-up
or relay station for information from distant parts of its body, thus
confirming the ‘stupid’ and ‘lowly’ status of dinosaurs beyond
reasonable doubt.
While the weight of comparative evidence undoubtedly sustained
this particular perception of the dinosaur, it ignored, or simply
glossed over, contradictory observations: many dinosaurs, such as
little Compsognathus (Figure 14), were known to be lightly built
and designed for rapid movement. By implication they should have
had rather un-reptile-like levels of activity.
Armed with this battery of prevailing opinion and Ostrom’s
observations and interpretations based on Deinonychus, it is easier
to appreciate how this creature must have been challenging his
mind. Deinonychus was a relatively large-brained, fast-moving
predator capable of sprinting on its hind legs and attacking its prey
– common sense said that this was no ordinary reptile.
One of Ostrom’s students, Robert Bakker, took up this theme by
aggressively challenging the view that dinosaurs were dull, stupid
creatures. Bakker argued that there was compelling evidence that
dinosaurs were more similar to today’s mammals and birds. It
should not be forgotten that this argument echoes the incredibly
far-sighted comments made by Richard Owen in 1842, when he
first conceived the idea of the dinosaur. Mammals and birds are
regarded as ‘special’ because they can maintain high activity levels
that are attributed to their ‘warm-blooded’, or endothermic,
physiology. Living endotherms maintain a high and constant body
temperature, have highly efficient lungs to maintain sustained
aerobic activity levels, are capable of being highly active whatever
the ambient temperature, and are able to maintain large and
sophisticated brains; all these attributes distinguish birds and
mammals from the other vertebrates on Earth.

The range of evidence Bakker used is interesting when considered
from our now slightly more ‘tuned’ palaeobiological perspective.
Using the anatomical observations made by Ostrom, he argued, in
agreement with Owen before him, that:
i) Dinosaurs had legs arranged pillar-like beneath the body (as do
mammals and birds), rather than legs that sprawl out sideways
from the body, as seen in lizards and crocodiles.

ii) Some dinosaurs had complex, bird-like lungs, which would have
permitted them to breathe more efficiently – as would be necessary
for a highly energetic creature.
iii) Dinosaurs could, based on the proportions of their limbs, run at
speed (unlike lizards and crocodiles).
However, borrowing from the fields of histology, pathology, and
microscopy, Bakker reported that thin sections of dinosaur bone,
when viewed under a microscope, showed evidence of a complex
structure and rich blood supply that would have allowed a rapid
turnover of vital minerals between bone and blood plasma – exactly
paralleling that seen in modern mammals.
Turning to the field of ecology, Bakker analysed the relative
abundances of predators and their supposed prey among samples
of fossils representing time-averaged communities from the fossil
record and the present day. By comparing modern communities
of endotherms (cats) and ectotherms (predatory lizards), he
estimated that endotherms consume, on average, ten times the
volume of prey during the same time interval. When he surveyed
ancient (Permian) communities, by counting fossils of this age in
museum collections, he observed rather similar numbers of
potential predators and prey. When he examined some dinosaur
communities from the Cretaceous period, he noticed that there was
a considerably larger number of potential prey compared to the
number of predators. He came to a similar conclusion after
studying Tertiary mammal communities.

Using these admittedly simple proxies, he suggested that dinosaurs
(or at least the predators) must have had metabolic requirements
more similar to mammals; for the communities to stay in some
degree of balance, there needed to be sufficient prey items to
support the appetites of the predators.

Within the fields of geology and the ‘new’ palaeobiology, he also
looked for macroevolutionary evidence (large-scale patterns of
change in fossil abundance) taken from the fossil record. Bakker
examined the times of origin and extinction of the dinosaurs for
evidence that might have had a bearing on their putative physiology.
The time of origin of the dinosaurs, during the Late Triassic
(225 Ma), coincided with the time of the evolution of some of
the most mammal-like creatures, with the first true mammals
appearing about 200 Ma. Bakker suggested that dinosaurs
evolved into a successful group simply because they developed an
endothermic metabolism slightly earlier than mammals. If not, or
so he argued, dinosaurs would never have been able to compete
with the first truly endothermic mammals. In further support of
this idea, he noted that true early mammals were small, probably
nocturnal insectivores and scavengers during the entirety of the
Mesozoic, when the dinosaurs ruled on land, and only diversified
into the bewildering variety that we know today once the dinosaurs
became extinct at the end of the Cretaceous. On that basis, so
Bakker argued, dinosaurs simply had to be endotherms, otherwise
the supposedly ‘superior’ endothermic mammals would have
conquered the land and replaced the dinosaurs in the Early
Jurassic. Moreover, when he considered the time of extinction of the
dinosaurs at the close of the Cretaceous (65 Ma), Bakker believed
that there was evidence that the world had been subjected to a
temporary period of low global temperatures. Since dinosaurs were,
in his opinion, large, endothermic, and ‘naked’ (that is, they were
scale-covered and had neither hair nor feathers to keep their bodies
warm), they were unable to survive a period of rapid climatic
cooling and therefore died out. This left the mammals and birds to
survive to the present day. Dinosaurs were too big to shelter in
burrows, as do the modern reptiles that evidently survived the
Cretaceous catastrophe.

Combining all these lines of argument, Bakker was able to propose
that far from being slow and dull, dinosaurs were intelligent, highly
active creatures that had stolen the world from the traditionally
superior mammals for the remaining 160 million years of the
Mesozoic. Rather than being ousted from the world by the
evolutionary rise of superior mammals, they had only given up their
dominance because of some freakish climatic event 65 million
years ago.

It should now be obvious that the palaeobiological agenda for
research is rather more intellectually broad-based. The ‘expert’ can
no longer rely upon specialist knowledge in his or her own narrow
area of expertise. However, this part of the story does not end here.
John Ostrom had another important part to play in this saga.

The discovery of ‘terrible claw’

2007-06-26T19:51:00.000-07:00

In the summer of 1964 John Ostrom was prospecting for fossils
in Cretaceous rocks near Bridger, Montana, and collected the
fragmentary remains of a new and unusual predatory dinosaur.
Further collecting yielded more complete remains, and by 1969
Ostrom was able to describe the new dinosaur in sufficient detail
and to christen it Deinonychus (‘terrible claw’) in recognition of
a wickedly hooked, gaff-like claw on its hind foot.
Deinonychus (Figure 16) was a medium-sized (2–3 metres in
length), predatory dinosaur belonging to a group known as the
theropods. Ostrom noted a number of unexpected anatomical
features; these prepared the intellectual ground for a revolution
that would shatter the then rather firmly held view of dinosaurs
as archaic and outmoded creatures that plodded their way to
extinction at the close of the Mesozoic world.
However, Ostrom was far more interested in understanding the
biology of this puzzling animal than in simply listing its skeletal
features. This approach is far removed from the pejorative epithet
‘stamp-collecting’ that palaeontology had attracted, and echoes the
method of Louis Dollo in his earlier attempts to understand the
biology of the first complete Iguanodon skeletons (Chapter 1). As an approach, it has more in common with modern forensic pathology,
driven as it is by a need to assemble broad ranges of facts from a
number of different scientific areas in order to arrive at rigorous
interpretation, or hypothesis, on the basis of the available evidence;
this is one of several driving forces behind today’s palaeobiology.

Dinosaur palaeobiology: a new beginning

2007-06-26T19:11:00.000-07:00

It was not until the 1960s and early 1970s that the study of fossils
began to re-emerge as the subject of wider and more general
interest. The catalyst for this re-awakening was a younger
generation of evolutionarily minded scientists eager to demonstrate
that the evidence from the fossil record was far from being a
Darwinian ‘closed book’. The premise that underpinned this
new work was that while evolutionary biologists are obviously
constrained by working with living animals in an essentially
two-dimensional world – they are able to study species, but they do
not witness the emergence of new species – palaeobiologists, by
contrast, work in the third dimension of time. The fossil record
provides sufficient time to allow new species to appear and others to
become extinct. This permits palaeobiologists to pose questions
that bear on the problems of evolution: does the geological
timescale offer an added (or different) perspective on the process of
evolution?; and, is the fossil record sufficiently informative that it
can be teased apart to reveal some evolutionary secrets?
Detailed surveys of the geological record began to demonstrate
rich successions of fossils (particularly shelled marine creatures) –
considerably richer than Charles Darwin could ever have imagined,
given the comparative infancy of palaeontological work in the
middle of the 19th century. Out of this work emerged observations
and theories that would challenge the views of biologists over the
modes of biological evolution over long intervals of geological
time. Sudden massive, worldwide extinction events and periods
of faunal recovery were documented which could not have been
predicted from Darwinian theory. Such events seemed to reset the
evolutionary timetable of life in a virtual instant, and this prompted
some theorists to take a much more ‘episodic’ or ‘contingent’ view
of the history of life on Earth. Large-scale, or macroevolutionary,
changes in global faunal diversity over time seemed to be
demonstrable; these again were not predicted from Darwinian
theory and required explanation.

Most notably, however, Niles Eldredge and Stephen Jay Gould
proposed the theory of ‘punctuated equilibrium’. They suggested
that modern biological versions of evolutionary theory needed to be
expanded, or modified, to accommodate patterns of change seen
repeatedly among species in the fossil record. These consisted of
prolonged periods of stasis (the ‘equilibrium’ period) during which
relatively minor changes in species were observable, and contrasted
with very short periods of rapid change (the ‘punctuation’). These
observations did not fit well with the Darwinian prediction of slow
and progressive change in the appearance of species over time
(dubbed ‘evolutionary gradualism’). These ideas also prompted
palaeobiologists to question the levels at which natural selection
might function: perhaps it could operate above the level of the
individual in some instances?

As a consequence, the whole field of palaeobiology became more
dynamic, questioning, and also outward-looking; it was also
prepared to integrate its work more broadly with other fields of
science. Even highly influential evolutionary biologists such as
John Maynard Smith, who had had little truck with fossils at all,
were prepared to accept that palaeobiology had valuable
contributions to make to the field.
While the general field of scientific palaeobiology was
re-establishing its credentials, the mid-1960s was also a time
of important new dinosaur discoveries; these were destined to
spark ideas that are still important today. The epicentre of this
renaissance was the Peabody Museum at Yale University, the
original workplace of ‘bone-fighter’ Othniel Charles Marsh.
However, this time it was in the person of John Ostrom, a young
professor of palaeontology with a strong interest in dinosaurs.

Dinosaur palaeontology in decline

2007-06-26T19:09:00.000-07:00

Paradoxically, the culmination of Dollo’s remarkable work on this
dinosaur and his international recognition as the ‘father’ of the new
palaeobiology in the 1920s marked the beginning of a serious
decline in the perceived relevance of this area of research within
the larger theatre of natural science.

In the interval between the mid-1920s and the mid-1960s,
palaeontology, and particularly the study of dinosaurs, rather
unexpectedly stagnated. The excitement of the early discoveries,
notably those in Europe, was succeeded by more the spectacular
‘bone wars’ that gripped America during the last three decades of
the 19th century. These centred on a furious – and sometimes
violent – race to discover and name new dinosaurs, and had all the
hallmarks of an academic equivalent of the ‘Wild West’. At its centre
were Edward Drinker Cope (a protégé of the polite and unassuming
Professor Leidy) and his ‘opponent’ Othniel Charles Marsh at Yale
University. They hired gangs of thugs to venture out into the
American mid-West to collect as many new dinosaur bones as
possible. This ‘war’ resulted in a frenzy of scientific publications
naming dozens of new dinosaurs, many of whose names still
resonate today, such as Brontosaurus, Stegosaurus, Triceratops,
and Diplodocus.

Equally fascinating discoveries were made, partly by accident,
during the early 20th century in exotic places such as Mongolia
by Roy Chapman Andrews of the American Museum of Natural
History in New York (the real-life hero/explorer upon whom was
based the mythical ‘Indiana Jones’); and in German East Africa
(Tanzania) by Werner Janensch of the Berlin Museum of Natural
History.

More new dinosaurs were continually being discovered and named
from various places around the world, and although they created
dramatic centrepieces in museums, palaeontologists seemed to be
doing little more than adding new names to the roster of extinct
creatures. A sense of failure took hold to the extent that some even
used dinosaurs as examples of a theory of extinction based on ‘racial
senescence’. The general thesis was that they had lived for so long
that their genetic constitution was simply exhausted and no longer
capable of generating the novelty necessary for the group as a whole
to survive. This supported the idea that dinosaurs were merely an
experiment in animal design and evolution that the world had
eventually passed by.

Not surprisingly, many biologists and theoreticians began to view
this area of research with an increasingly jaundiced eye. New
discoveries, though undeniably exciting, did not seem to be
providing data that would lead in any particular direction.
Discovery required the scientific formalities of description and
naming of these creatures, but beyond that all interest seemed
essentially museological: to be brutal, the work was seen as the
equivalent of ‘stamp collecting’. Dinosaurs, and many other fossil
discoveries, offered glimpses of the tapestry of life within the fossil
record, but beyond this their scientific value seemed questionable.
Several factors justified this change of perception: Gregor Mendel’s
work (published in 1866, but overlooked until 1900) on the laws of
particulate inheritance (genetics) provided the crucial mechanism
to support Darwin’s theory of evolution by means of natural
selection. Mendel’s work was elegantly merged with Darwin’s
theory in order to create ‘Neodarwinism’ in the 1930s. At a stroke,
Mendelian genetics solved one of Darwin’s most fundamental
worries about his theory: how favourable characteristics (genes
or alleles in the new Mendelian language) could be passed
from generation to generation. In the absence of any better
understanding of the mechanism of inheritance in the mid-19th
century, Darwin had assumed that characters or traits, the features
subject to selection according to his theory, were blended when
inherited by the next generation. This, however, was a fatal flaw,
because Darwin realized that any favourable traits would simply be
diluted out of existence if they were blended during reproduction
from generation to generation. Neodarwinism clarified matters
enormously, Mendelian genetics provided a degree of mathematical
rigour to the theory, and the revitalized subject rapidly spawned
new avenues of research. It led to the new sciences of genetics and
molecular biology, culminating in Crick and Watson’s model of
DNA in 1953, as well as huge developments in the fields of
behavioural evolution and evolutionary ecology.
Unfortunately, this fertile intellectual ground was not so obviously
available to palaeontologists. Self-evidently, genetic mechanisms
could not be studied in fossil creatures, so it seemed that they could
offer no material evidence to the intellectual thrust of evolutionary
studies during much of the remainder of the 20th century. Darwin
had already foreseen the limitations of palaeontology in the context
of his new theory. Using his inimitable reasoning, he noted the
limited contribution that could be made by fossils to any of the
debates concerning his new evolutionary theory. In a chapter of
the Origin of Species devoted to the subject of the ‘imperfections
of the fossil record’, Darwin noted that although fossils provided
material proof of evolution during the history of life on Earth
(harking back to the older progressionists’ arguments), the
geological succession of rocks, and the fossil record contained
within in it, was lamentably incomplete. Comparing the geological
record to a book charting the history of life on Earth, he wrote:

Reconstructing Iguanodon

2007-06-26T19:08:00.000-07:00

In 1878 remarkable discoveries were made at a coal mine in the
small village of Bernissart in Belgium. The colliers, who were mining
a coal seam over 300 metres beneath the surface, suddenly struck a
seam of shale (soft, laminated clay) and began to find what
appeared to be large pieces of fossil wood; these were eagerly
collected because they seemed to be filled with gold! On closer
inspection, the wood turned out to be fossil bone, and the gold
‘fool’s gold’ (iron pyrites). A few fossil teeth were also discovered
among the bones, and these were identified as similar to those
described as belonging to Iguanodon by Mantell many years before.
The miners had accidentally discovered not gold, but a veritable
treasure trove of complete dinosaur skeletons.
Over the next five years, a team of miners and scientists from the
Royal Belgian Museum of Natural History in Brussels (now the Royal Institute of Natural Sciences) excavated nearly 40 skeletons
of the dinosaur Iguanodon, as well as a huge number of other
animals and plants whose remains were preserved in the same
shales. Many of the dinosaur skeletons were complete and fully
articulated; they represented the most spectacular discovery that
had been made anywhere in the world at the time. It was the good
fortune of a young scientist in Brussels, Louis Dollo (1857–1931), to
be able to study and describe these extraordinary riches, and this he
did from 1882 until his retirement in the 1920s.
The complete dinosaur skeletons unearthed in Bernissart proved
finally that Owen’s model of dinosaurs such as Iguanodon was
incorrect. As Mantell had suspected, the front limbs were not as
large and strong as the back legs, while the animal had a massive
tail (see Figure 12), and the overall proportions of a giant kangaroo.
The skeletal restoration, and the process by which it was arrived at,
are particularly revealing because they show how the influence of the contemporary interpretations about the appearance and
affinities of dinosaurs affected Dollo’s work. Owen’s ‘elephantine
reptile’ vision of the dinosaur had been questioned as early as 1859
by some tantalizingly incomplete dinosaur discoveries made in New
Jersey and studied by Joseph Leidy, a man of equivalent scientific
stature to Owen who was based at the Philadelphia Academy of
Natural Sciences. However, Owen was to be far more roundly
criticized by a younger, London-based, and ambitious rival:
Thomas Henry Huxley (1825–95).

By the late 1860s, a series of new discoveries had been made that
added considerably to the debate over the relationships of dinosaurs
to other animals. The earliest well-preserved fossil bird (called
Archaeopteryx, or ‘ancient wing’) had been discovered in Germany
(Figure 13). It was eventually bought from its private collector
by the Natural History Museum in London, and described by
Richard Owen in 1863. The specimen was unusual in that it had
well-preserved impressions of feathers, the key identifier for any
bird, forming a halo in the matrix around its skeleton; however,
unlike any living bird, and rather disconcertingly similar to modern
reptiles, it also had three long fingers ending in sharp claws on each
hand, teeth in its jaws, and a long bony tail (some living birds might
seem to have long tails, but this is just the profile of their feathers
that are anchored in a short remnant of the tail).
Not long after the discovery of Archaeopteryx, another small,
well-preserved skeleton was found in the same quarries in Germany
(Figure 14). It bore no feather impressions and its arms were far
too short to have served as wings in any case; anatomically, it was
clearly a small, predatory dinosaur and was named Compsognathus
(‘pretty jaw’).
These two discoveries emerged at a particularly sensitive time
scientifically speaking. In 1859, just a year or so before the first
skeleton of Archaeopteryx was unearthed, Charles Darwin
published a book entitled On the Origin of Species. This book provided a very detailed discussion of the evidence in favour of
the ideas being put forward by the transmutationists and
progressionists referred to earlier. Most importantly, Darwin
suggested a mechanism – natural selection – by which such
transmutations might occur and how new species appear on Earth.
The book was sensational at the time because it offered a direct
challenge to the almost universally accepted authority of biblical
teachings by suggesting that God did not directly create all the species known in the world. Darwin’s ideas were vigorously
opposed by pious establishment figures such as Richard Owen.
In contrast, the radical intellectuals reacted very positively to
Darwin’s ideas. Thomas Huxley is reputed to have declared, after
reading Darwin’s book, ‘How very stupid of me not to have thought
of that!’

While not wishing to become too involved in Darwinian matters, it
is nevertheless the case that dinosaurian discoveries featured
in some of the arguments. Huxley was quick to realize that
Archaeopteryx and the small predatory dinosaur Compsognathus
were anatomically very similar. By the early 1870s, Huxley was
proposing that birds and dinosaurs were not only anatomically
similar, but used this evidence to support the theory that birds had
evolved from dinosaurs. In many ways, the stage was set for the
discoveries in Belgium. By the late 1870s, Louis Dollo, as a bright
young student, would have been fully aware of the Owen–Huxley/
Darwin feuds. One burning question must have been: did these new
discoveries have any bearing on the great scientific controversy of
the day?

Careful anatomical study of the full skeleton of Iguanodon revealed
that it had a hip structure known as ornithischian (‘bird-hipped’);
furthermore, it had long back legs that ended in massive, but
decidedly bird-like, three-toed feet (very similar in shape to the feet
of some of the biggest known land-living birds such as emus). This
dinosaur also had a rather bird-like curved neck, and the tips of its
upper and lower jaws were toothless and covered by, yet again, a
bird-like horny beak or bill. Given the task of description and
interpretation faced by Dollo in the immediate aftermath of these
exciting discoveries, it is intriguing to note that, in the early
photographs taken at the time of the reconstruction of the first
skeleton in Brussels, just beside the huge dinosaur skeleton can be
seen skeletons of two Australian creatures: a wallaby (a small
variety of kangaroo) and a large, flightless bird known as a
cassowary.
The influence of the debate raging in England cannot be doubted.
This new discovery pointed to the truth implicit in Huxley’s
arguments and made it clear that Mantell had been on the right
track in 1851. Iguanodon was no lumbering, scaly rhinoceros
lookalike as portrayed by Owen in his grand models of 1854; rather
it was a huge creature with a pose similar to that of a resting kangaroo, but with a number of bird-like attributes, just as Huxley’s
theory predicted.
Dollo proved to be tirelessly inventive in his approach to the fossil
creatures that he described – he dissected crocodiles and birds in
order to better understand the biology and detailed musculature of
these animals and how it could be used to identify the soft tissues
of his dinosaurs. In many respects, he was adopting a decidedly
forensic approach to understanding those mysterious fossils. Dollo
was regarded as the architect of a new style of palaeontology that
became known as palaeobiology. Dollo demonstrated that
palaeontology should be expanded to investigate the biology, and by
implication ecology and behaviour, of these extinct creatures. His
final contribution to the Iguanodon story was a paper he published
in 1923 to honour the centenary of Mantell’s original discoveries.
He succinctly summarized his views on the dinosaur, identifying it
as the dinosaurian ecological equivalent of the giraffe (or indeed
Mantell’s giant ground sloth). Dollo concluded that its posture
enabled it to reach high into trees to gather its fodder, which it was
able to draw into its mouth by using a long, muscular tongue; the
sharp beak was used to nip off tough stems, while the characteristic
teeth served to pulp the food before it was swallowed. So firmly
was this authoritative interpretation adopted, based as it was on a
set of complete articulated skeletons, that it stood, literally and
metaphorically, unchallenged for the next 60 years. This was
reinforced by the distribution of replica, mounted skeletons
of Iguanodon from Brussels to many of the great museums
around the world during the early years of the 20th century,
and also by the many popular and influential textbooks written
on the subject.

The ‘invention’ of dinosaurs

2007-06-26T19:06:00.000-07:00

Fourteen years younger than Mantell, Richard Owen also studied
medicine, but concentrated in particular on anatomy. He gained a
reputation as a skilled anatomist, and acquired a position at the
Royal College of Surgeons in London, which gave him access to
a great deal of comparative material and, through considerable
industry and skill, allowed him to foster a reputation as the ‘English
Cuvier’. During the late 1830s, he was able to persuade the British
Association to grant him money to prepare a detailed review of all that was then known of British fossil reptiles. This eventually
resulted in the publication of a stream of large, well-illustrated
volumes that would mimic the hugely important works (notably
the multi–volume Ossemens Fossiles) published by Cuvier
earlier in the century, and further cemented Owen’s scientific
reputation.
This project resulted in two important publications: one in 1840 on
mostly marine fossils (Conybeare’s Enaliosauria) and another in
1842 on the remainder, including Mantell’s Iguanodon. The 1842
report is a remarkable document because of Owen’s invention of
the new ‘tribe or sub-order . . . which I . . . name . . . Dinosauria’.
Owen identified three dinosaurs in this report: Iguanodon and
Hylaeosaurus, both discovered in the Weald and named by
Mantell; and Megalosaurus, the giant reptile from Oxford.
He recognized dinosaurs as members of a unique and hitherto
unrecognized group on the basis of several detailed and distinctive
anatomical observations. These included the enlarged sacrum
(a remarkably strong attachment of the hips to the spinal column),
the double-headed ribs in the chest region, and the pillar-like
construction of the legs (see Figure 10).
In reviewing each dinosaur in turn, Owen trimmed their
dimensions considerably, suggesting that they were large, but in the region of 9 to 12 metres, rather than the more dramatic lengths
suggested by Cuvier, Mantell, and Buckland on previous occasions.
Furthermore, Owen speculated a little more on the anatomy and
biology of these animals in words that have an extraordinary
resonance in the light of today’s interpretations of the biology
and way of life of dinosaurs.

Owen’s conception was therefore one of very stout, but egg-laying
and scaly (because they were still reptiles) creatures resembling the
largest mammals to be found in the tropical regions of the Earth
today; his dinosaurs were in effect the crowning glory of a time
on Earth when egg-laying and scaly-skinned reptiles reigned
supreme. Owen’s dinosaurs were the ancient world’s equivalents of
present-day elephants, rhinos, and hippos. Looked at purely from
the logic of scientific deduction, based on such meagre remains, this
was not only brilliantly incisive, but an altogether revolutionary
vision of creatures from the ancient past. Such breathtaking vision
is all the more remarkable when it is juxtaposed to the ‘gigantic
lizard’ models, though these were entirely reasonable and logical
interpretations built on established and respected Cuvierian
principles of comparative anatomy.

The creation of the Dinosauria had other important purposes at the
time. The reports also offered a sweeping refutation of the general
progressionist and transmutationist movements within the fields
of biology and geology during the first half of the 19th century.
Progressionists noted that the fossil record seemed to show that life
had become progressively more complex: the earliest rocks showed
the simplest forms of life, while more recent rocks showed evidence
of more complex creatures. Transmutationists noted that members
of one species were not identical and pondered whether this
variability might also allow species to change over time. Jean
Baptiste de Lamarck, a colleague of Cuvier in Paris, had suggested
that animal species might transmute, or change, in form over time
through the inheritance of acquired characteristics. These ideas
challenged the widely held, biblically inspired belief that God had
created all creatures on Earth, and were being widely and
acrimoniously discussed.

Dinosaurs, and indeed several of the groups of organisms
recognized in the God-fearing Owen’s reports, provided evidence
that life on Earth did not demonstrate an increase in complexity
over time – in fact quite the reverse. Dinosaurs were anatomically
reptiles (that is to say, members of the general group of egg-laying,
cold-blooded, scaly vertebrates); however, the reptiles living today
were a degenerate group of creatures when compared to Owen’s
magnificent dinosaurs that had lived during Mesozoic times. In
short, Owen was attempting to strangle the radical, scientifically
driven intellectualism of the time in order to re-establish an
understanding of the diversity of life that had its basis closer to the
views espoused by Reverend William Paley in his book entitled
Natural Theology in which God held centre-stage as the Creator
and Architect of all Nature’s creatures.
Owen’s fame grew steadily through the 1840s and 1850s, and he
became involved in the committees associated with the planning of
the relocated Great Exhibition of 1854. It is a curious fact that
Owen, for all his burgeoning fame, was not first choice as the
scientific director for the construction of the dinosaurs – Gideon
Mantell was. Mantell refused on the grounds of persistent
ill-health, and also because he was exceedingly wary of the risks
associated with popularizing scientific work, particularly the risk of
misrepresentating imperfectly developed ideas.
Mantell’s story ended in tragedy: his obsession with fossils and
the development of a personal museum led to the collapse of his
medical practice, and his family disintegrated (his wife left him and
his surviving children emigrated once they were old enough to leave
home). The diary that he kept for much of his life makes melancholy
reading; in his final years he was left lonely and racked by chronic
back pain, and he died of a self-administered overdose of
laudanum.
Although outflanked by the ambitious, brilliant, and crucially
full-time, scientist Owen, Mantell spent much of the last decade
of his life continuing research on ‘his’ Iguanodon. He produced
a series of scientific articles and extremely popular books
summarizing many of his new discoveries, and he was the first to
realize (in 1851) that Owen’s vision of the dinosaurs (or at least
Iguanodon) as stout ‘elephantine reptiles’ was probably wrong.
Further discoveries of jaws with teeth, and further analysis of the
partial skeleton (the ‘Mantel-piece’), revealed that Iguanodon had
strong back legs and smaller, weaker front limbs. As a result, he
concluded that its posture may have had much more in common
with the ‘upright’ reconstructions of giant ground sloths
(paradoxically inspired by Owen’s detailed description of the fossil
ground sloth Mylodon). Unfortunately, this work was overlooked,
largely because of the excitement and publicity surrounding Owen’s
Crystal Palace dinosaur models. The truth of Mantell’s suspicions,
and the strength of his own intellect, were not to be revealed for a
further 30 years, and through another amazing piece of serendipity.

Dinosaur discovery: Iguanodon

2007-06-26T19:05:00.000-07:00

Once you have found your fossil, it needs to be studied scientifically
in order to reveal its identity, its relationship to other known
organisms, as well as more detailed aspects of its appearance,
biology, and ecology. To illustrate a few of the trials and tribulations
inherent in any such programme of palaeontological investigation,
we will examine a rather familiar and well-studied dinosaur:
Iguanodon. This dinosaur has been chosen because it has an
interesting and appropriate story to tell, and one with which I am
familiar, because it proved to be the unexpected starting point
for my career as a palaeontologist. Serendipity seems to have a
significant role to play in palaeontology, and this is certainly true
for my own work.
The story of Iguanodon covers almost the entire history of scientific
research on dinosaurs and also the entire history of the science
now known as palaeontology. As a result, this animal unwittingly
illustrates the progress of scientific investigation on dinosaurs
(and other areas of palaeontology) during the past 200 years. The
story also reveals scientists as human beings, with passions and
struggles, and the pervasive influence of pet theories at times in
the history of the subject.
The first bona fide records of the fossil bones that were later to
be named Iguanodon date back to 1809. They comprise, among
indeterminable broken fragments of vertebrae, the lower end of a
large, very distinctive tibia (shin bone) collected from a quarry at
Cuckfield in Sussex (Figure 6). This particular fossil was collected
by William Smith (often referred to as the ‘father of English
geology’). Smith was then researching the first geological map of
Britain, which he completed in 1815. Although these fossil bones
were clearly sufficiently interesting to have been collected and preserved (they are still in the collections of the Natural History
Museum, London), no further study was made of them. The bones
languished unrecognized until I was asked to establish their
identity in the late 1970s.
Yet 1809 was a remarkably opportune moment for such a discovery
to be made. Things were happening in Europe in the branch of
science concerned with fossils and their meaning. One of the
greatest and most influential scientists of this age, Georges
Cuvier (1769–1832), was a ‘naturalist’ working in Paris and
an administrator in the Emperor Napoleon’s government.
‘Naturalist’ was, in these times, a broad category denoting the
philosopher-scientist who worked on a wide range of subjects
associated with the natural world: the Earth, its rocks and minerals,
fossils, and all living organisms. In 1808, Cuvier redescribed a
renowned gigantic fossil reptile collected from a chalk quarry at
Maastricht in Holland; its renown stemmed from the fact that it
had been claimed as a trophy of war during the siege of Maastricht
in 1795 by Napoleon’s army. The creature, originally mistaken for a
crocodile, was identified correctly by Cuvier as an enormous marine
lizard (later named Mosasaurus by the English cleric and naturalist
the Revd William D. Conybeare). The effect of this revelation – the
existence of an unexpectedly gigantic fossil lizard of a former time
in Earth history – was truly profound. It encouraged the search for,
and discovery of, other giant extinct ‘lizards’; it established, beyond
reasonable doubt, that pre-biblical ‘earlier worlds’ had existed; and
it also determined a particular way of viewing and interpreting such
fossil creatures: as gigantic lizards.
Following the defeat of Napoleon and the restoration of peace
between England and France, Cuvier was finally able to visit
England in 1817–18 and meet scientists with similar interests. At
Oxford he was shown some gigantic fossil bones in the collections
of the geologist William Buckland; these seemed to belong to
a gigantic, but this time land-living, lizard-like creature, and
they reminded Cuvier of similar bones that had been found in Normandy. William Buckland eventually named this creature
Megalosaurus in 1824 (with a little help from Conybeare).
However, from the perspective of this particular story, the really
important discoveries were not made until around 1821–2 and at
the same quarries, around Whiteman’s Green in Cuckfield, visited
by William Smith some 13 years earlier. At this time, an energetic
and ambitious medical doctor, Gideon Algernon Mantell
(1790–1852), living in the town of Lewes, was dedicating all his
spare time to completing a detailed report on the geological
structure and fossils in his native Weald district (an area
incorporating much of Surrey, Sussex, and part of Kent) in
southern England. His work culminated in an impressively large,
well-illustrated book that he published in 1822. Included in this
book were clear descriptions of several unusual, large reptilian teeth
and ribs that he had been unable to identify properly. Several of
these teeth were purchased by Mantell from quarrymen, while
others had been collected by his wife, Mary Ann. The next three
years saw Mantell struggling to identify the type of animal to which
these large fossil teeth might have belonged. Although not trained
in comparative anatomy (the particular specialism of Cuvier), he
developed contacts with many learned men in England in the hope
of gaining some insight into the affinity of his fossils; he also sent
some of his precious specimens to Cuvier in Paris for identification.
At first, Mantell’s discoveries were dismissed, even by Cuvier, as
fragments of Recent animals (perhaps the incisor teeth of a
rhinoceros, or those of large, coral-chewing, bony fish). Undeterred,
Mantell continued to investigate his problem, and finally found a
likely solution. In the collections of the Royal College of Surgeons in
London he was shown the skeleton of an iguana, a herbivorous
lizard that had recently been discovered in South America. The
teeth were similar in general shape to those of his fossils and
indicated to Mantell that they belonged to an extinct, herbivorous,
giant relative of the living iguana. Mantell published a report on the
new discovery in 1825 and the name chosen for this fossil creature
was, perhaps not surprisingly, Iguanodon. The name means, quite literally, ‘iguana tooth’ and was created yet again, at the suggestion
of Conybeare (clearly the latter’s classical training and turn of mind
gave him a natural facility in the naming of many of these early
discoveries).
Not surprisingly, given the comparisons then available, these early
discoveries confirmed the existence of an ancient world inhabited
by improbably large lizards. For example, a simple scaling of the
minute teeth of the living (metre-long) iguana with those of
Mantell’s Iguanodon yielded a body length in excess of 25 metres.
The excitement, and personal fame, engendered by the description
of Iguanodon drove Mantell to greater efforts to discover more
about this animal and the fossil inhabitants of the ancient Weald.
For several years after 1825, only fragments of Weald fossils
were discovered; then, in 1834, a partial, disarticulated skeleton
(Figure 8) was discovered at a quarry in Maidstone, Kent.
Eventually purchased for Mantell, and christened the
‘Mantel-piece’, it proved to be the inspiration behind much of
his later work and resulted in some of the first visualizations of
dinosaurs ever produced (Figure 9). He continued probing the
anatomy and biology of Iguanodon in his later years, but much of
this was, alas, overshadowed by the rise of an extremely able,
well-connected, ambitious, and ruthless personal nemesis:
Richard Owen (1804–1892).

Searching for dinosaurs

2007-06-26T19:03:00.000-07:00

From the very outset, we need to approach the search for dinosaurs
systematically. On the basis of what we have learned so far, it is first
necessary to check where to find rocks of the appropriate age by
consulting geological maps of the country that is of interest. It is
equally important to ensure that the rocks are of a type that is
at least likely to preserve the remains of land animals; so some
geological knowledge is required in order to predict the likelihood
of finding dinosaur fossils, especially when visiting an area for the
first time.

Mostly, this involves developing a familiarity with rocks and their
appearance in the area being investigated; this is rather similar to
the way in which a hunter needs to study intently the terrain in
which the prey lives. It also requires the development of an ‘eye’ for
fossils, which comes simply from looking until fossil fragments are
eventually recognized, and this takes time.
Discovery provides the adrenaline-rush of excitement, but is also
the time when the discoverer needs to be most circumspect. All too
often fossil discoveries have been ruined, scientifically speaking, in
the frantic rush to dig the specimen up, so that it can be displayed
by its proud finder. Such impatience can result in great damage to
the fossil itself. Even worse, the object might be part of a larger
skeleton that might be far more profitably excavated carefully by a
larger team of trained palaeontologists. And, as the sleuth might
point out, the rocks in which the fossil was embedded may also have
important tales to tell concerning the circumstances under which
the animal died and was buried, in addition to the more obvious
information concerning the actual geological age of the specimen.
The search for, and discovery of, fossils can be a personally exciting
adventure as well as a technically fascinating process. However,
finding fossils is just the beginning of a process of scientific
investigation that can lead to an understanding of the biology
and way of life of the fossilized creature and the world in which it
once lived. In this latter respect, the science of palaeontology
exhibits some similarities to the work of the forensic pathologist:
both clearly share an intense interest in understanding the
circumstances surrounding the discovery of a body, and use science
to interpret and understand as many of the clues as possible in an
effort to leave, quite literally, no stone unturned.

Why dinosaur fossils are rare

2007-06-26T19:02:00.000-07:00

It is important, at the outset, for the reader to realize that the fossil
record is incomplete and, perhaps more worryingly, decidedly
patchy. The incompleteness is a product of the process of
fossilization. Dinosaurs were all land-living (terrestrial) animals,
which poses particular problems. To appreciate this, it is necessary
first to consider the case of a shelled creature living in the sea, such
as an oyster. In the shallow seas where oysters live today, their
fossilization potential is quite high. They are living on, or attached
to, the seabed and are subjected to a constant ‘drizzle’ of small
particles (sediment), including decaying planktonic organisms,
silt or mud, and sand grains. If an oyster should die, its soft tissues
would rot or be scavenged by other organisms quite quickly and its
hard shell would be gradually buried under fine sediment. Once
buried, the shell has the potential to become a fossil as it becomes
trapped under an increasingly thick layer of sediment. Over
thousands or millions of years, the sediment in which the shell was buried is gradually compressed to form a silty sandstone, and this
may become cemented or lithified (literally, turned to stone) by
the deposition of calcium carbonate (calcite) or silica (chert/flint)
carried through the fabric of the rock by percolating water. For the
fossil remains of the original oyster to be discovered, the deeply
buried rock would need to be lifted, by earth movements, to form
dry land, and then subjected to the normal processes of weathering
and erosion.
Land-living creatures, by contrast, have a far lower probability of
becoming fossilized. Any animal dying on land is likely, of course, to
have its soft, fleshy remains scavenged and recycled; however, for
such a creature to be preserved as a fossil it would need to be subject
to some form of burial. In rare circumstances creatures may be
buried rapidly in drifting dune sand, a mud-slide, under volcanic
ash, or some by other catastrophic event. However, in the majority
of cases the remains of land animals need to be washed into a
nearby stream or river, and eventually find their way into a lake or
seabed where the process of slow burial, leading to fossilization,
can commence. In simple, probabilistic terms the pathway to
fossilization for any land creature is that much longer, and fraught
with greater hazard. Many animals that die on land are scavenged
and their remains become entirely scattered so that even their hard
parts are recycled into the biosphere; others have their skeletons
scattered, so that only broken fragments actually complete the path
to eventual burial, leaving tantalizing glimpses of creatures; only
very rarely will major parts, or even whole skeletons, be preserved
in their entirety.
So, logic dictates that dinosaur skeletons (as with any land-living
animal) should be extremely rare and so they are, despite the
impression sometimes given by the media.
The discovery of dinosaurs and their appearance within the fossil
record is also a decidedly patchy business, for rather mundane
reasons. Fossil preservation is, as we have just come to appreciate, a
Dinosaurs
16
chance-laden, rather than design-driven, process. The discovery of
fossils is similarly serendipitous in the sense that outcrops of rocks
are not neatly arranged like the pages of a book to be sampled
perhaps in sequence, or as fancy takes us.
The relatively brittle surface layers of the Earth (its crust, in
geological terms) have been buckled, torn, and crumpled by huge
geological forces acting over tens or hundreds of millions of years
that have wrenched landmasses apart or crushed them together. As
a result, the geological strata containing fossils have been broken,
thrown up, and frequently destroyed completely by the process of
erosion throughout geological time, and further confused by later
periods of renewed sedimentation. What we, as palaeontologists,
are left with is an extremely complex ‘battlefield’, pitted, cratered,
and broken in a bewildering variety of ways. Disentangling this
‘mess’ has been the work of countless generations of field geologists.
Outcrops here, cliff-sections there, have been studied and slowly
assembled into the jigsaw that is the geological structure of the
land. As a result, it is now possible to identify rocks of Mesozoic age
(belonging to the Triassic, Jurassic, and Cretaceous Periods) with
some accuracy in any country in the world. However, that is not
sufficient to aid the search for dinosaurs. It is also necessary to
disregard Mesozoic rocks laid down on the sea floor, such as the
thick chalk deposits of the Cretaceous and the abundant limestones
of the Jurassic. The best types of rocks to search in for dinosaur
fossils are those that were laid down as shallow coastal or estuarine
environments; these might have trapped the odd, bloated carcasses
of land-living creatures washed out to sea. But best of all are river
and lake sediments, environments that were physically much closer
to the source of land creatures.

Dinosaurs in perspective

2007-06-26T19:01:00.000-07:00

The fossilized remains of dinosaurs (with the notable exception of
their lineal descendants the birds – see Chapter 6) have been found
in rocks identified as belonging to the Mesozoic Era. Mesozoic rocks
range in age from 245 to 65 million years ago (abbreviated to Ma
from now on). In order to put the time during which dinosaurs lived
into context, since such numbers are so large as to be quite literally
unimaginable, it is simpler to refer the reader to the geological
timescale.

During the 19th and a considerable part of the 20th centuries, the
age of the Earth, and the relative ages of the different rocks of which
it is composed, had been the subject of intense scrutiny. During the
early part of the 19th century it was becoming recognized (though
not without dispute) that the rocks of the Earth, and the fossils that
they contained, could be divided into qualitatively different types.
There were rocks that appeared to contain no fossils (often referred
to as igneous, or ‘basement’). Positioned above these apparently
lifeless basement rocks was a sequence of four types of rocks that
signified four ages of the Earth. During much of the 19th century
these were named Primary, Secondary, Tertiary, and Quaternary –
quite literally the first, second, third, and fourth ages. The ones that
contained traces of ancient shelled and simple fish-like creatures
were ‘Primary’ (now more commonly called Palaeozoic, literally
indicative of ‘ancient life’). Above the palaeozoics was a sequence of
10
rocks that contained a combination of shells, fish, and land-living
saurians (or ‘crawlers’, which today would include amphibians
and reptiles); these rocks were designated broadly as ‘Secondary’
(nowadays Mesozoic, ‘middle life’). Above the mesozoics were found
rocks that contain creatures more similar to those living today,
notably because they include mammals and birds; these were
named ‘Tertiary’ (now also called Cenozoic, ‘recent life’). And finally,
there was the ‘Quaternary’ (or Recent) that charted the appearance
of recognizably modern plants and animals and the influence of the
great ice ages.

This general pattern has stood the test of time remarkably well.
All modern geological timescales continue to recognize these
relatively crude, but fundamental, subdivisions: Paleozoic,
Mesozoic, Cenozoic, Recent. However, refinements in the way
the fossil record can be examined for example, through the use
of high-resolution microscopy, the identification of chemical
signatures associated with life, and the more accurate dating of
rocks enabled by radioactive isotope techniques have led to a more
precise timescale of Earth history.

The part of the timescale that we are most concerned with in this
book is the Mesozoic Era, comprising three geological periods:
the Triassic (245–200 Ma), the Jurassic (200–144 Ma), and the
Cretaceous (144–65 Ma). Note that these periods of time are not by
any means equal in duration. Geologists were not able to identify a
metronome-like tick of the clock measuring the passing of Earth
time. The boundaries between the periods were mapped out in the
last two centuries by geologists who were able to define particular
rock types and, very often, their constituent fossils, and this is
usually reflected in the names chosen for the periods. The term
‘Triassic’ originates from a triplet of distinctive rock types
(known as the Lias, Malm, and Dogger); the ‘Jurassic’ hails from
a sequence of rocks identified in the Jura Mountains of France;
while the name ‘Cretaceous’ was chosen to reflect the great
thickness of chalk (known as Kreta in Greek) such as that which forms the White Cliffs of Dover and is found widely across Eurasia
and North America.

The earliest dinosaurs known have been identified in rocks dated
at 225 Ma, from the close of the Triassic (a period known as the
Carnian), in Argentina and Madagascar. Rather disconcertingly,
these earliest remains are not rare, solitary examples of one type of
creature: the common ancestor of all later dinosaurs. To date at
least four, possibly five, different creatures have been identified:
three meat-eaters (Eoraptor, Herrerasaurus, and Staurikosaurus),
a tantalizingly incomplete plant-eater named Pisanosaurus, and an
as-yet-unnamed omnivore. One conclusion is obvious: these are not
the earliest dinosaurs. In the Carnian there was clearly a diversity of
early dinosaurs. This indicates that there must have been dinosaurs
living in the Middle Triassic (Ladinian-Anisian) that had ‘fathered’
the Carnian diversity. So we know for a fact that the story of
dinosaur origins, both the time and the place, is incomplete.

More facts on Dinasaurs

2007-06-26T18:58:00.000-07:00

A remarkable piece of evidence in support of the notion that there
is a relationship between the latent appeal of dinosaurs and the
human psyche can be found in mythology and folklore. Adrienne Mayor has shown that as early as the 7th century bc the Greeks had
contact with nomadic cultures in central Asia. Written accounts
at this time include descriptions of the Griffin (or Gryphon): a
creature that reputedly hoarded and jealously guarded gold; it was
wolf-sized with a beak, four legs, and sharp claws on its feet.
Furthermore, Near East art of at least 3000 bc depicts Griffin-like
creatures, as does that of the Mycenaean. The Griffin myth arose in
Mongolia/north-west China, in association with the ancient caravan
routes and gold prospecting in the Tienshan and Altai Mountains.
This part of the world (we now know) has a very rich fossil heritage
and is notable for the abundance of well-preserved dinosaur
skeletons; they are remarkably easy to find because their white
fossil bones stand out clearly against the soft, red sandstones in
which they are buried. Of even greater interest is the fact that the
most abundant of the dinosaurs preserved in these sandstones is
Protoceratops, which are approximately wolf-sized, and have a
prominent hooked beak and four legs terminated by sharp-clawed
toes. Their skulls also bear strikingly upswept bony frills, which
might easily be the origin of the wing-like structures that are often
depicted in Griffin imagery (compare the images in Figure 3).
Griffins were reported and figured very consistently for more
than a millennium, but beyond the 3rd century ad they became
defined increasingly by allegorical traits. On this basis it would
appear to be highly probable that Griffins owe their origin to
genuine observations of dinosaur skeletons made by nomadic
travellers through Mongolia; they demonstrate an uncanny link
between exotic mythological beasts and the real world of
dinosaurs.

Looked at through the harsh lens of objectivity, the cultural
pervasiveness of dinosaurs is extraordinary. After all, no human
being has ever seen a living non-avian dinosaur (no matter what
some of the more absurd creationist literature might claim). The
very first recognizably human members of our species lived about
500,000 years ago. By contrast, the very last dinosaurs trod our
planet approximately 65 million years ago and probably perished, along with many other creatures, in a cataclysm following a giant
meteorite impact with Earth at that time (see Chapter 8).
Dinosaurs, as a group of animals of quite bewildering variety,
therefore existed on Earth for over 160 million years before their
sudden demise. This surely puts the span of human existence, and
our current dominance of this fragile planet (in particular, the
debates concerning our utilization of resources, pollution, and
global warming), into a decidedly sobering perspective.
The very fact of the recognition of dinosaurs, and the very different
world in which they lived, today is a testament to the extraordinary
explanatory power of science. The ability to be inquisitive, to probe
the natural world and all its products, and to keep asking that
beguilingly simple question – why? – is one of the essences of being
human. It is hardly surprising that developing rigorous methods in
order to determine answers to such general questions is at the core
of all science.

Dinosaurs are undeniably interesting to many people. Their very
existence incites curiosity, and this can be used in some instances as
a means of introducing unsuspecting audiences to the excitement of
scientific discovery and the application and use of science more
generally. Just as fascination with bird songs could lead to an
interest in the physics of sound transmission, echolocation, and
ultimately radar, on the one hand, or linguistics and psychology
on the other; so it can be that an interest in dinosaurs can open
pathways into an equally surprising and unexpectedly wide range of
scientific disciplines. Outlining some of these pathways into science
is one of the underlying purposes of this book.
Palaeontology is the science that has been built around the study of
fossils, the remains of organisms that died prior to the time when
human culture began to have an identifiable impact on the world,
that is more than 10,000 years ago. This branch of science
represents our attempt to bring such fossils back to life: not literally,
as in resuscitating dead creatures (in the fictional Jurassic Park
7
Introduction
mode), but by using science to understand as fully as we can what
such creatures were really like and how they fitted into their
world. When a fossil of an animal is discovered, it presents the
palaeontologist with a series of puzzles, not unlike those faced by
the fictional sleuth Sherlock Holmes:
• What type of creature was it when it was alive?
• How long ago did it die?
• Did it die naturally of old age, or was it killed?
• Did it die just where it was found, buried in the rock, or was its body
moved here from somewhere else?
• Was it male or female?
• How did the creature look when it was alive?
• Was it colourful or drab?
• Was it fast-moving or a slow-coach?
• What did it eat?
• How well could it see, smell, or hear?
• Is it related to any creatures that are alive today?
These are just a few examples of the questions that might be asked,
but all tend towards the piecemeal reconstruction of a picture of
the creature and of the world in which it lived. It has been my
experience, following on from the first broadcasting of the
television series called Walking with Dinosaurs, with their
incredibly realistic-looking virtual dinosaurs, that many people
were sufficiently intrigued by what they saw or heard in the
commentary to ask: ‘How did you know that they moved like
that? . . . looked like that? . . . behaved like that?’
Questions driven by uncomplicated observations and basic
common sense underpin this book. Every fossil discovery is in and
of itself unique and has the potential to teach the inquisitive among
us something about our heritage as members of our world. I should,
however, qualify this statement by adding that the particular type of
heritage that I will be discussing relates to the natural heritage that
we share with all other organisms on this planet. This natural
8
Dinosaurs
heritage spans a period of time that exceeds 3,800 million years
according to most modern estimates. I will be exploring only a tiny
section of this staggeringly long period of time: just that interval
between 225 and 65 million years ago, when dinosaurs dominated
most aspects of life on Earth.

Dinosaurs: facts and fiction

2007-06-26T18:56:00.000-07:00

Dinosaurs were ‘borne’ officially in 1842 as a result of some truly
brilliant and intuitive detective work by the British anatomist
Richard Owen (Figure 1), whose work had concentrated upon
the unique nature of some extinct British fossil reptiles.
At the time of Owen’s review, he was working on a surprisingly
meagre collection of fossil bones and teeth that had been discovered
up to that time and were scattered around the British Isles.
Although the birth of dinosaurs was relatively inauspicious
(first appearing as an afterthought in the published report of the
11th meeting of the British Association for the Advancement of
Science), they were soon to become the centre of worldwide
attention. The reason for this was simple. Owen worked in London,
at the Museum of the Royal College of Surgeons, at a time when the
British Empire was probably at its greatest extent. To celebrate such
influence and achievement, the Great Exhibition of 1851 was
devised. To house this event a huge temporary exhibition hall
(Joseph Paxton’s steel and glass ‘Crystal Palace’) was built on Hyde
Park in central London.

Rather than destroy the wonderful exhibition hall at the end of 1851
it was moved to a permanent site at the London suburb ofSydenham (the future Crystal Palace Park). The parkland
surrounding the exhibition building was landscaped and arranged
thematically, and one of the themes depicted scientific endeavour
in the form of natural history and geology and how they had
contributed to unravelling the Earth’s history. This geological
theme park, probably one of the earliest of its kind, included
reconstructions of genuine geological features (caves, limestone
pavements, geological strata) as well as representations of the

2
Dinosaurs
inhabitants of the ancient world. Owen, in collaboration with
the sculptor and entrepreneur Benjamin Waterhouse Hawkins,
populated the parkland with gigantic iron-framed and
concrete-clad models of dinosaurs (Figure 2) and other prehistoric
creatures known at this time. The advance publicity generated
before the relocated ‘Great Exhibition’ was re-opened in June
1854 included a celebratory dinner held on New Year’s Eve 1853
within the belly of a half-completed model of the dinosaur
Iguanodon and this ensured considerable public awareness of
Owen’s dinosaurs.
The fact that dinosaurs were extinct denizens of hitherto
unsuspected earlier worlds, and were the literal embodiment of
the dragons of myth and legend, probably guaranteed their
adoption by society at large; they even appeared in the works of
Charles Dickens, who was a personal acquaintance of Richard
Owen. From such evocative beginnings public interest in dinosaurs
has been nurtured and maintained ever since. Quite why the appeal
should have been so persistent has been much speculated upon;
it may have much to do with the importance of story-telling as a
means of stimulating human imaginative and creative abilities. It
strikes me as no coincidence that in humans the most formative
years of intellectual growth and cultural development, between the
ages of about 3 and 10 years, are often those when the enthusiasm
for dinosaurs is greatest – as many parents can testify. The buzz of
excitement created when children glimpse their first dinosaur
skeleton is almost palpable. Dinosaurs, as the late Stephen Jay
Gould – arguably our greatest popularizer of scientific natural
history – memorably remarked, are popular because they are ‘big,
scary and [fortunately for us] dead’, and it is true that their gaunt
skeletons exert a gravitational pull on the imaginative landscape of
youngsters.