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.
Wednesday, June 27, 2007
Ancient biomolecules and tissues
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.
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
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.
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
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.
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
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.
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
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.
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
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?
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
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.
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
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’.
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’.
Tuesday, June 26, 2007
Legs, heads, hearts, and lungs of Dinasaurs
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.
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
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.
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
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.
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
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.
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
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.
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
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
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
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.
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.
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