Wednesday, June 27, 2007

K-T extinctions: the end of dinosaurs?

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

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

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

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

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

Ancient biomolecules and tissues

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

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

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

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

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

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

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

Dinosaur mechanics: how Allosaurus fed

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
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

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
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’

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

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.


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

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

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?