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.

Deducing the biology and natural history of Deinonychus

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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