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.