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.