Dinosaurs hot cold or lukewarmblooded

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]

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 o intermediate between Dromaeus [an emu] and Compsognathus [a §

dinosaur] ... and the hypothesis that the ... class Aves has its root §

in the Dinosaurian reptiles; ... d

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 ofpalaeobiology, 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.

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