AboutDavid Prus Expertise I can answer any general questions about dinosaurs and on prehistoric mammals in the Cenozoic. I also know a bit about the media's various depictions of dinosaurs and their inaccuracies. I don't know much about "microfauna"-small animals, or about Paleozoic besides some knowledge about Permian animals. Plants are right out, I'm afraid.
Experience I have been interested in dinosaurs for most of my life, own a large collection of scientific papers and books on the subject, and am a member of the Field Museum in Chicago.
Education/Credentials I am in college, but studied biology and geology both in high school and as a personal effort.
Question Hi David! Many thanks for your answer about Ornithosuchus, which makes things much clearer! I knew I had to be missing something. Would you mind if I Pester with another archosaur question which has bothered me for years?
Robert Bakker, in "The Dinosaur Heresies" makes a convincing case for endothermic or "warm-blooded" dinosaurs, but also extends his arguments right back to the early Triassic, describing how the "prot-mammals" which had filled most of the large predator and herbivore niches were effectively ousted by the Erythrosuchids (vividly described as "the shock troops of the rival empire") thus establishing the archosaurian dominance of terrestrial ecosystems which lasted until the end of the Mesozoic. Dr Bakker asserts that the "proto-mammals" (cynodonts etc) already had relatively high metabolic rates and that in order to out-compete them the erythrosuchids must have been similarly endowed. He also claims physical evidence in support of this hypothesis, with erythrosuchids and their relatives exhibiting microscopic bone-structure consistent with an endothermic metabolism.
However, Bakker also accepts that erythrosuchids et al were basal forms, ancestral to both the dinosaurian and crocodilian lineages. This would have to mean that "cold-blooded" crocodiles are descended from a "warm-blooded" ancestor! On the face of it, this sounds like a backward step. But I was thinking about how a low metabolic rate may be crucial to a modern crocodilian's way of life. As ambush predators rather than active hunters, they clearly need to be able to endure long periods without feeding. Also their killing strategy depends largely on being able to stay under water for long enough to drown large animals that they would probably be unable to overpower on land. It seems to make a sort of sense for a warm-blooded croc-ancestor, having opted for this mode of life, to re-evolve a slower metabolic rate.
Dr Bakker's book was published in 1986, and I haven't been able to find much recent literature on the Erythrosuchids. Are they still considered ancestral to both crocodilians and dinosaurs? And have the claims for their "warm-bloodedness" stood the test of time thus far? Is it possible for a warm-blooded lineage to revert to a slower metabolism, and is this what is believed to have happened with the crocodilians? Once again, your advice would be greatly appreciated!
Paul :)
Answer Thing is, Erythrosuchids were basal but not ancestral to anything. They were ambush predators, but they were fully erect and probably not completely cold-blooded. However, this idea of a secondary adaptation is accurate considering the erect, active stance of early Crocodylomorphs like Hesperosuchus. Using a cladogram, there is a point between Crocodylomorpha and the clade Crocoyliformes where the entire body system changes to a lower metabolism, so I do think you're on the right track. Archosaur endothermy was not well developed until the Mesozoic, so I think your hypothesis is very likely.
