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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
crocodiles or closer to birds based on morphological evidence? And which of the adaptations—cold blood with relatively simple tidal lungs or warm blood with radically transformed and efficient (for low oxygen) lungs did the first dinosaurs possess?
With regard to metabolism, the chapter by dinosaur experts Kevin Padian and Jack Horner points out that dinosaurs were probably more than “damned good reptiles,” citing the rapid growth rates observed in dinosaurs. But were they warm-blooded? Birds are, and it is inferred by Padian and Horner that the direct ancestors of birds, the nonavian bipedal saurischians of the middle Jurassic, may have already evolved this characteristic. But by the time of the first birds (Archaeopteryx) of the late Jurassic, oxygen levels had dramatically risen from their late Triassic nadir. The world was a very different place from the Triassic world, it was one closer to our own, where oxygen was no longer a limiting factor for metabolism.
As far as lungs and respiration go, the chapter by Anusuya Chinsamy and Willem Hillenius took a supposedly dispassionate look at the possibility of an air sac system in dinosaurs that ended up sounding very much as if it had been written by John Ruben. They took the conservative route—no air sacs. And what about the possible presence of air sacs? We know that air sacs ultimately evolved in birds and that the presence of bone pneumaticity makes a strong case for the air sac system. A fully evolved air sac system would have been the best way to deal with the low-oxygen conditions at the time of the dinosaurs’ first appearance and early history, from the late Triassic to the upper Jurassic. But even the true believers, such as Gregory Paul, acknowledge that the basal dinosaurs such as Herrerasaurus showed no bone pneumaticity that would argue for the air sac system and that, if it were present, air sacs in these first dinosaurs would necessarily have been abdominal, thus leaving no fossil record. The evidence for or against air sacs in dinosaurs comes down to rib morphology, rather than bone pneumaticity. As we have seen, the fantastic air sac system in birds can work because their ribs are highly mobile, being jointed or hinged in such a way that would allow the required pattern of avian respiration. Another possible breathing pattern has been proposed, one in which the dinosaurs used dermal ossifications in the abdominal wall to ventilate