and the carnivorous rauisuchians. Endothermy evolved somewhere on the other great branch, known as archosaurs, the lineage leading to dinosaurs and birds. We know that birds are warm-blooded, and in recent decades there has been a great deal of research and speculation as to whether the ancestors of birds, the saurischian dinosaurs, were themselves endothermic. Several camps of dinosaur specialists have formed around this fundamental question about dinosaur metabolism. One group that includes Jack Horner, Robert Bakker, and A. de Riqules argue that dinosaurs were endotherms. More recently, however, a new faction has come forward suggesting that dinosaurs and even the earliest birds were all ectoderms and that endothermy in birds did not arise until at least the Cretaceous Period.
Recently a new hypothesis has been put forward by the same group that argued that late Permian archosaurs had a four-chambered heart and were at least primitively endothermic. This idea has arisen from recognition that, like the contemporaneous therapsids, the late Permian and early Triassic archosaurs had a more upright posture with legs beneath the body, rather than sprawled to the side in lizard-like fashion. The skeletons of both groups suggest an active life style of high mobility. In this model, all the basal archosaurs had warm blood. But later, perhaps in the middle Triassic, the crocodile and crocodile-like lineages returned to a largely aquatic life style, and re-evolved ectothermy, while maintaining the crocodilian four-chambered heart. The argument here is that ectothermy was thus secondarily re-evolved in this lineage for a simple reason: ectothermy aids diving by enabling the animal to stay underwater longer. By reducing oxygen uptake, an aquatic predator can remain underwater longer than can a similarly sized endotherm. Large size also favors diving and breath holding. For every order of magnitude body mass increases, diving time is doubled. Another adaptation to diving is blood “shunting,” where oxygenated blood is mixed with less oxygenated blood during dives.
This latter view fits well with the history of the archosaurs. Modern crocodiles have four-chambered hearts, a trait associated with endothermy. Additionally, crocodiles came from ancestors that had an upright rather than sprawling posture. This upright posture is found today only in endotherms.
While the major radiation of the early archosaurs took place in the