an avian-style air sac system. This kind of breathing pattern, called cuirassal breathing, could be used to inflate any air sacs and thus might be evidence that air sacs were present. But again, this system, if it were present, seems less developed in the first bipedal saurischians than in later ones.

Where does this lead us? What systems would be optimal and what systems seem allowable by the osteological evidence of the first dinosaurs? First, metabolism. A warm-blooded dinosaur would have an advantage over a cold-blooded dinosaur—in our world. All modern (cold-blooded) reptiles have to warm up at the start of the day, and thus there is little early-morning activity, other than behavioral movement, in order to acquire heat from the external environment. If the first bipedal dinosaurs—all predators—did not have to do this, they would have been able to forage freely on the slower ectotherms in the cooler morning or nighttime hours. But what is the price for this? At rest, all endo-therms use as much as 15 times the amount of oxygen as do ectotherms (there is a 5 to 15 times range based on experimental observation). In our oxygen-rich world this is not a problem for the warm-blooded animals. So much oxygen is available that there is no penalty. But in the oxygen-poor mid-Triassic, such was surely not the case. And the energy and oxygen necessary for endothermy would not have been necessary if the dinosaurs moved toward large size. With larger body size, the ratio of surface area (from which heat is lost) to body volume becomes increasingly favorable. Truly large-sized animals could have remained essentially homoeothermic in their environment even during cooler nighttime temperatures. While a lizard rapidly loses body heat in a cooler night, a 100-pound reptile does not. And the conditions of the Triassic may have been such that, thanks to highly elevated carbon dioxide levels, greenhouse heating may have kept the temperatures virtually equal day and night—and hot to boot. The Triassic climate was one suited for reptiles—hot. That heat would actually have been a problem for very large endotherms. Large dinosaurs (greater than a ton, such as most sauropods) would have overheated in even moderate temperatures, and the Triassic environment was anything but moderate. So here it is suggested that cold-blooded dinosaurs would have been a condition actually more favorable than warm-bloodedness for dinosaurs, mainly because of the large differ-



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