Summary

Physiologists avoid terms like "warm-blooded" and "cold-blooded" and instead replace them by more meaningful terms like endothermy and ectothermy, which describe the heat source, and homeotherm and poikilotherm, which describe the degree to which temperatures fluctuate. All of these terms should be thought of as endpoints on spectra of metabolic strategies.

A dispassionate look at the evidence for dinosaur endothermy gives an apparently mixed signal. Anatomical indicators in dinosaurs suggestive of an endothermic metabolism include: the erect stance, which among living vertebrates is exclusively posessed by endotherms; various adaptations to more efficiently process food (particularly within Genasauria), presumably to support a higher metabolic rate; the inferred presence of a four-chambered heart, necessary to sustain the higher blood pressures associated with an endothermic metabolism; and the relatively high EQs of some dinosaurs.

Each of these indicators, however, is inconclusive: the erect stance argument has been criticized as being purely coincidental (and not causal); the food processing in genasaurs is inconsistent and, as we have seen, somewhat contradictory (for example, the pairing in stego-saurs and ankylosaurs of cheeks and poor occlusion); and the high EQs of some dinosaurs are matched by strikingly low EQs in others. Moreover, the absence in non-avian dinosaurs of respiratory turbinates has suggested that they perhaps didn't maintain the high rates of ventilation seen in living endotherms.

The presence in dinosaurs of Haversian canals appears to suggest endothermy. These, however, are coupled with LAGs, which suggest a greater dependence upon external temperatures than would be expected in an endotherm. Moreover, Haversian canals can arise as a result of longevity as well as the from possession of an endothermic metabolism. Yet, as juveniles, many dinosaurs likely experienced rapid growth rates that today are known only in endotherms. These suggest that, as juveniles at least, some dinosaurs may have possessed endothermic metabolisms.

Because endothermy is, in terms of energy, quite costly to maintain, it was suggested that the ratio of the biomassess of predators and prey in endothermic ecosystems ought to be significantly smaller than that ratio in ectothermic ecosystems. Several attempts were made to calculate such ratios for dinosaurs. None ultimately proved definitive for a variety of reasons, including the unreliability of museum collections as accurate indicators of ancient communities, the fact that endothermic predators sometimes eat ectothermic prey (and vice versa), and the fact that the limiting factor on prey populations is not generally predation.

The existence of polar-dwelling dinosaurs has been interpreted as suggestive of an endothermic metabolism, since today large ectotherms don't get much above 20° N or below 20° S latitude. Yet, a high-latitude temnospondyl also preserved suggests that polar climates were warmer than they are today.

The conclusion that birds are dinosaurs suggests that endothermy happened at least once within Dinosauria; the question is, how phylogenetically basal was this innovation? Feathered non-flying theropods show that endothermy must have occurred below Avialae, because the development of insulation in an ectotherm makes little sense. Speculations about the extent of dinosaur endothermy have spanned all of Dinosauria to a just a few highly evolved theropods.

18O : 16O ratios are temperature sensitive, and have been used as a kind of paleother-mometer in well-preserved fossil bone. The idea was that ectotherms would show a greater range of temperature fluctuations from core to extremities than endotherms. In fact, dinosaurs (and even some mammals) produced a somewhat mixed signal: while some dinosaurs, such as hadrosaurs, showed little temperature variation, ankylosaurs and two of the large theropods tested showed ectotherm-like variations. Reinforcing the point that endothermy and ectothermy are actually endpoints in a spectrum and that metabolisms among vertebrates are not easily predictable, the tail of an opossum, a living marsupial (mammal), also showed the kind of variation expected in an ectotherm.

The apparent inconclusiveness of all of these studies is best interpreted as reflective of a variety of metabolic strategies in Dinosauria. Large dinosaurs, especially sauropods, are not particularly good candidates for human-style homeothermic endothermy and their very size may have precluded high metabolic rates. In other groups, the rapid growth rates of some juveniles may have slowed significantly as adults (they do in mammals, after all!), and such groups of dinosaurs may have experienced a conversion from a dominantly endother-mic metabolism to a dominantly ectothermic metabolism. Small bipedal theropods and orni-thopods may have been closer to homeothermic endothermy throughout their lives. While it is clear that the old "cold-blooded" lizard or crocodile model of dinosaur metabolism is defunct, the record suggests that dinosaurs likely enjoyed a range of metabolic strategies.

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