The adaptive value of a permanent and high constant body temperature depends upon the action of enzymes. A complex, multi-enzyme system must evolve and function, because each individual enzyme system is constant and therefore predictable. Highly complex biological organisation depends upon multi-enzyme systems. No single characteristic that responded to numerous condition could evolve unless it were accompanied by all the other characteristics that were necessary. There must therefore have been a gradual evolution of all such systems, and many of the reptiles in the fossil record provide evidence of poor homeostasis. The disadvantages of tachymetabolism included, among others, greater food requirements, and the necessity for more complex digestive systems, the evolution of methods for cooling as well as for heating the body and enhanced locomotor abilities (Kemp 1982).
The large size of most dinosaurs obscures the distinction between tachy-metabolic and bradymetabolic thermoregulation, for which most criteria are size-dependent. Likewise, predator:prey ratios do not distinguish between tachymetabolism and bradymetabolism because these converge as the sizes of the animals concerned increase. Bone histology shows only that the dinosaurs were different from living reptiles, not that they were similar to mammals. Water conservation and locomotor refinements were more important than thermal strategy to dinosaur success. Finally, size distinguishes dinosaur evolution from that of extant endotherms. Large size led to reduced rates of heat-transfer between the reptile and its environment, resulting in inertial homeothermy. If necessary, smaller dinosaurs could hibernate or aestivate; larger dinosaurs would not need to do either (Hotton 1980). (Indeed, hibernation and aestivation would probably not have been necessary, even in the case of the smaller dinosaurs.) Hotton (1980) also pointed out that all really large dinosaurs would have had to have been nomadic in order to be able to gather enough food. As seasonal climates became widespread and persistent during the Mesozoic Era (as indicated by growth rings in fossil timber) they would have influenced the adaptive evolution of the dinosaurs and engendered long range seasonal migrations. Evidence for this is, he claimed, readily obtained from elements present in Upper Cretaceous palaeogeography. Migration and random drift over large distances depended on size, locomotor ability, and also on continuous activity for the great distances that would have had to have been travelled.
It would seem that the larger dinosaurs must have been homeothermic and to some extent tachymetabolic, but they could not possibly have been active to the degree indicated by Bakker (1971a, 1972,1987) and Desmond (1975). The controversy has been reviewed very clearly by Fastovsky and Weishampel (1996) and Benton (2004). Farlow (1997), Reid (1997), in addition to various authors in Currie and Padian (1997) and in Ruben et al. (1997), are recommended for further reading.
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