The Evidence for
Ectothermy in Dinosaurs
It may seem that little work has been done to support ectothermy in dinosaurs (until very recently), most likely because ectothermy in dinosaurs is assumed to be the default condition; the ancestors of dinosaurs (as reptiles) were ectothermic, so some might say that endothermy should be demonstrated, not ectothermy. We shall objectively examine the hypotheses advocating ectothermy.
Top six hypotheses
Given: Dinosaurs were quite large. Hypothesis: Dinosaurs did not need to be endotherms; they could be inertial homeotherms — they would warm up slowly and cool very slowly, so they could maintain a near-constant body temperature. Problem: Not all adult dinosaurs were large — the weight range was from about 10 pounds (5 kg) to 80 or so tons. Even the largest dinosaurs would have had small babies. Also, there are large mammals (e.g., whales, elephants, and the extinct Paraceratherium) which are endotherms, not inertial homeotherms.
Given: The climate of the Mesozoic era was milder and warmer. Hypothesis: Dinosaurs did not need to be endothermic; it was always warm enough to keep them warm. Problem: The climate in the Mesozoic era was varied, and dinosaur fossils are found in some areas that were cooler and less mild then, including polar regions.
Given: Ectotherms are generally scaly. Hypothesis: Dinosaur fossilized skin impressions show somewhat scaly hides, so they were ectotherms. Problem: Birds (which are endotherms) are scaly, too! Look at their legs; these scales are indicative of their archosaurian ancestry. Bird feathers apparently are modified scales. Also, there are few known dinosaur skin impressions, so we don't know what the skin of most dinosaurs was like. Most endotherms do have fur or feathers for insulation, though.
Given: Dinosaurs were quite large. Hypothesis: Large endotherms such as mammals and whales have problems with heat; since the Mesozoic was hot, dinosaurs would have cooked if they were endotherms. Problem: Again, not all dinosaurs were big. What of them? Also, we have no gigantic (e.g., 30 tons) terrestrial endotherms alive today whose physiology we can study to see if they have heat stress problems. Elephants and whales do quite well with their large body size, although they do seem to have to more problems with heat than smaller animals do. Animals can have ways of regulating heat that the fossil record does not preserve — behavioral (stay out of the sun), physiological (special enzymes), or anatomical (big ears to radiate heat).
Given: The bone structure of some dinosaur bones exhibit lines of arrested growth (LAGs), showing that growth was seasonal. Hypothesis: Because modern endotherms do not have LAGs, and modern ectotherms do, dinosaurs were ectotherms. Problem: Modern endotherms can develop LAGs during periods of stress (malnutrition, etc.). Modern ectotherms may lack LAGs. The correlation between LAGs and ectothermy, then, is not rock solid. We're still not sure why and when certain animals develop LAGs, and whether their presence demonstrates anything except sporadic growth. Still, a compelling line of evidence.
The latest controversy:
Given: All terrestrial endotherms have respiratory turbinates (folded bones in the sinuses; apparently important in conserving water loss). Hypothesis: Dinosaurs did not have respiratory turbinates, so they could not have functioned as endotherms. Problem: It has not yet been shown that no dinosaurs had turbinates; many birds have them, so turbinates in birds must have evolved sometime in the Mesozoic. Also, the relative importance of turbinates has yet to be shown; some few endotherms get along fine without them. Are there alternative solutions to the problem of water loss? The matter is not closed yet, but the respiratory turbinate factor is an interesting one, and good science too. Lately some researchers have noticed that some dinosaurs do have structures in their nasal passages that do appear to be respiratory turbinates, but this has not yet been established, and followup reports have cast some doubts on those interpretations.
Return to the Introduction
Evidence for endothermic dinosaurs
Summary of current hypotheses
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