Where Do We Go From Here?

SUMMARY OF THE DEBATE

The problem is far from solved, as you may guess from what you've seen so far. Many questions about endothermy and ectothermy in modern animals remain to be solved, such as:

"How did endothermy evolve in mammals?"
"When did it evolve in birds?" or
"What alternatives are there to simple endothermic or ectothermic strategies?"

There is much, much work to be done before a satisfying answer is available; maybe that time will never come — dinosaurs (except birds) are long dead and gone; all we have left are bits of fossilized bone and some poor modern analogs (birds and mammals, or lizards and crocodilians).

Commentary

Yes, it's a confusing and frustrating controversy, and the two (among the many) diametrically opposed sides of the argument help to make it even more complex. Are some of them missing something that paleontologists outside of the debate see? Maybe. There is strong evidence from both sides that dinosaurs in general had a different physiology from either mammals or "typical reptiles." Endothermy did evolve from ectothermy, and birds (endotherms) did evolve from dinosaurs, which we know came from ectothermic ancestors sometime in the distant past. Dinosaurs were quite diverse in size and form; their physiologies must have differed as well, just like whales, bats, and horses have different physiologies. We have found examples of physiologies outside of the artificial "endotherm-ectotherm" dichotomy: elevated metabolic rates and even homeothermy exist in some species of sea turtles, sharks, pythons, tuna, and even insects. Some mammals, such as the monotremes have lower metabolic rates, and seem closer to an ectothermic condition. As long as we do not understand endothermy and how it evolves, we have little chance of understanding what animals were endotherms or not. And, above all, there is no reason why dinosaurs could not have had some sort of intermediate physiology between endothermy and ectothermy. In fact, it seems more likely than anything that they did have some unknown sort of physiological system that worked well (they did dominate the realm of the terrestrial vertebrates for some 170 million years!). Some dinosaurs could have been normal ectotherms, and some could have been endotherms. Yes, it's a messy issue.

We'll close by listing the five main sides that paleontologists have taken since the issue began. Feel free to make up your own mind who may be right!

Top five current hypotheses

Dinosaurs were complete endotherms, just like birds, their descendants.
Some or all dinosaurs had some intermediate type of physiology between endothermy and ectothermy.
We know too little about dinosaurs to hazard a guess at what their physiology was like.
Dinosaurs were mostly inertial homeotherms; they were ectothermic but maintained a constant body temperature by growing large. Small dinosaurs were typical ectotherms, maybe with a slightly elevated metabolic rate.
All dinosaurs were simple ectotherms, enjoying the warm Mesozoic climate. But that's okay; many ectotherms are quite active, so dinosaurs could be active, too.

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