Dinosaur Thermoregulation Other Considerations

The controversy over whether dinosaurs were endothermic, ectothermic, or some sort of physiology that did not qualify as either (or varied on a species-to-species basis) has caused the death of many trees because of the large number of papers written on the subject during the last 30 years. Of all topics in dinosaur studies, the popularity of the dinosaur thermoregulation discussion is rivaled only by the enduring debates over theropod ancestry and origin of flight in birds, and the causes of dinosaur extinctions at the end of the Cretaceous. One of the fortunate outcomes of this continuing research is that the question of which mode of thermoregulation dominated among dinosaurs has provoked a multi-faceted approach to answering it. One of the less fortunate outcomes is a common dilemma in science: more questions are generated by the research than are answered.

The other clues used to test for endothermy and ectothermy in dinosaurs include the following, most of which involve comparison to living terrestrial vertebrates (including birds):

■ Neurophysiology, where the encephalization quotient (EQ) is calculated and brain complexity is described (Chapter 11).

■ Geochemistry of bone as related to oxygen-isotope ratios.

■ Social behavior (degree of brooding or nurturing, gregariousness, intra-specific competition).

■ Cranial anatomy related to respiration.

■ Overall posture and locomotion.

■ Body size (including the lengths of some body parts).

■ Soft-part anatomy (organs and feathers).

■ Paleobiogeographic distribution.

■ Phylogenetic "closeness" to endothermic or ectothermic animals.

None of these approaches has resulted in 100% agreement among researchers. However, the evidence suggests strongly that some, but not necessarily all, dinosaurs were likely endothermic but showed this thermoregulatory mode while growing up. Figuring out which thermoregulatory mode was the case after adulthood is less certain. With that warning in mind, arguments and counterarguments for each facet of thermoregulation are presented briefly here. Readers should look for points of consensus, then peruse the voluminous literature on this subject for the details.

Neurophysiology

When EQs are plotted for dinosaurs against a 1.0 standard for modern crocodiles, the values range from extremely low (0.2) to substantially higher (6.5). The less "brainy" dinosaurs include sauropods, thyreophorans, and ceratopsians, which all rank below 1.0. Ornithopods and theropods show higher values, the highest being from coelurosaurs (especially troodontids), a clade that presumably includes the ancestors of birds. In fact, the EQs at the higher end of the scale for non-avian theropods overlap with some modern flightless birds (Chapter 15). The reasoning behind using EQs as a measure of thermoregulation is that modern endothermic organisms have high EQs. However, this does not necessarily mean that a cause-and-effect relationship can be inferred between EQ and endothermy. Also, less than 5% of dinosaur genera have had their EQs calculated, so the data set should be regarded as preliminary. Independent neurophysiological evidence related to EQ, such as brain complexity, also reveals that dinosaur brains are rather simple, comparable in morphology to those of modern reptiles. Nevertheless, a few (such as Tyrannosaurus) show enlarged olfactory bulbs, which suggests a superior sense of smell but denotes nothing about thermoregulation.

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