According to phylogenetic analyses, birds and crocodilians are the closest living relatives to dinosaurs, but crocodilians are ectothermic and birds are endothermic. According to present paleontological knowledge, the common ancestor of cro-codilians and dinosaurs probably lived in the Early Triassic. In contrast, the common ancestor of dinosaurs and birds probably lived in the Middle to Late Jurassic. The common ancestry of crocodiles and dinosaurs was originally used as evidence of ectothermy in dinosaurs. Now, however, the common ancestry of dinosaurs and birds is used as evidence for endothermy in dinosaurs. Furthermore, pterosaurs (flying reptiles: Chapter 6) are now regarded as endotherms, and they had a common ancestor with dinosaurs. Some paleontologists default to the more recent evolution of birds as evidence of a minimum time when endothermy emerged in archosaur lineages. Nevertheless, this still does not answer exactly when endothermy developed. Endothermy also was independently developed in mammalian lineages throughout the Mesozoic, and it most likely began in mammals well before birds.
The only clear conclusion from using a phylogenetic argument for the timing of endothermy is that it developed in some archosaur lineages sometime in the Mesozoic and most likely in some small theropods during the Jurassic. Nonetheless, this conclusion still does not entirely pertain to other dinosaurs, even other saurischians. As a result, the mystery of dinosaur thermoregulation is not yet firmly answered by either "big picture" or "small picture" perspectives, but the answers have been good in provoking further research.
Paleontologists who study dinosaur physiology examine how dinosaurs converted and transferred matter and energy in their daily lives, which is especially related to their \ metabolism. This is a topic of much interest to paleontologists because of what it reveals about dinosaur biology and evolutionary history Much of the controversy surrounding dinosaur physiology centers on whether they were endothermic ("warm blooded"), ectothermic ("cold blooded"), or some other type of physiology that was in between. Lines of evidence used in interpreting dinosaur physiology are mostly associated with reproduction, growth, and feeding. Reproductive behavior, types of eggs, clutch sizes, and the biogeochemistry of eggs all lend clues to dinosaur physiology, particularly for female dinosaurs. Studies of bone mineralization and dinosaur bone histology provide information about their rates of growth. These indicate rapid growth rates consistent with endothermy in a few dinosaur species, although it is unknown whether most or all dinosaurs had the same sort of physiology. Dinosaur nutritional needs, adaptations to feeding (such as teeth and jaws), and how they fed were intimately related to dinosaur physiology. Dinosaurs consisted of both herbivores and carnivores, although evidence from toothmarks, stomach contents, gastroliths, and coprolites can sometimes indicate exactly what a dinosaur ate or which of them were eaten. Other lines of evidence related to dinosaur physiology include EQ (encephalization quotient), geochemistry of bone, social behavior, cranial anatomy related to respiration, overall posture and locomotion, body size, soft-part anatomy, paleobiogeographic distribution, and phy-logenetic relatedness to endothermic or ectothermic animals. Collectively, these data point toward dinosaurs as a physiologically varied group of animals that cannot be easily categorized.
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