| There is little, if any, direct fossil evidence of the cardiovascular, respiratory, reproductive or digestive biology of dinosaurs. However, a variety of data can be used to draw reasonable inferences about the physiology of the carnivorous theropod dinosaurs (Archosauria: Theropoda).;Extant archosaurs, birds and crocodilians, possess regionally differentiated, vascularized and avascular lungs, although the crocodilian lung is less specialized than the avian lung air-sac system. Essential components of avian lungs include the voluminous, thin-walled abdominal air-sacs which are ventilated by an expansive sternum and specialized ribs. Inhalatory, paradoxical collapse of these air-sacs in birds is prevented by the synsacrum, pubes and femoral-thigh complex. The present work examines the theropod abdomen and reveals that it lacked sufficient space to have housed similarly enlarged abdominal air-sacs as well as the skeleto-muscular modifications requisite to have ventilated them. There is little evidence to indicate that theropod dinosaurs possessed a specialized bird-like, air-sac lung and, by extension, that theropod cardiovascular function was any more sophisticated than that of crocodilians.;Conventional wisdom holds that theropod visceral anatomy was similar to that in birds. However, exceptional soft tissue preservation in Scipionyx samnitcus (Theropoda) offers rare evidence of in situ theropod visceral anatomy. Using computed tomography, close comparison of Scipionyx with gastrointestinal morphology in crocodilians, birds and lizards indicates that theropod visceral structure and "geography" was strikingly similar only to that in Alligator.;In modern birds, there exists a unique osseous tissue, medullary bone, which forms only in adult females as a source of calcium for developing hard-shelled, calcified eggs. Although crocodilians also produce calcified eggshells, no medullary bone is formed. Recent supposed identification of medullary bone in Tyrannosaurus rex (Theropoda) suggests an avian style of reproduction was present in dinosaurs and, if correct, would seem to support a close relationship between dinosaurs and birds. However, scanning electron microscopy demonstrates that claims of medullary bone in theropod dinosaurs should be regarded with skepticism as the tissues recovered are not unique from those recovered in growing Alligator.;These data provide evidence of significant but heretofore unrecognized differences between avian and theropodan dinosaur internal structure and function. |
