The respiratory and metabolic biology of dinosaurs is poorly, if at all, reflected in the fossil record. By comparing anatomical features of modern taxa that are functionally linked to specific biology with the remains of theropod dinosaurs (Archosauria: Theropoda) and early birds, we may reasonably infer the physiology of long extinct taxa. This dissertation uses comparative anatomy and experiments with living crocodilians to investigate dinosaur respiratory biology.

Modern crocodilians ventilate a relatively unspecialized lung using both costal action as well as fore-aft movement of the transversely oriented liver. During inhalation, the diaphragmaticus muscles, which attach to the pubis and invest the digestive viscera, pull the liver caudally. Interestingly, the unique crocodilian pubis is mobile at its articulation with the pelvic girdle and is capable of dorso-ventral movement. In my experiments, I surgically fixed the pubis and found that this decreased the movement of the liver and tidal volume but did not increase intrabdominal pressure. Thus I infer that theropod dinosaurs with immobile pubes could have utilized a crocodilian-like ventilatory mechanism without suffering excessively high intrabdominal pressure that would limit venous return. Furthermore, the skeleton of theropod dinosaurs strongly suggests the presence of crocodilian-like ventilatory mechanisms.

Modern birds ventilate a highly derived lung air-sac system using movements of a specialized thoracic skeleton. Requisite to the proper function of this system are their voluminous, thin-walled abdominal air-sacs that are supported by a specialized synsacrum, pubes and femoral-thigh complex. This lung air-sac system facilitates increased oxygen delivery to serve the demands of highly active tissues characteristic of endothermy. The earliest bird, Archaeopteryx , lacked the skeletal modifications indicative of a modern bird-like lung air-sac system and therefore was likely not endothermic, yet may have been capable of powered flight. Endothermy likely did not evolve in birds until the mid-Cretaceous ornithiurine birds Hesperornis and Ichthyornis.