VASCULAR ANATOMY AND PHYSIOLOGICAL IMPLICATIONS IN EXTANT AND EXTINCT THEROPOD DINOSAURS

A paradigm shift holds that extinct dinosaurs may have been active and potentially tachymetabolic animals. But even beyond potentially high metabolic rates, the great size and low surface-to-volume ratios of many dinosaurs would have resulted in increased heat loads that required efficient thermoregulatory mechanisms. In extant taxa, vascular systems play a key role in such thermal phenomena as evaporative and convective cooling and basking. Thus, we hypothesize that dinosaurs had vascular physiological devices to deal with heat. First, the general pattern of vascular anatomy of dinosaurs must be elucidated, which has not been the focus of major study until recently. The vascular anatomy of dinosaurs is being investigated and characterized using modern CT scanning and the Extant Phylogenetic Bracket approach. By comparing specimens of extant taxa (turkey, pheasant, alligator, iguana), the vascular anatomy was studied and osteological correlates (OCs) were identified. These OCs are being surveyed in extinct theropods, and the results of our preliminary findings are presented using the Cretaceous abelisaurid Majungasaurus crenatissimus (FMNH PR 2100). Vascular OCs were identified in the fossils and CT data, and the vascular anatomy was restored in a 3D environment using Avizo 6.3. The vasculature of the orbit displays a more avian condition, with the supraorbital artery anastomosing with the ethmoid artery. Venous drainage of the braincase was found to form an anastomotic loop around the parietal and otoccipital. The nasal vasculature is hypothesized to anastomose with the vasculature of the palate, creating an extensive loop, similar to that displayed in both birds and alligators. The presence of a palatal plexus is a less robust inference, but still probable, due to the presence of a palatal plexus in both extant relatives of dinosaurs. The presence of a narial plexus is also reconstructed in Majungasaurus based on its presence in extant taxa. These preliminary results indicate that study of vascular anatomy in dinosaurs has potential for further refinement, resulting in a robust model, and with the inclusion of more taxa, a more accurate picture of the circulatory system and physiology in dinosaurs.