Evidence for Blood Vessels and Physiological Heat Exchange in the Heads of Dinosaurs

Abstract

Extant diapsids share broad vascular patterns with known roles in thermoregulation, but different clades emphasize different sites of thermal exchange (oral, nasal, and orbital regions). We tested the hypothesis that dinosaurs emphasized different sites of thermal exchange. Evidence for dinosaur vascular anatomy was collected, and blood vessels were restored in Maya. Cross-sectional areas of canals offering evidence for blood flow (dorsal alveolar, cerebral carotid, premaxillary, and canals for palatine and nasal vessels) were measured and analyzed using principal components analysis and partial least squares. Small-bodied dinosaurs were compared with larger-bodied members of the same clade to assess the evolution of vascular networks associated with enhanced heat-exchange regions. Smaller-bodied dinosaurs with modest oral and nasal regions displayed balanced blood flow patterns. Large dinosaurs had less capacity to shed heat, resulting in selective pressure to evolve elaborate physiological heat-exchange regions. These regions displayed blood flow patterns with different sizes and combinations of blood vessels. For example, the large subnarial foramen in sauropods indicates that both the oral and nasal regions were highly vascularized, with little supply from the maxillary vessels. Yet, ankylosaurs show evidence for enhanced nasal and maxillary vessels, and unexpanded vascularization of the oral cavity. Theropods generally showed balanced blood flow patterns, but evidence indicates they may have used paranasal sinuses as a site of heat exchange. The observed phylogenetic patterns are consistent with the hypothesis that larger-bodied forms evolved anatomically expanded regions with enhanced blood flow to facilitate evaporative cooling.