SEDLMAYR, J C: Encephalic Blood Vessels and Evolution of the Archosaur Braincase

As the vertebrate braincase houses the brain, cranial nerves, etc., it can provide critical information central to our understanding of vertebrate cephalic and physiological evolution. However, study of bony remains and endocasts is complicated by encephalic vasculature, a system of physiologically important vessels that occupies much of the endocranial space. Few comparative data exist for encephalic vascular structures and their osteological correlates, especially in archosaurs, which greatly inhibits our ability to address key aspects of amniote evolution. To redress the above problems, I performed an analysis of the encephalic vasculature of Archosauria using the extant phylogenetic bracket approach. Vascular-injected extant amniotes were dissected and studied to generate hypotheses of homology and to determine osteological correlates. Many vessels exhibited clear osteological correlates. Fossil archosaur specimens were then examined for osteological correlates and for congruence testing of homologies (complicated by differential ossification of the braincase in various fossils). Encephalic vascular structures and patterns appear to be relatively consistent across amniote clades and were highly dependent upon brain morphology. Crocodilians and birds possess highly derived brains, altering the positions of the blood vessels and their osteological correlates relative to the ancestral condition (i.e., basal suchians and non-avian dinosaurs). Archosaurs apomorphically share a dual endocranial venous system of ventral vessels flowing from the cavernous sinus into the jugular system, and dorsal sinuses that drain into the intravertebral venous plexus. These sinuses can be greatly enlarged and in close contact with arteries supplying neural tissues, potentially allowing for selective brain temperature regulation.