HOLLIDAY, CM*; WITMER, LM; Ohio Univ: Evolutionary morphology of the orbitotemporal region in Archosauria

The orbitotemporal region is a structurally complex, functionally important, and phylogenetically informative cephalic space built by elements of the palate and braincase which includes trigeminal nerves, adductor muscles, and other topologically conservative structures. However, little is known about the region�s evolution in Archosauria, obscuring knowledge of regional patterns among amniotes. Tests of similarity and congruence among soft and bony adductor chamber contents were conducted in a large survey of extant and fossil archosaur taxa revealing a mosaic of structural patterns associated with the braincase and palate. Iterative losses of the epipterygoid�within seemingly disparate adaptive contexts�occurred along the lines to Crocodylia and Neornithes, as well as within at least three terminal non-avian dinosaur clades (e.g., Ornithopoda, Ceratopsidae, and Sauropodomorpha). Complementing these changes, neomorphic bony walls of the laterosphenoid developed, altering the topology of the trigeminal nerves, cavum epiptericum, and temporal fossa, as well as impacting the functional properties of the palatocranial junction. Associated modifications in muscular structures include the loss of m. levator pterygoideus through Tetanurae, an enlarged m. protractor pterygoideus in Tyrannosauridae, and the development of a complex, multidirectional suite of protractor muscles in Ornithopoda. These functional characteristics suggest adaptations for maintaining intracranial rigidity via a passive, soft-tissue, stay system, which is interpreted to be a plesiomorphic feature that birds exapted for powered cranial kinesis.