HOLLIDAY, Casey; Ohio University: Homology of extant archosaur jaw muscles: soft tissues and their osteological correlates.

Divergent morphological specializations of extant groups and fragmentary descriptions of soft- and hard-tissue topology have hampered understanding of the evolution and homology of archosaur jaw musculature, hindering interpretation of feeding evolution in non-avian dinosaurs. A similarity test was conducted to develop hypotheses of jaw muscle homology in extant archosaurs (birds and crocodilians) in order to identify potentially useful osteological characters that can be tracked in fossil taxa via a congruence test. Anatomical techniques, such as dissection and serial sectioning, were coupled with CT scanning and 3D image analysis to identify causal associations between jaw muscles, other relevant soft tissues, and the bones they modify in extant archosaurs and outgroup taxa. In addition to classically used, topological characters (e.g., trigeminal nerve divisions), several new, consistent positional relationships (e.g., n. anguli oris passing between adductor internus and externus muscle groups, caudal ramus of mandibular nerve passing between quadrate muscles and braincase muscles) and particular osteological correlates (e.g., adductor tubercle, cotylar crest, and lateral mandibular fossa) were incorporated to unite extant archosaur jaw muscles and to identify clade-specific soft-tissue phylogenetic characters. Major changes in temporal fossa functional morphology led to marked changes in bony structures such as the quadrate orbital process, palate, and laterosphenoid cotylar crest. However, soft-tissue topological patterns remain constant in that the pseudotemporalis and profundus muscles of the braincase, superficial and caudal muscles of the quadrate, and palatal muscles all maintain consistent relationships among themselves and other structures implying homologies of muscle groups previously considered disparate.