62 - Non-avian Theropods Did Not Have Laterally-wrapping Pterygoideus Muscles

Poster Board Number: 62
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Co-authors:

Casey Holliday, Dr. - University of Missouri

Abstract Body : All vertebrate skulls are under significant feeding-related selective pressures. Their jaw musculature must generate sufficient force to seize and/or process food, while their skull bones must withstand feeding stresses. While we have a good idea where most jaw muscles in archosaurs attach, m. pterygoideus ventralis (mPTv) poses a particular challenge. Dinosaurs like T. rex are often reconstructed with large, crocodilian-like laterally-wrapping pterygoideus muscles, which are hypothesized to resist long-axis rotation of the mandible. We evaluated specimens for osteological correlates of this muscle attachment and found equivocal signal. Extant phylogenetic bracketing is equally equivocal. Without clear osteological or phylogenetic support for an attachment site, we resorted to mechanical testing to determine mPTv’s attachment site and role in skull function. We modeled Alligator and Tyrannosaurus rex with a laterally wrapping and non-wrapping mPTv to analyze the relationships between muscle loads and skull forces. Because Alligator mPTv volume is ~3 times greater than predicted by the area-frustum method, we tripled both modeled Alligator and T. rex mPTv volume to generate more accurate contractile forces for mPTv. To mimic a laterally wrapping mPTv fibers, we oriented contractile forces towards the rostralmost extent of its insertion site, as this most closely mimics the orientation of muscle fibers as determined by iodine-contrast CT data. Alligator models with laterally wrapping mPTv show reduced rotational moment (torque), especially about the intramandibular joint, and lowered jaw joint reaction forces compared to models without laterally wrapping mPTv. In contrast, T. rex models display comparable long axis rotational moments, regardless of whether mPTv laterally wraps or not. The laterally wrapping mPTv in the dorsoventrally flattened skull of Alligator helps resist jaw joint tension and long axis rotation of the hemimandible. Non-avian theropods like T. rex, with their comparatively tall and narrow mandibles, evidently did not face this problem, as both wrapping and non-wrapping models display comparable rotational moments. We thus conclude that non-avian theropods likely did not possess crocodilian-like laterally wrapping pterygoideus muscles.