Function and evolution of the crocodyliform feeding apparatus
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The evolution of crocodylians from their suchian ancestors represents one of the great transformations in vertebrate evolution. Modern crocodylians have flat skulls and generate high forces during feeding, but crocodylian ancestors have tall skulls and lack most of the characters that help crocodylians generate and resist high forces. Thus, the evolution of crocodylians involved a substantial reorganization of the feeding apparatus. Although changes to skull shape in the lineage leading to crocodylians have received a great deal of attention, the functional consequences of shape change on feeding biomechanics are unclear. This dissertation addresses this gap in our knowledge by building high-fidelity biomechanical models to ask questions about the evolution of skull shape and feeding performance in the lineage leading from early suchians to extant crocodylians. I use detailed 3D muscle attachment sites to estimate muscle forces and distribute forces on 3D finite element models using an approach validated against in vivo data. These finite element models are used to estimate bite and joint forces. I use traditional linear morphometrics to characterize skull flattening. I also develop novel methods to quantify and visualize joint articular surface shape. These results are analyzed using phylogenetic comparative methods and ancestral character state reconstruction. My results show that skull flatness is linked with inefficient muscle geometries and that these geometries developed stepwise in the lineage leading to crocodylians. I found that joint shape best reflects the highest-magnitude loads that the skull experiences, that the orientation of joint loading tracks with skull flatness, and that peak joint pressure falls within the range predicted by chondral modeling. This study shows that extant crocodylians rely on extra muscle mass to produce high bite force with inefficient muscle configurations and that the derived, suturally-immobilized cranial joints are key features of the feeding apparatus that mitigate mechanical inefficiencies imposed by a flat skull. Overall, these results depict a coordinated evolution of skull shape, muscle anatomy, joint surface shape, and biomechanical performance in the lineage leading to Crocodylia and have broad implications and applicability to all vertebrate musculoskeletal systems. This dissertation research represents an important step in improving our understanding of the biomechanics of musculoskeletal transitions.
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Ph. D.