Meeting Abstract
142.6 Monday, Jan. 7 Form, function, and evolution of archosaur mandibular symphyses. HOLLIDAY, CM*; GANT, CA; NESBITT, SJ; University of Missouri; University of Missouri; University of Washington hollidayca@missouri.edu
Archosaurs radiated into numerous trophic niches during the Mesozoic, resulting in a diversity of cranial adaptations and feeding behaviors. The mandibular symphysis is a poorly understood cranial joint which may offer significant insight into cranial development and function, feeding ecology, and evolution in these vertebrates. Using imaging, histology, and dissection data from extant and fossil sauropsids, we investigated the anatomy and evolution of archosaur symphyses with a focus on Alligator mississippiensis and crocodyliforms. Adapting Scapino’s classification scheme, character complexes of specific clades were identified and their evolution was mapped using a current phylogeny of archosauriforms. During ontogeny, alligators rapidly develop a complex, interdigitated, Class III symphysis coupled with fused Meckel’s cartilages. This morphology is a derived for mesoeucrocodylians as protosuchians possess non-interdigitated Class II symphyses. Extinct taxa with the simple Class I condition (e.g., proterochampsids, rauisuchians), rugose Class II (aetosaurs, silesaurids, derived dinosaurs), and interdigitating Class III symphyses (e.g., phytosaurs, crocodyliforms, basal birds) and finally fused Class IV (Neoaves) build the joints in expected ways, though they differ in contributions of bony elements and Meckel’s cartilage. Optimization of the different classes of symphyses across a archosauromorph clades indicate that major iterative transitions from plesiomorphic Class I to derived, rigid Class II-IV symphyses and beaks occurred along the lines to phytosaurs, aetosaurs, poposauroids, crocodyliforms, pterosaurs, ornithischians, and birds. These transitions in symphyseal morphology appear to correlate with changes in dentition, the origin of beaks, and potentially inferred diet.
