THE ROLE OF SYMPHYSEAL TISSUES AND SUTURES ON MANDIBLE FUNCTION IN ARCHOSAURS

The mandible is a biomechanically important structure wherein forces are transferred to and from the organism during biting behaviors. Because reptile mandibles are composed of numerous bones and joints, they have evolved diverse adaptations to the competing pressures of mandibular development and function. Archosaurs generally possess unfused dentary symphyses and prominent intramandibular joints (IMJs) whose roles in mediating mandibular forces are not yet fully appreciated. Moreover, archosaurs variably possess laterally wrapping pterygoideus, depressor mandibulae and intramandibularis jaw muscles whose roles in mandibular mechanics remain unclear. Theropods dinosaurs are an ecologically diverse archosaur clade with great diversity in mandibular shape and many taxa have been described as possessing forms of mandibular kinesis. This however presents a biomechanical paradox for stiff-skulled, osteophagous theropods like Tyrannosaurus rex that must use its jaw muscles to both produce extreme bite forces and mediate flexibility of the dentary about the IMJ. We use the jaw muscle architecture of Alligator and Struthio mandibles to bracket inferences of structure and function in non-avian dinosaurs such as T. rex. We used anatomical techniques, contrast imaging, and computational modeling to test how the mandible of T. rex performs under differing hypothesized soft tissue properties and loads. Because the osteological correlates of pterygoideus ventralis in non-avian dinosaurs are poorly defined we built different finite element models with differing hypotheses of muscle attachments. Our results found that medial bending and long axis rotation about the IMJ induced by a laterally-wrapping m. pterygoideus ventralis inserting onto the mandibular shelf, and medially-inserting m. adductor mandibulae externus medialis, is best reduced by a dorsomedial orientation of m. intramandibularis that is contiguous with m. pseudotemporalis superficialis. These results also suggest intramandibularis prevents lateral bending and long axis rotation of the mandible of theropods with prominent IMJs, and that the developmentally primitive IMJ may present biomechanical problems for archosaurs and other non-mammalian tetrapods.