Biomechanics and the Evolution of the Crocodyliform Skull

Abstract

The evolution of the derived crocodyliform skull from that of basal suchians involved the acquisition of a number of functionally salient features. Skull flattening resulted in a reorganization of the jaw muscles and a rotation of the quadrate, placing the jaw joint caudal to the braincase. These changes were preceded by an expansion of the pterygoid buttress, which in crocodyliforms is elaborated into a second articulation of the skull with the mandible known as the pterygomandibular joint (PMJ). While the fossil record demonstrates the pattern of morphological changes, the biomechanics of the system is less well known. This study uses high‐fidelity biomechanical modeling to investigate muscle, bite, and joint force magnitude and orientation in Prestosuchus, protosuchians, and Alligator. We made finite element and free body models to estimate how individual muscles contribute to cranial forces. Osteological correlates of jaw muscle attachments informed reconstructed muscle maps on 3D models built through the Boneload computational modeling workflow. Three‐dimensional muscle force vectors were projected into ternary space to visualize how muscular geometry changes with osteology. The orientations of joint surface and joint reaction force were also compared. Results show a progressive increase in medially‐oriented muscle force along the lineage leading to crocodyliforms concomitant with the expansion of the pterygoid buttress and PMJ. Results also reveal that jaw joint reaction forces align with the quadrate across the transition in quadrate orientation, suggesting that a shift in muscular geometry in more derived taxa increased the rostral and medial components of jaw joint reaction force, thereby avoiding excessive bending of the suspensorium.Support or Funding InformationNSF IOS‐1457319; NSF‐EAR 1631684; Missouri ResearchBoard; Missouri Research Council; University of Missouri Department of Pathology and Anatomical Sciences