Skull Shape, Muscle Orientation, And Joint Loading In A Biomechanical Transformation: Evolution Of The Suchian Skull

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. To assess the biomechanical effects of changing configurations of muscles and cranial joints, we used CT data to create 3D models of extant and fossil suchians that demonstrate the evolution of the crocodylian skull, using osteological correlates to reconstruct muscles. Muscle forces were distributed with the computational package Boneload and used as input for finite element analysis. We found successive transformation of cranial joints preceded muscle shifts. After the pterygoid buttress expanded and the quadrate and palate sutured to the braincase, jaw muscles were free to expand and shift attachments through suchian evolution. Muscle orientations became more mediolateral as the skull flattened, the pterygoideus ventralis muscle began inserting on the lateral mandible, and the depressor mandibulae muscle expanded its attachment on the enlarged retroarticular process. We found that working side joint force is lower during rostral bites; the jaw joint is likely loaded in tension during feeding events such as shaking bites or death roll. We also found that joint force orientation tracks with articular surface metrics. We show that joint area and joint force scale isometrically with body size. Joint pressures are remarkably consistent both with values previously reported in vivo and in vitro in other joint systems and with the range of joint pressures predicted to result in stable joint morphology by chondral modeling theory.