The effects of skull flattening on suchian jaw muscle evolution.

SELLERS, KALEB C.; NIETO, MAURO N.; DEGRANGE, FEDERICO J.; CLARK, JAMES M.; MIDDLETON, KEVIN M.; HOLLIDAY, CASEY M.

Congreso; 81 st Annual meeting Society of Vertebrate Paleontology; 2021

Jaw muscles are key features of the vertebrate feeding apparatus and often show adaptations to aspects of diet. However, the jaw musculature is housed in the skull, with morphology that reflects a compromise between multiple functions, and the skull and its contents constrain the geometry of jaw muscles. Thus, jaw muscle anatomy is often suboptimally constructed. Crocodylians are a group of vertebrates that generate the highest bite forces ever measured, with a flat skull suited to their aquatic ambush predatory style. However, fossil ancestors of crocodylians (e.g. Prestosuchus) were terrestrial predators with plesiomorphic skull shapes, and thus the origin of modern crocodylians involved a substantial reorganization of the feeding apparatus and its jaw muscles. Here, we reconstruct jaw muscle anatomy in select crocodylians and fossil suchians and investigate the impact of skull flattening on jaw muscle anatomy. 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. We hypothesize that taxa with flatter skulls have less efficient jaw muscle resultants and that modern crocodylians will have larger size-standardized gross muscle masses or longer lever arms in order to generate high bite forces with inefficient muscle configurations. We found that jaw muscle anatomy in early fossil suchians reflected the ancestral archosaur condition but experienced progressive shifts in the lineage leading to Mesoeucrocodylia (e.g., Neosuchia+Notosuchia). In early fossil suchians, Musculus Adductor Mandibulae Posterior (mAMP) and Musculus Pterygoideus (mPT) were of comparable size, but by Mesoeucrocodylia, the jaw musculature is dominated by mPT. We found that taxa with flatter skulls have less efficient muscle orientations. However, extant crocodylians did not have either larger size-standardized muscle masses or lever arms for jaw muscles, suggesting that crocodylians use alternate means to offset their inefficient muscle orientations. This study highlights the diversity and evolution of jaw muscles in one of the great transformations in vertebrate evolution.