Meeting Abstract
Mesoeucrocodylia (alligators, crocodiles, gharial, and their extinct relatives) is a surprisingly diverse clade of archosaurs that includes carnivorous, omnivorous, herbivorous forms that once occupied a wide range of habitats. Whereas the crown group is comparatively uniform in their morphology, this apparent lack of variation may be a result of functional or developmental bias or homoplasy due to shared semi-aquatic ecology. Here, we investigate the dynamics of skull shape evolution to understand how rates of evolution and disparity have varied across mesoeucrocodylian lineages through time. Previous investigations of skull evolution in the group have focused on subgroups or on 2D geometric morphometric approaches that exclude key aspects of cranial variation (e.g., palate and pterygoid). We quantify whole-skull morphology across the diversity of Mesoeucrocodylia (n=45, including 19 fossils with 7 notosuchians, 2 thalattosuchians, and 10 crocodylians) using high-dimensional geometric morphometrics (1291 surface landmarks and semilandmarks). The most variable parts of the skull are the pterygoid and ectopteryoid, as well as the premaxilla, illustrating that other parts of skull shape beyond craniocaudal elongation are important parts of morpho-functional diversification in this clade. Modelling phenotypic evolution under a variable-rates Brownian motion model reveals high posterior probability of increased rates of evolution at the split between Crocodilia and Alligatoridae and on the terminal branches, suggesting rapid evolution in the crown group. Phenotypic integration is negatively correlated with disparity, supporting the hypothesis that intrinsic bias has constrained cranial evolution. Together, this whole-skull approach illustrates the importance of constraint and convergence in crocodyliform skull evolution.
