CROCODYLIAN SNOUTS IN SPACE AND TIME REVISITED: NEW INSIGHTS USING GEOMETRIC MORPHOMETRICS, DIETARY SURVEYS, AND FEEDING TRACES

Crocodyliform snout shape is often used as an indicator of dietary niche. Slender-snouted taxa are interpreted as piscivores, or small-prey specialists. Blunt-snouted species often have anvil-shaped teeth associated with durophagy, and broad, triangular snouts denote ecological generalists. The associations between snout form and function are so entrenched that broad similarities have been used to infer diet in distantly related taxa (e.g. phytosaurs; dinosaurs). Functional studies seem to support these groupings, with wider jaws being more resistant to torsional forces, but increasing drag under water, a morphology that benefits targeting larger prey, while slender snouts are less resistant to torsion, but minimize drag, which would align with a diet of smaller, swifter prey. However, these ecomorphological groupings are problematic. Bite force, which seemingly should vary with snout shape and dietary strategy, is surprisingly uniform across crocodylians of similar body sizes. Death roll behavior, once viewed as characteristic of generalists and absent in slender-snouted taxa, has now been observed in nearly every living species. Dietary surveys of “specialist” groups reveal a surprising variety of prey items. What was once portrayed as an ideal exemplar of the interplay of form and function is actually more complicated. Here we present a new snout shape classification scheme based on geometric morphometric analyses of skull outline. Partnered with characteristics of dentition, this yields seven shape classes. These results are combined with a meta-analysis of maximum body mass in predator and prey groupings, based on ecological surveys in modern groups and feeding traces associated with fossil taxa. This study demonstrates that the discrete dietary classifications assigned to crocodylian snout shapes are, in reality, much less distinct. Durophagy has been observed in groups lacking specializations for that diet. Slender-snouted forms do focus on comparatively smaller prey, but are much more generalist than previously recognized. Our results demonstrate that diet and prey selection are not easily circumscribed into discrete categories in living animals, who can be observed directly, much less fossil ones, whose behavior must be studied through indirect means.