MAMMAL-LIKE CROCS: DO EXTINCT CROCODYLOMORPHS OVERLAP WITH THE MORPHOSPACE OF LIVING MAMMALS?

Crocodylomorpha is an extremely successful clade of living reptiles, having repeatedly radiated into novel ecological roles over the past 230 million years, survived two mass extinctions, and displayed a remarkable morphological disparity throughout their evolutionary history. Relative to modern crocodylians, disparate ecologies, skull morphologies, and tooth shapes of some extinct taxa has led to comparisons with another clade of successful living animals, mammals. In particular, it has been hypothesized that fossil crocodylomorphs filled ecological roles and morphospace otherwise occupied by mammals due to similarities in complex jaw movement cycles, dental occlusion, and tooth morphology. We utilize a combination of linear and geometric morphometrics to quantitatively test for overlap between the skull morphology of extinct crocodylomorphs and living amniotes (i.e., mammals, squamates, and crocodylians). We find that there is little overlap between the extant amniotes, with each clade occupying separate regions of morphospace. Within each clade, closely related taxa typically fall in similar regions of morphospace, suggesting phylogenetic relationships play an important role in skull shape. Fossil crocodylomorphs span a much greater morphospace than their extant relatives. Marine and semiaquatic taxa overlap with extant crocodylians, whereas some small extinct taxa fall among living squamates. Many terrestrial taxa (including both protosuchians and notosuchians) independently evolve skull shapes that plot in morphospaces not exhibited in sampled extant amniotes. However, despite the morphological range of extinct crocodylomorphs, there is no significant overlap with mammals, even though other features, such as tooth morphology and complex jaw movement cycles, occur in both clades. These results demonstrate that extant clades have partitioned skull morphospace and despite their disparate ecologies and skeletal morphologies, extinct crocodylomorphs do not converge with living mammal morphospace.