Comparative endocranial anatomy in the crocodylians Leidyosuchus canadensis and Stangerochampsa mccabei from the upper Cretaceous of Alberta, Canada.

Crocodylians evolved a high variety of rostral morphologies during their evolutionary history, highlighting the strong links between morphological plasticity and environmental and ecological parameters. Two Late Cretaceous alligatoroids, the mesorostrine Leidyosuchus canadensis Lambe, 1907, and the brevirostrine Stangerochampsa mccabei Wu et al., 1996, from Alberta, Canada, preserve a large groove-shaped recess on the posterior part of the maxilla that has not been documented in other alligatoroids. Despite the potential phylogenetic and paleoecological significance of this neurovascular feature, internal and endocranial structures remain under-explored among stem alligatoroids. The endocranial morphology, including the paratympanic sinus system of Leidyosuchus canadensis and Stangerochampsa mccabei, was compared to those of extant crocodylians and of the extinct alligatoroid Diplocynodon ratelii based on computed tomography data. The Cretaceous alligatoroids share endocranial features, such as a posteroventral neurovascular projection of the labiolateral canal that connects to the groove-like recess at the posterior edge of the maxilla and a paratympanic sinus system most similar to those of small-bodied and young extant crocodylians, suggesting that these pedomorphic features may reflect the ancestral crocodylian condition. Future phylogenetic studies should consider internal and endocranial characters alike to improve our understanding on the relationships among crocodylians.