Digital reconstruction of the endocranial anatomy of species of the Cenozoic European crocodylian Diplocynodon

With the exception of Alligator sinensis in eastern China, extant members of Alligatoroidea are restricted to the Americas. However, the fossil record indicates a broader distribution that includes at least Europe. The latter is mostly based on Diplocynodon, which is typically recovered as one of the earliest diverging alligatoroids. It was seemingly endemic to Europe, and also the last known alligatoroid taxon to have inhabited the continent, with dozens of occurrences representing eleven currently recognised species, spanning the late Paleocene to the middle Miocene. Specimens of Diplocynodon have usually been recovered from freshwater environments, but remains of at least one species have been collected from a marine environment. This potentially suggests a capability for salt tolerance, contrasting with extant alligatoroids, that might have been key to the dispersal of early members of the clade. However, some phylogenetic analyses have recently cast doubt on the monophyly of these species and have even suggested that Diplocynodon might instead lie outside of crown-group Crocodylia. One possible avenue to address these phylogenetic and ecological questions is to evaluate the endocranial anatomy of Diplocynodon, which can potentially shed light on osteological correlates for oceanic capability, as well as provide a source of currently neglected anatomical features that can be incorporated into phylogenetic analysis. Here we present preliminary results based on CT scans of specimens referable to two different species, D. ratelii from the Miocene of France, and D. hantoniensis from the late Eocene of the UK. The high level of preservation of the fossils of both species has permitted a detailed description of their endocranial anatomy, providing information that can help infer their ecology and phylogeny. Based on a series of specimens of D. hantoniensis, representing three ontogenetic stages, we have been able to reconstruct how endocranial anatomy changes with ontogeny in a fossil crocodylian for the first time. Our study of D. ratelii has also facilitated the first analysis of the inner ear of a Diplocynodon species, with its morphology showing no clear evidence of a possible pelagic adaptation, at least for this species. Equivalent study of additional specimens of Diplocynodon will be critical in determining whether such features vary interspecifically.