Endocranial anatomy of the earliest Cretaceous European neosuchian crocodyliform Pholidosaurus purbeckensis provides new evidence for the ecological evolution of Pholidosauridae

Abstract

The neosuchian crocodyliform clade Pholidosauridae had a near-cosmopolitan distribution, spanning the Late Jurassic to the early Paleocene. Representatives of the group inhabited aquatic environments, ranging from freshwater to potentially fully marine forms. The phylogenetic placement of Pholidosauridae within Neosuchia remains debated: whereas most analyses place it as closely related to Dyrosauridae, some studies argue for a closer relationship with Goniopholididae. One skeletal region that could shed light on both the phylogenetic position of Pholidosauridae, as well as how it achieved its broad distribution, is the internal cranial anatomy, which has been shown to document morphological features with both an ecological and phylogenetic signal in other crocodyliforms. However, a natural endocast is currently the only available information on the internal cranial anatomy of a pholidosaurid. Here, we present new insights into the internal cranial anatomy of Pholidosaurus purbeckensis, based on CT-scan data of material from the lowermost Cretaceous Purbeck Limestone Group, southern UK. Overall, the endocranial anatomy of P. purbeckensis is more similar to that of goniopholidids than dyrosaurids, especially the morphology of the olfactory tract and the cerebrum, which might represent a phylogenetic rather than ecological signal. However, this might merely reflect retention of the ‘standard’ crocodyliform skull and endocranial shape in pholidosaurids and goniopholids, rather than necessarily a close relationship, with the cranial anatomy of dyrosaurids instead representing an apomorphic departure from this morphology. We identify paired dorsolateral expansions in the olfactory region of the skull of P. purbeckensis, which have been interpreted as osteological correlates of nasal salt glands in some marine thalattosuchians, dyrosaurids, and extinct gavialoid crocodylians. If this interpretation is correct, it would suggest a higher tolerance for saltwater than previously hypothesised in Pholidosaurus, which would provide support for oceanic capabilities early in the evolution of Pholidosauridae that potentially enabled the group's near-cosmopolitan distribution. Finally, we demonstrate external cranial anatomical variation amongst specimens attributed to P. purbeckensis, particularly in contemporaneous French remains provisionally referred to this species. However, this might best be regarded as individual variation: we therefore tentatively support the attribution of this material to P. purbeckensis pending the much-needed revision of the type species of Pholidosaurus, P. schaumburgensis, from the earliest Cretaceous of Germany.