The endocranial anatomy of ‘thoracosaur’ crocodyliforms: implications for their ecology and phylogenetic affinities

Abstract

‘Thoracosaurs’ are Late Cretaceous to early Palaeogene longirostrine eusuchian crocodyliforms from coastal marine deposits in Europe and North America. Their phylogenetic placement is debated; whereas most morphological studies place them within Gavialidae, implying extensive ghost lineages inconsistent with molecular‐based divergence estimates, analyses applying tip‐dated Bayesian inference in a total evidence framework retrieve thoracosaurs outside of Crocodylia. Recent advances in computed tomography (CT) scanning of eusuchian skulls have allowed exploration of endocranial anatomy, an additional morphological resource that could contribute to resolving conflicting phylogenetic hypotheses, and shed light on their ecology. Here, we use CT scanning to reconstruct the endocranial cavities of three thoracosaur skulls: Thoracosaurus isorhynchus from the Maastrichtian of France and Danian of Sweden, and Eothoracosaurus mississippiensis from the Maastrichtian of the USA. Dorsal expansions of the olfactory region within the lacrimals and prefrontals, enlarged semicircular canals of the inner ear, and the absence of a bony wall between the vestibule and the encephalic endocast support a marine ecology for thoracosaurs, consistent with their depositional record and biogeographic distribution. Expansive nasopharyngeal ducts indicate that thoracosaurs were probably incapable of deep diving; instead, they may have used them for acoustic signalling. Comparison of endocranial features of thoracosaurs to extinct and extant crocodyliforms suggests that many of these characteristics are affected by longirostry and/or ecological adaptation. Our results do not clearly support either phylogenetic hypothesis for the position of ‘thoracosaurs’. Rather, our study highlights the need for increased taxon sampling to clarify the phylogenetic distribution of endocranial features across Crocodyliformes.