Revision of the longirostrine crocodylian Thoracosaurus isorhynchus from the Maastrichtian–Danian of northwestern Europe

A highly debated topic in eusuchian systematics has been the placement of “thoracosaurs,” an assemblage of Cretaceous–Paleogene longirostrine crocodylians from Europe and North America. The position of “thoracosaurs” varies from one study to the next, from being the sister taxon of the extant Gavialis to being excluded from the crown group Crocodylia entirely. Despite these issues, “thoracosaurs” have been understudied, with many taxa suffering from incomplete or outdated descriptions and dubious taxonomic assignment. Here, we provide a comprehensive study of Thoracosaurus isorhynchus from Maastrichtian–Danian deposits of northwestern Europe. We present and describe new material from the type locality of T. isorhynchus. In addition, we redescribe and figure the lectotype and paralectotypes of the species, as well as the material from the Maastrichtian type area, the Netherlands. Moreover, comparison of T. isorhynchus with the type material of Thoracosaurus “scanicus” of the Danian of southern Sweden indicates that the latter might be a junior synonym of T. isorhynchus, suggesting the survival of this species across the Cretaceous–Paleogene boundary. An array of phylogenetic analyses points to a close relationship between T. isorhynchus and the Cenomanian Portugalosuchus azenhae. This implies an early emergence of “thoracosaurs” and causes extensive ghost lineages within Gavialoidea, highlighting the stratigraphic inconsistency of this group and a need for a thorough revision of other “thoracosaurs.”