A new Late Cretaceous gavialoid crocodylian from eastern North America and the phylogenetic relationships of thoracosaurs

Abstract

Eothoracosaurus mississippiensis, gen. et sp. nov., is based on a skull and partial skeleton from the Upper Cretaceous (early Maastrichtian) Ripley Formation of Mississippi. Less complete material from the late Campanian or early Maastrichtian of western Tennessee is referred to this taxon. It can be distinguished from late Maastrichtian Thoracosaurus neocesariensis on the basis of a wider distance between the supratemporal fenestrae, a long anterior frontal process, and closer apposition of the third and fourth dentary alveoli. Longirostrine crocodylian remains from the later Maastrichtian and earliest Paleocene of New Jersey pertain to a single species (Thoracosaurus neocesariensis), as do remains from the early Paleocene of France and Sweden (Thoracosaurus macrorhynchus). The basisphenoid in these animals is still an anteroposteriorly thin lamina wedged between the basioccipital and pterygoid, reflecting the “verticalized” condition seen in extant non-gavialoid crocodylians. At least some late Paleocene occurrences from New Jersey, Maryland, and Virginia can be referred to Thecachampsoides minor (Marsh, 1870), and a second larger gavialoid may also be present in these units. North American “thoracosaurs” lacked antorbital fenestrae. “Thoracosaurinae” are a para-phyletic grade at the base of Gavialoidea, with Thecachampsoides being closer to Gavialis than are Thoracosaurus or Eothoracosaurus. Prior referral of “thoracosaurs” to Tomistominae reflects a typological approach to taxonomy, with longirostrine crocodylians maintaining plesiomorphic rostral states being regarded as tomistomines.