A New Species of Alligatoroid with European Affinities from the earliest Paleocene (Puercan) Ludlow Formation (Fort Union Group) of North Dakota

BOYD, Clint A; North Dakota Geological Survey, Bismarck, ND. HOUSEHOLDER, Mindy L; North Dakota Geological Survey, Bismarck, ND.

Abstract 

Paleogene rocks in North Dakota are rich sources of crocodylian fossils, previously providing the first records of Borealosuchus formidabilis, Wannaganosuchus brachymanus, and Chrysochampsa mlynarskii. While crocodylian fossils from the Bullion Creek and Sentinel Butte Formations of the Fort Union Group have received much attention, little effort has been focused on the collection and study of crocodylian fossils from the earliest Paleocene Ludlow Formation immediately following the Cretaceous-Paleogene extinction. Two crocodylian specimens collected from two different localities within the lower-most portion of the Ludlow Formation in southwestern North Dakota are the focus of this study. While these specimens are generally similar to some species of Borealosuchus, most of their shared traits are plesiomorphic for Crocodylia and all derived traits of the Borealosuchus clade (e.g., lateral curvature of the posterior portion of the maxillary toothrow) are absent in these specimens. These specimens do display several apomorphic traits typically only seen in alligatoroids, including subequal anterior processes of the surangular and an articular foramen aerum that is inset from the medial margin of the retroarticular process. Alternatively, they lack traits typically seen in globidontan alligatoroids such as a plate-shaped intercentrum and modest entry of frontoparietal suture into the supratemporal fenestrae, indicating a basal placement of the Ludlow taxon within Alligatoroidea. Preliminary phylogenetic analysis of these two specimens produced the following results: 1) these specimens are conspecific; 2) this taxon represents a previously undescribed non-globidontan alligatoroid lineage; and, 3) this taxon forms a clade with Diplocynodon remensis from the Paleocene of Europe. Diplocynodon remensis was the only Paleocene species referred to the diverse European taxon Diplocynodon and was previously cited as evidence for the dispersal of the Diplocynodon lineage into Europe prior to the Paleocene-Eocene Thermal Maximum. If correct, the removal of D. remensis from the Diplocynodontinae suggests that D. remensis represents the dispersal of a separate alligatoroid lineage into Europe during the Paleocene, making it uncertain if the Diplocynodon lineage dispersed to Europe before or after the Paleocene-Eocene Thermal Maximum. Further study of the Ludlow taxon will provide important insights into the early history of alligatoroid evolution and biogeographic dispersal.