A REVIEW OF THE PALEOGENE EUSUCHIAN CROCODYLIFORM BOREALOSUCHUS WILSONI BROCHU 1997 FROM WESTERN NORTH AMERICA

Borealosuchus Brochu, 1997 was erected for a group of Late Cretaceous through middleocene eusuchian crocodyliforms from North America formerly assigned to Leidyosuchus Lambe 1907. In phylogenetic analyses, Borealosuchus has been retrieved as either a basal crocodylian (e.g., Salisbury and Willis, 1996; Brochu, 1997; Wu et al., 2001; Buscalioni et al., 2011; Brochu et al., 2012; Narváez et al., 2016) or a close crocodylian outgroup (e.g. Pol et al., 2009; Turner and Pritchard, 2015). Its phylogenetic placement, and the large amount of well-preserved material for several species, gives Borealosuchus a central role in the resolution of ongoing crocodylian phylogenetic disputes.

Borealosuchus wilsoni (Mook 1959) was originally based on a dorsoventrally crushed skull from the early Eocene (Wasatchian) Green River Formation of Wyoming, and referred to Leidyosuchus. Specimens from several sites, ranging in age from late Paleocene to middle Eocene, have subsequently been referred to B. wilsoni, including Diplocynodon stuckeri Mook 1960, from the middle Eocene (Bridgerian) of Wyoming. B. wilsoni is stratigraphically the youngest species of Borealosuchus, and as such, it figures prominently in our understanding of crocodylian response to climate change near the end of the Eocene. However, whether all of the specimens referred to B. wilsoni pertain to a single species is an open question.

This study focuses on a more thorough description of the skeletal morphology of Eocene Borealosuchus and a revision of the taxonomy of this assemblage, including the conspecific nature of Wasatchian B. wilsoni and referred Bridgerian B. wilsoni, including Diplocynodon stuckeri. A phylogenetic re-assessment of B. wilsoni will hopefully provide further resolution within Borealosuchus and among other closely related taxa.

Specimens were coded with a matrix of 189 morphological characters and subjected to maximum parsimony analysis. This study supports species-level separation of Wasatchian and Bridgerian B. wilsoni. Specimens from the Bridgerian are diagnosable and distinct from those of the preceding Wasatchian stage. Some specimens referred to constratigraphic taxa (e.g., Brachyuranochampsa eversolei and Crocodylus affinis) may also be referable to Borealosuchus, and would increase the known diversity of the lineage.