Borealosuchus Brochu 1997 was erected for a group of Late Cretaceous through early Eocene eusuchian crocodyliform species formerly assigned to Leidyosuchus Lambe 1907. Borealosuchus wilsoni was originally described by Mook (1959) based solely on a dorsoventrally crushed skull from the early Eocene (Wasatchian) Green River Formation of Wyoming, and assigned to Leidyosuchus. Later analyses referred specimens from the late Paleocene and middle Miocene to the species (Brochu, 1997). 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. Benton and Clark, 1988; Pol et al., 2009; Turner and Pritchard, 2015). More exact phylogenetic placement of Borealosuchus remains unclear. Borealosuchus wilsoni is stratigraphically the youngest species of Borealosuchus, and one of the most completely known, and as such, it figures prominently in ongoing debates over the phylogenetic relationships and origin of crown group Crocodylia. Given its long stratigraphic range in the literature, the question of whether specimens currently referred to B. wilsoni all pertain to a single species is open.
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 Borealosuchus wilsoni and middle Eocene (Bridgerian) fossils referred to Borealosuchus wilsoni, including material of Diplocynodon stuckeri Mook 1960. 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 190 morphological characters and subjected to phylogenetic analysis. The diversity of Borealosuchus during the Eocene has been underestimated and specimens from the Bridgerian are diagnosable and distinct from those of the preceding Wasatchian stage. A new species will be erected to contain this material. The erection of a new species of Eocene Borealosuchus renders the holotype of D. stuckeri non-diagnostic at the species level, and a new holotype USNM 12990 is designated for Bridgerian Borealosuchus. Some specimens assigned to Brachyuranochampsa eversolei and Crocodylus affinis may also be referable to Borealosuchus. The addition of new material to Borealosuchus increases the diversity of this group during the Eocene.