Borealosuchus is a significant genus as it is considered either a basal crocodylian or close outgroup. A new species identified from the early Campanian Mooreville Chalk of Alabama is one of the oldest known crocodylians and extends the stratigraphic range of Borealosuchus from the Maastrichtian minimally into the early Campanian. It co-occurs with the giant alligatoroid Deinosuchus in deposits formed in a marginal marine setting with fluvial input. Its age, along with the phylogenetic position of Borealosuchus, renders this material critical for understanding the morphological conditions and relationships at the root of Crocodylia. The most complete specimen consists of partial cranial, mandibular, and postcranial remains preserving a unique combination of characters including a short dentary symphysis, splenial participation in the symphysis, lack of discrete concavity on the angular dorsal margin for an external mandibular fenestra, and bipartite ventral osteoderms. The external mandibular fenestra was either absent or very small, raising questions about the ancestral condition for this structure in Crocodylia. The new species is, surprisingly, closer phylogenetically to more derived species of Borealosuchus, including B. wilsoni, B. threeensis, and B. acutidentatus, than to B. sternbergii and B. formidabilis. This creates many range extensions and ghost lineages in the clade and suggests substantial unsampled diversity within Borealosuchus. Additional mandibular material from the Santonian Eutaw Formation may be referable to this new species, further increasing its stratigraphic range. A specimen consisting of a partial post-cranial skeleton from the Mooreville Chalk is significantly larger than the holotype of the new species, and is referable to Borealosuchus sp., but is too incomplete for more precise referral. These newly described specimens provide additional information about the faunal assemblages of Alabama during the Late Cretaceous and have broader implications because they are some of the earliest crocodylian material known. Indeed, the Eutaw material appears to be the oldest known occurrence of Crocodylia.

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.

The Eocene Green River and Bridger Formations of Wyoming represent lacustrine and fl uvial environments noteworthy for an extremely diverse crocodylian fauna (at least eight species in seven genera). This paper discusses a fragmentary crocodylian jaw from the Bridger Formation, and also notes possible ecological partitioning among these sympatric crocodylians. The jaw fragment can be assigned confi dently to Borealosuchus based on the exclusion of the splenial from the mandibular symphysis and the presence of occlusal grooves between the alveoli, and it is referred tentatively to Borealosuchus cf. B. wilsoni. To examine the paleoecology of these crocodylians, variables based on habitat, body size, and inferred diet were formulated and species placed within respective categories. The research found that while there were more sympatric crocodylians in the early to mid Eocene of Wyoming than in any present-day biota, direct interspecifi c competition for resources is presumed to have been relatively low.