First record of Borealosuchus sternbergii from the lower Paleocene Denver Formation (lower Danian), Colorado (Denver Basin)

The Late Cretaceous through Paleogene fossil record for North American eusuchians is remarkable in that it indicates high levels of survivorship across the Cretaceous–Paleogene (K–Pg) boundary, as well as an increase in diversity through the early Eocene. Despite the clade’s continued existence, gaps remain in its fossil record, particularly near the K–Pg boundary and in Colorado. A eusuchian from the lower Paleocene Denver Formation of Corral Bluffs is represented by specimens identified as Borealosuchus sternbergii. Here, we provide anatomical descriptions and comparisons supported by micro-CT scan data of three partial crania. We conduct a phylogenetic analysis, the results of which are influenced by the potential sub-adult ontogenetic stage of the specimens and species-level assignment is preliminary. The specimens fill both geographic and temporal gaps in the record of the Borealosuchus species complex as the first specimens from the earliest Paleocene of Colorado. These additions highlight the uncertainty in phylogenetic relationships among the Borealosuchus species complex, the importance of taking ontogenetic stage into account when assessing such relationships, and help constrain biogeographic dispersal patterns and ecological niche occupation of the clade.