New alligatorid, Mandrasuchus milleri , from the Early Paleocene (Danian) of the Denver Basin supports earliest Paleocene North American alligatorid radiation

The North American eusuchian fossil record through the Cretaceous–Paleogene (K–Pg) mass extinction shows a remarkable increase in diversity across the K–Pg boundary through the early Eocene. This is in contrast to most other vertebrate clades. Despite the group’s uninterrupted existence across the K–Pg boundary, gaps remain in the alligatorid fossil record, particularly near the K–Pg boundary and geographically in Colorado. Here we describe a new alligatorid, Mandrasuchus milleri, gen. et sp. nov., from the lower Paleocene Denver Formation of Corral Bluffs, Colorado, represented by over 15 specimens. We identify a globidontan alligatorine by the globular caudal dentition and blunted rostrum. Regardless of precise taxonomic assignment, the alligatorid is stratigraphically the earliest confirmed alligatorine, with diagnostic crania from ∼350 kyrs (Puercan 2 North American Land Mammal Age [NALMA]) and referred globular teeth from ∼200 kyrs (Puercan 1 NALMA) post-K–Pg mass extinction. The alligatorine is a new representative of a largely unresolved portion of the crocodylian phylogeny, and its addition could help resolve relationships of early diverging alligatorids and taxa historically referred to Allognathosuchus. This taxon’s distinct rostral morphology and globidont dentition suggest its ability to engage in a macrogeneralist diet. Dietary niche partitioning is likely present in this ecosystem as M. milleri lived alongside the generalist macrocarnivore Borealosuchus sternbergii, the other known Denver Formation crocodylian. The eusuchian survivorship across the K–Pg mass extinction and the morphological diversity of these apex freshwater predators underscores the degree to which freshwater terrestrial ecosystems were buffered from Earth's last mass extinction.