Crocodylian diversity during the early Eocene climatic optimum in the Golden Valley Formation of North Dakota, U.S.A.

Rich bone beds from the lower Eocene strata of the Golden Valley Formation of Stark County, North Dakota reveal a speciose sympatric crocodylian fauna. However, analyses demonstrate limited phylogenetic diversity among these co-occurring taxa, and of the four species known for the locality, three are alligatorids and one is a crocodyloid. Phylogenetic hypotheses recover <i>Chrysochampsa mylnarskii</i> as a late lived member of Brachychampsini—a stem-based clade including <i>Brachychampsa montana</i> and all alligatorids more closely related to it than to <i>Caiman crocodilus</i> or <i>Alligator mississippiensis</i>—and a new genus and species, <i>Ahdeskatanka russlanddeutsche</i> groups with species of <i>Allognathosuchus</i>. The crocodylians, partitioned by body size and plan, would have occupied an array of ecological niches and feeding strategies. Whereas the large-bodied alligatorid <i>Chrysochampsa mylnarskii</i> preserves a generalist morphology, <i>Ahdeskatanka russlanddeutsche</i> bears a short, broad snout and globular distal teeth. Contemporaneous with a peak in alligatoroid diversity during this interval, <i>Ahdeskatanka russlanddeutsche</i> is an exemplar of a radiation of small-bodied alligatorids with crushing dentition and preserves the ancestral alligatorid feeding strategy. Trophic dynamics of the locality diverge from modern environments and the abundant crocodylians may have filled the ecological niche of large mammalian carnivores conspicuously absent here. This alligatorid-rich crocodylian fauna evolved in swampy lowlands and meandering streams flanked by subtropical forests during one of the hottest sustained intervals in Earth history. The lush, highly productive ecosystems preserved in the Golden Valley Formation inform the evolutionary history of North American alligatorids and preserve significant biodiversity following the Paleocene–Eocene Thermal Maximum.