SYSTEMATICS OF EOCENE ZIPHODONT CROCODYLIFORMS OF THE NORTH AMERICAN WESTERN INTERIOR

Eusuchian crocodyliforms with flattened, serrated teeth (the ziphodont condition), laterally compressed snouts, and hooflike terminal phalanges are known from several Paleogene localities in western North America and Eurasia. They are often perceived as less aquatic than their living relatives. These are usually referred to Pristichampsus Gervais 1853, but questions about the diagnosability and relationships of North American and Eurasian forms complicate our understanding of their systematics. Fragmentary fossils establish the presence of the lineage in North America as far back as the Paleocene, but well-preserved material first appears in the Middle Eocene, where at least two species are represented. The best known is P. vorax from the Bridgerian of Wyoming. Younger Uintan fossils from two units in Texas – the Devil’s Graveyard Formation in the Big Bend region and the Laredo Formation from southern Texas – can also be distinguished from P. vorax and represent at least one distinct species, though whether they are conspecific with each other is unclear. They share derived states with contemporary material from Germany not found in P. vorax, though a close phylogenetic relationship between them remains tenuous because of the fragmentary nature of the Texas specimens. Pristichampus has also been reported from the Uintan of California and Oregon; at least some of the Oregon material is actually from a crocodyloid, and the California specimens are fragmentary and difficult to assess, but Pristichampsus nonetheless appears to have been widespread throughout western North America in the Uintan. Its absence from younger deposits suggests extinction as climatic conditions moved away from the environmental tolerances of living crocodylians in the later Middle Eocene.