First Record of Stygimys (Mammalia, Multituberculata, Eucosmodontidae) from the Paleocene (Puercan) Part of the North Horn Formation, Utah, and a Review of the Genus

A single tooth referred to Stygimys kuszmauli recovered at the Gas Tank Hill locality from Puercan strata of the North Horn Formation represents the first record of Stygimys from Utah. S. kuszmauli now occurs in both proposed north–south late Puercan biogeographic provinces, which reduces statistical support for provinciality. A new species from Texas, Stygimys vastus, is described. Of the five species previously referred to Stygimys, S. kuszmauli, Stygimys camptorhiza, Stygimys jepseni, and Stygimys teilhardi are valid, but Stygimys cupressus is synonymous with S. kuszmauli. The type of S. jepseni is restricted to a single p4 as the m1 thought to be from the same individual represents another multituberculate genus. Five sites, including Gas Tank Hill that comprise the Gas Tank Hill Local Fauna are widely dispersed in slumped strata which may differ in age, are located in a mostly floodplain depositional setting, and are small surface collected samples. Therefore, biogeographic analyses that compare the Gas Tank Hill Local Fauna to large samples collected from channel fills using screenwash techniques should be employed with caution. Stygimys has a relatively broad geographic and geochronologic distribution, but it is usually rare in Late Cretaceous and Early Paleocene mammalian assemblages. The recent recovery of Stygimys after more than 60 years of collecting in the North Horn Formation suggests that further sampling in other non-marine basins in Western North America will eventually yield additional specimens of the genus.