Microcosmodon

Abstract: A low diversity of multituberculates in the Clarkforkian (late Paleocene-early Eocene) of North America is documented for the first time. Included in the Clarkforkian fauna of the Bighorn Basin, northwestern Wyoming, are Prochetodon cf. P. cavus, Ectypodus powelli, Parectypodus lay- toni, Microcosmodon rosei n. sp., and Neoliotomus conventus. Ectypodus, Parectypodus, and Neo- liotomus were known previously from both older and younger horizons and Microcosmodon only from the Tiffanian. Previous records of Prochetodon from the Wasatchian are almost certainly in- correct; thus, the latest known records of both Prochetodon and Microcosmodon are from the Clark- forkian. A juvenile specimen of Ectypodus powelli from the Bighorn Basin provides the only known as- sociated upper and lower dentitions for the species and provides additional information regarding the deciduous dentition and sequence of tooth replacement in multituberculate mammals. The inferred sequence of tooth eruption in the lower dentition of E. powelli, a species that lacks P3, is dI1-P4-M,- II-M2. Clarkforkian multituberculates remain poorly known from outside the Bighorn Basin. Tentative identifications include ?Prochetodon sp. from Buckman Hollow, west-central Wyoming; Ectypodus sp., Microcosmodon rosei?, and ?Neoliotomus sp. from Bitter Creek, southern Wyoming; and Ptil- odus sp. from Plateau Valley, west-central Colorado. The latter is the first Clarkforkian record of the genus Ptilodus. A dramatic reduction in multituberculate species diversity is reflected in Clarkforkian mammalian faunas when compared with those from the Tiffanian. The decline in relative abundance of multi- tuberculates appears to have occurred in the late Tiffanian.