Over the last two decades, mammalian faunas that bracket the Torrejonian–Tiffanian transition have been reported by others from Wyoming’s Hanna, Carbon, and Bighorn basins and are important for assessing mammalian diversity and evolution during the middle Paleocene. Here, we describe a new early Tiffanian (Ti1) fauna, and the first fossil mammals from the Overland Member of the Fort Union Formation in the Great Divide Basin, south-central Wyoming. The sample of the fauna, which is primarily composed of isolated teeth, includes at least 18 species in 11 families and five mammalian orders. With exception of Pantolambda, the faunal composition is skewed toward smaller-bodied mammalian species, probably because the majority of specimens were collected from anthills. The taxa identified from the localities in the basal Overland Member include: the pantodont Pantolambda cavirictum; ‘condylarths’ Mimotricentes sp., Haplaletes disceptatrix, Haplaletes pelicatus, Litomylus dissentaneus, Litaletes disjunctus, Promioclaenus acolytus, Promioclaenus sp., Phenacodus sp., and Ectocion cf. E. collinus; cimolestids Acmeodon hyoni and Gelastops sp.; pantolestids Paleotomus sp. cf. P. milleri and Paleotomus senior as well as Bessoecetor sp.; leptictids Prodiacodon concordiarcensis and Prodiacodon puercensis; and the primates Nannodectes intermedius, Torrejonia sirokyi, and Paromomys depressidens.
Like The Breaks local fauna from the Hanna Basin, the Overland fauna contains multiple taxa characteristic of the late Torrejonian, including Acmeodon hyoni, Prodiacodon puercensis, and Litaletes disjunctus. However, it also contains the plesiadapid Nannodectes intermedius, an index taxon for the earliest Tiffanian (Ti1), and Torrejonia sirokyi, Haplaletes pelicatus, Ectocion cf. E. collinus, and Paleotomus senior, which are restricted elsewhere to the Tiffanian. Plesiadapis praecursor, the index species on which Ti1 is defined, has not yet been recovered from the Overland localities. Nevertheless, based upon presence of N. intermedius and the other Tiffanian taxa, we assign the basal Overland fauna an earliest Tiffanian (Ti1) age. Temporal range extensions include Haplaletes pelicatus, whose range is extended from Ti2–Ti3 back into Ti1, while Acmeodon hyoni and Paleotomus cf. P. milleri is from To3 to Ti1. In sum, the basal Overland fauna confirms the presence of earliest Tiffanian in the Great Divide Basin.