Extensive outcrops in the badlands of Makoshika State Park near Glendive, Montana, and surrounding areas expose a continuous sequence of fossiliferous, fluviodeltaic sedimentary rock spanning both sides of the Cretaceous-Tertiary (K/T) boundary. This fortuitous circumstance has enabled us to document change in the composition of mammalian faunas during this time, to relate observed patterns to palynomorph and molluscan changes in composition, and to test hypotheses of faunal turnover at the K/T boundary. Of particular interest is the appearance and diversification of archaic ungulate mammals, since simultaneous dinosaur extinction along with ungulate radiation has been invoked in models of gradual faunal change at the K/T boundary. Fossil mammals have been recovered from six localities in and near Makoshika State Park. The known samples represent three successive local faunas. Composition of the Muddy Tork Local Fauna from the Hell Creek Formation (Upper Cretaceous) resembles those of the Lancian North American Land Mammal "Age" (NALMA) from elsewhere in Montana and Wyoming. Composition of the Hiatt Local Fauna recovered from lower parts of the Ludlow Member of the Fort Union Formation (Paleocene) resembles faunas of the Puercan NALMA at northern latitudes. The diverse assemblage of archaic ungulates present in the Hiatt Local Fauna in conjunction with negative evidence (absence of the multituberculate Taeniolabis taoensis, the appearance of which marks the advent of the late Puercan) suggests that the Hiatt Local Fauna is no younger than middle Puercan (Pu2). The School Well Local Fauna (tentatively, Torrejonian NALMA) occurs higher in the Ludlow Member of the Fort Union Formation. Strata yielding typical Late Cretaceous mammals are separated stratigraphically from those yielding early Paleocene mammals by about 20 m. The K/T boundary in Makoshika State Park has been recognized in more detail on the basis of palynomorphs. Pollen studies have led to preliminary identification of a fern spike, noted elsewhere by others immediately above the K/T boundary. Lack of any type of Bug Creek vertebrate assemblage suggests that the controversial "Bugcreekian" biochron (Pu0) cannot be extrapolated to this area. Evolutionary radiation of archaic ungulates may not have begun until after dinosaur extinction.