Knowledge of Late Cretaceous vertebrate faunas in the Western Interior is less than for Tertiary vertebrates. This is particularly true for mammalian faunas of the very poorly known "Edmontonian" age. A new fauna from the Williams Fork Formation, NW Colorado, is the first definitive mammal-producing fauna of this age. The mammals will serve as a basis for a review of species that span the poorly known "Edmontonian" and will also serve as a basis for analyses of higher-level relationships as warranted by the material. Zonations based upon ammonities and pollen argue very strongly that the vertebrates are best regarded as "Edmontonian" in age. Four ammonite zones, Didymoceras cheyennense through Baculites reesidei, bracket the vertebrate faunas, which so far are from the lower half of the Williams Fork Fm. Large microvertebrate samples to be collected over the next three years using screenwashing techniques and heavy liquid separation will more fully document this poorly know faunal interval. Analysis of both sedimentology and stratigraphy (and soon palynology and hopefully magnetostratigraphy) will be helpful in establishing a biostratigraphic framework.
