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A new latest Paleocene mammal fauna from the Great Divide Basin in southern Wyoming is described and compared with fossil assemblages of similar age elsewhere in Wyoming. The Twelvemile Gulch local fauna is currently documented by 182 mammalian (127 identifiable) specimens from two localities representing 10 orders, 18 families, and 22 species of mammals, including Phenacolemur cavatus, new species. Several characteristic taxa indicate a Clarkforkian age, but some taxa that co-occur at Twelvemile Gulch show disjunct stratigraphic ranges in the well-documented stratigraphic sequence exposed in the Clarks Fork Basin of northern Wyoming. Anachronistic occurrences of Clarkforkian taxa in northern and southern Wyoming have now been documented repeatedly. These unexpected faunal associations likely reflect changing climates and associated taxon-specific range shifts across a latitudinal gradient in the Rocky Mountain Interior. The apparently asynchronous first and last appearances of certain taxa across this latitudinal gradient highlight the utility of immigrant clades over endemic taxa in biostratigraphy. A previous biozonation scheme for the Clarkforkian of the Clarks Fork Basin emphasized endemic Plesiadapis cookei and Copecion as index taxa for Clarkforkian biozones Cf-2 and Cf-3, respectively. However, the applicability of this zonation to Clarkforkian faunas from other parts of Wyoming has been problematic because Plesiadapis cookei occurs in all of them, despite substantial evidence for age disparity among these faunas. A revised biozonation for the Clarkforkian leverages first appearances of invasive Coryphodontidae and Miacidae to discriminate later Clarkforkian faunas including Twelvemile Gulch from earlier Clarkforkian faunas such as Big Multi Quarry.