VARIATION AND ITS IMPLICATIONS IN A POPULATION OF CUPIDINIMUS (HETEROMYIDAE) FROM HEPBURNS MESA, MONTANA

New specimens of the heteromyid genus Cupidinimus have been found at t he Barstovian (middle Miocene) Hepburn's Mesa site in the Yellowstone Valley, southwestern Montana. The population is not placed within a ne w or existing species because the high coefficients of variation of se veral dental measurements indicate the presence of more than one speci es. However, lack of any clear character differentiation (qualitative or quantitative) within the population makes it impossible to distingu ish separate species. Possible explanations proposed for this high var iation and lack of specific diagnosibility are: morphologically simila r species (including the products of a recent speciation event or intr ogression); sexual dimorphism; geographic variation; and temporal mixi ng. The age, location, and size of the surrounding Cupidinimus taxa, t he lack of any size differentiation of the population into two species , and the geological history of the area suggest that the variation in the Hepburn's Mesa Cupidinimus population might be the result of the presence of multiple species or forms during a speciation event.