Variation and its implications in a population ofCupidinimus(Heteromyidae) from Hepburn's Mesa, Montana

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Abstract
New specimens of the heteromyid genus Cupidinimus have been found at the Barstovian (middle Miocene) Hepburn's Mesa site in the Yellowstone Valley, southwestern Montana. The population is not placed within a new or existing species because the high coefficients of variation of several dental measurements indicate the presence of more than one species. However, lack of any clear character differentiation (qualitative or quantitative) within the population makes it impossible to distinguish separate species. Possible explanations proposed for this high variation and lack of specific diagnosibility are: morphologically similar species (including the products of a recent speciation event or introgression); sexual dimorphism; geographic variation; and temporal mixing. The age, location, and size of the surrounding Cupidinimus taxa, the 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.