Intraspecific versus interspecific variation in Miocene Great Basin mylagaulids: implications for systematics and evolutionary history

The Mylagaulidae are a family of burrowing rodents abundant in Miocene faunas from western North America. Recent taxonomic revisions of mylagaulids from the Great Plains suggest that their systematics may be best understood on a regional basis. Previous studies addressed the taxonomy and evolutionary history of mylagaulids from the Great Basin, but recent discoveries of specimens, new phylogenetic data, and more detailed stratigraphical information necessitate a thorough reanalysis of their relationships and occurrences. We present a revision of the systematics of the mylagaulids from the Great Basin. In addition to rare large mylagaulids of uncertain taxonomic affinity, we recognize four species of mylagaulids distributed throughout Oregon and Nevada from the late Hemingfordian through to the early late Hemphillian: Alphagaulus vetus, Hesperogaulus gazini, Hesperogaulus wilsoni, and a new species from the genus Hesperogaulus. All species are known from large sample sizes of isolated premolars, allowing consideration of ontogenetic variation in determining the key morphological differences that allow recognition of different species. Although the number of enamel lakes varies within a given taxon, the presence of some of these lakes is taxonomically significant. This result emphasizes the importance of understanding ontogeny in describing species of fossil hypsodont mammals.