Additional specimens of the problematical rodent PipestoneomysDonahoe, 1956, have allowed for recognition of a new family, Pipestoneomyidae. A new genus and species of pipestoneomyid is recognized from the late middle Eocene (Duchesnean North American Land Mammal Age; Bartonian), Argorheomys septendrionalis, which is morphologically more primitive than Pipestoneomys and demonstrates that this new family has been distinct since the Duchesnean. The Pipestoneomyidae share a number of derived characters with the Geomorpha, especially the two-part inner layer of incisor enamel of the Eoymidae. The Pipestoneomyidae differ from the Eomyidae in lacking the basic “omega” pattern of the cheek teeth of the former, so are in the Eomyoidea as a distinct family.

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