NEW ISCHYROMYIDS (RODENTIA) FROM THE BRIDGERIAN (MIDDLE EOCENE) OF WYOMING: CLARIFIED SPECIES BOUNDARIES FOR PARAMYS, UINTAPARAMYS, AND THISBEMYS

Ischyromyids are a diverse group of primitive rodents known from the middle Eocene of North America. Although ischyromyids are relatively easy to distinguish from other mammals collected from the same assemblage, species level classification can be challenging. This is especially true for specimens recovered from the Bridger Formation, of the Green River Basin, Wyoming, where several ischyromyid species of similar size and morphology are found to co-occur. Six of these species which have been confused with one another in the past are: Paramys delicatior, P. delicatus, Thisbemys plicatus, T. corrugatus, Uintaparamys parvus, and U. bridgerensis. Species such as Uintaparamys parvus and U. bridgerensis are very similar morphologically and known to vary in size over time which led some workers to delimit these species using stratigraphic boundaries. Later workers followed the same practice for other similar sized, contemporaneous ischyromyid species such as Thisbemys plicatus and T. corrugatus as well as Paramys delicatior and P. delicatus. Currently, when two or more of any of the aforementioned species are found together, it is difficult to sort them out using morphology alone. Newly discovered ischyromyid fossils from the Green River Basin, specimens collected with more detailed stratigraphic data, make it possible to complete a revision for these particular species. Because ischyromyids were abundant and geographically widespread during the Eocene of North America, they are key taxa useful for comparing mammalian faunas from distinct depositional environments. Results of this study include data essential to further our understanding of middle Eocene rodent evolution including the interplay between morphological traits, size, and depositional environment.