The genus Ischyromys in the Brule Formation, Oligocene, Nebraska, appears to be monophyletic and ranges from the lower Orella Member into the basal Whitney Member. The Whitney specimen is the first to be recovered from unreworked sediments in that member in Nebraska. Changes in the lineage occurred with time but appear to have been gradual, and speciation is facilitated by the presence of breaks in the geologic record and unfossiliferous strata. These breaks occurred at well-defined stratigraphic markers known as fossil soils or paleosols which divide Orella Member iinto lower, middle, and upper. Ischyromys parvidens seems to be represented in the lower, I. typus in the middle and I. pliacus in the upper Orella. A wide range of variation is evident in the materials from each level. Population studies show recognizable differences between the three species.

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