A New Genus and Species of Fossil Kangaroo Rat and Its Burrow

A fragmentary skeleton of a heteromyid found in a burrow in the Ash Hollow formation (late Clarendonian) in northeastern Nebraska is the earliest record of a rodent known to have the following combination of “desert-adapted” or “steppe-adapted” features: a) greatly reduced forelimbs and elongated hind limbs; b) markedly inflated auditory bullae; c) well-developed dentine tracts on the cheekteeth. In both a) and b) the specimen (described as a new genus and species) is intermediate between perognathines and advanced dipodomyines but is much closer to the latter. In its possession of dentine tracts the new form is as advanced as living Dipodomys, but its un-grooved upper incisors, rooted cheekteeth, and elongate P4 readily distinguish it from all known species of the modern genus. The cheekteeth of the new kangaroo rat closely resemble those of certain late Tertiary geomyines (Pliogeomys and Parapliosaccomys) but cranial and mandibular features as well as the shape of the incisors show that the resemblance is superficial. The presence of an anatomically precocious dipodomyine in the Clarendonian indicates that the Hemphillian-Blancan genus Prodipodomys, which is less advanced dentally than the new form, may be an aberrant side-branch of the Dipodomys lineage. Paleo-ecologically, the presence of a “desert-adapted” rodent in the presumably moist lowland savannahs of eastern Nebraska in the late Tertiary is puzzling. Modern Dipodomys populations sometimes colonize accumulations of bare sand along streams; perhaps this was the habitat in which typical dipodomyine characters first evolved.