New Aplodontid Rodent from Montana Oligocene

Abstract

The Chicago Natural History Museum collection of Pipestone Springs Oligocene material contains two jaw fragments, an upper and a lower, originally collected by Dr. E. C. Olson and party in 1941, that represent a new form of rodent. It seems best to describe this form on the basis of these two specimens because the materials from Pipestone Springs are usually fragmentary and the deposits have been sufficiently investigated and reported upon so that it is unlikely that better materials will be soon available. Although the upper molars of the Pipestone specimen lack the prominent mesostyles that are generally characteristic of the Aplodontidae, the form is assigned to that family on the basis of general tooth structure and tooth relationships. Most of the peculiarities of the dental pattern in this new form are shared with one or another of the early Tertiary genera of aplodontids. The general tooth shape, the posteriorly diminishing tooth row, and the cross-crests and apical basins of the upper molars are all characteristic of the Aplodontidae. The discovery of an aplodontid in the Montana Oligocene extends our knowledge of the group in two respects. Aplodontids have been described from the Late Eocene and the Early Miocene, but no Oligocene forms have been previously reported. All American records of the family have been from the Pacific coastal states and Nevada. I wish to thank Bryan Patterson of the Chicago Natural History Museum for permission to study the specimens and Dr. E. C. Olson of the University of Chicago for advice and direction. Diagnosis .—Cheek teeth high crowned, rooted, lacking prominent styles or stylids. M1 and M2 approximately equal size, quadrangular; M3 smaller, subcircular. Hypocone and protocone of M1 and M2 subequal, separated by a deep persistent groove. Deep groove separating anteroloph and paracone. …