Notes on the classification and evolution of some North American fossilMicrotus(Mammalia; Rodentia)

Abstract

Students of fossil arvicoline rodents are challenged to conform to a higher level of modern statistical and phylogenetic analysis. The terminology of the occlusal surface of arvicoline teeth of van der Meulen (1973, 1978) is proposed as the standard for future research. The current taxonomy of North American fossil voles referred to Allophaiomys, Pedomys, and Pitymys is reviewed and critiqued. These taxa are combined in the genus Microtus. Phaiomys (= Allophaiomys; = Orthriomys) and Pitymys (= Pedomys; = Herpetomys) are recognized as subgenera. A Pleistocene replacement chronology for Microtus dental morphotypes is presented, and a cladogram based on dental character states is constructed for North American Microtus. Microtus meadensis is considered to be an Old World immigrant and not closely related to M. quasiater. The Late Pleistocene Microtus reported by Repenning (1983) from El Tajo de Texquixquiac, Mexico as M. meadensis is tentatively referred to M. quasiater. Microtus quasiater is further considered to be distinct from M. pinetorum. Microtus (Pitymys) mcnowni is not a valid species and is tentatively combined under M. ochrogaster. Microtus (Pitymys) aratai is a valid species. Pleistocene specimens reported by Repenning (1983) from Centerville Beach, California now lack proper species reference. Microtus oaxacensis and M. guatemalensis, here classified in Pitymys, demonstrate both confluent T4–5 and closed T4–5 on m1, thus providing a possible evolutionary scenario deriving some species of the subgenus Microtus from Pitymys. This scenario is contrasted with that of Guthrie (1965), in which Microtus is descended from a species such as M. paroperarius, which has predominantly four closed triangles on m1.