Evidence of late root formation of molars in Anderson’s red-backed vole, Eothenomysandersoni (Thomas, 1905) (Cricetidae, Rodentia), and arguments for its generic allocation

We evaluated the molars in Anderson’s red-backed vole (n = 114) from the Kii Peninsula of Honshu, Japan. Two of the specimens are considered extremely old aged based on their dimensions and on the loss of alveolar capsules of M(2), and a third one is also old based on its strongly worn left M(3) and M(1). Of the former two individuals, one showed an incipient closure of re-entrant angles at its basal end, as estimated from the difference between the occlusal patterns of the occlusal and basal surfaces of the left M(2). The latter individual also showed a complete closure of the basal end in the left M(3). These patterns differ from incipient roots observed in other vole taxa but were similar to a previous example of incipient roots in Anderson’s red-backed vole. Therefore, we suggest that molar roots in this species form at an extremely late age or by strong wear. Root formation in molars is considered an important diagnostic character, as Eothenomys molars lack roots, while Craseomys molars develop roots at a late age. However, this dental character may be particularly difficult to assess in voles under natural conditions. Considering previous phylogenetic findings based on molecular analyses, Craseomys is the most appropriate genus for Anderson’s and other Asiatic red-backed voles.