Phenacomyine. Apart from Arborimus, nearest generic kin uncertain—placed as Arvicolinae incertae sedis (Chaline et al., 1999; Gromov and Polyakov, 1977); or with Phenacomyini, including Arborimus (Zagorodnyuk, 1990); Arvicolini, including Phaiomys and certain extinct genera (Repenning et al., 1990); or Myodini (McKenna and Bell, 1997). The rooted molars and lack of cement in reentrant angles are plesiomorphic traits that suggest an early differentiation of Phenacomys within the arvicoline radiation, and paleontologists have proposed its origin from a lineage of Beringian Mimomys in the early Pliocene (Repenning and Grady, 1988; Repenning et al. 1987). Early cladistic separation is also suggested by phylogenetic analysis of highly repetitive DNA (LINE-1) elements, in which Phenacomys forms an unresolved basal trichotomy with Dicrostonyx and a third branch subtending seven other genera surveyed (Modi, 1996). Revised by Howell (1926) and Hall and Cockrum (1953), then including species assigned to Arborimus (see that account).
We acknowledge phenacomyine in an informal sense because Phenacomyini, as coined by Zagorodnyuk (1990:27), is a nomen nudum and unavailable. Zagorodnyuk used the formal name in a paragraph listing of arvicoline tribes and member genera, without indicating its status as new and lacking any statement of differentiation. In view of the old and pronounced phyletic separation of Phenacomys suggested by the above studies, taxonomic sampling should be broadened and the family-group clade properly named and diagnosed should such a conclusion prove sustainable.