Microtus umbrosus Merriam 1898

Microtus umbrosus Merriam 1898, Proc. Biol. Soc. Wash., 12: 107.

Type Locality: México, Oaxaca, Mt Zempoaltépec, 8200 ft (2500 m).

Vernacular Names: Zempoaltepec Vole.

Distribution: Cerro Zempoaltépec, 1800-3000 m, NC Oaxaca, México.

Conservation: IUCN – Lower Risk (lc).

Discussion: Type species and sole member of Orthriomys, a taxon typically identified as a subgenus of Microtus (Anderson, 1959; Bailey, 1900; Hall and Cockrum, 1953; R. A. Martin, 1995; Zagorodnyuk, 1990). Nonbanded karyotype reported by Cervantes et al. (1994), who noted its agreement with the complement hypothesized as ancestral for Arvicolinae . Viewed as cladistically basal to other North American pitymyines (subgenera Herpetomys, Pedomys, and Pitymys) based on phylogenetic study of molar traits (R. A. Martin, 1995); more removed from pitymyine species according to mitochrondrial gene sequences and weakly associated with M. chrotorrhinus (Conroy et al., 2001) . Conroy et al. (2001) suggested that M. umbrosus represents an independent invasion of southern latitudes, the other consisting of species related to M. pinetorum, subgenus Pitymys ( M. guatemalensis, M. oaxacensis, M. quasiater). See Frey and Cervantes (1997, Mammalian Species, 555).