Zempoaltépec vole
[2] Microtus umbrosus is limited to a semi-isolated mountain range southeast of the Cajones River in the Mixes district of Oaxaca, Mexico.The intraorbital region is broad; zygomatic arches are slender and not widely spreading; incisive foramina are relatively short and widest in the middle; the palate is low with slender or incomplete lateral pits, and posterior median ridge sloping and grooved; the interpterygoid fossa is wide and quadrate.The karyotype of M. umbrosus has features similar to the hypothesized primitive pattern for the genus Microtus (Arvocolinae) and may prove that this species is older than its counterpart M. mexicanus.The patterns displayed by the karyotypes of M. umbrosus and M. quasiater are closer to the hypothetical ancestor than other members of the Microtus family.Comparing dental morphologies using electron scanning on enamel occlusal surfaces, and tooth measurements have been used to construct cladograms for the Microtus family.The species has been considered an ancient relict North American survivor of the subgenus Phaiomys from an early invasion from Asia.