Pitymys meadensisHibbard from the Valley of Mexico and the classification of North American species ofPitymys(Rodentia: Cricetidae)

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Abstract
The microtine mouse Pitymys meadensis Hibbard, a species indicative of the late Irvingtonian mammal age in the United States, is present in a fauna of Rancholabrean age in the Valley of Mexico at El Tajo de Tequixquiac. It is closely related to the living Pitymys quasiater of Sierra Madre Oriental. Following the reasoning of Van der Meulen, the species group of Pitymys pinetorum is recognized as a distinct North American lineage and the North American species P. meadensis (fossil), P. quasiater, and P. nemoralis are considered more closely related to Old World species of Pitymys, some of which have been placed in the genus Neodon. Pitymys and its immediate ancestor Allophaiomys are known from Europe and Asia in early Irvingtonian time. Allophaiomys also dispersed southward from Beringia into North America in early Irvingtonian time, where the pinetorum species group developed from it in late Irvingtonian time. The pinetorum species group seems always to have been restricted to the dry steppes of the northern Great Plains and to the forests well east of the Mississippi River. The quasiater species group dispersed from Beringia into North America in late Irvingtonian time and now survives partly in geographic asylum in central Mexico and the southern United States, as well as in southern Asia and southern Europe. It also survives partly in ecologic asylum with Microtus, a more dominant microtine genus, in the more northerly parts of the United States and Europe. The genus is distinctly separate from Microtus and is here recognized as having full generic status.