Abstract
ABSTRACT Fossil voles (Pliophenacomys primaevus, Ophiomys meadensis), a muskrat (Pliopotamys near meadensis), and a lemming (Synaptomys) are described from the Shooting Iron Formation in Jackson Hole, Teton County, Wyoming. These microtines indicate that the base of the formation is of late Blancan age (=latest Gauss or early Matuyama magnetic epochs, between 2.4 and 1.8 million years old). All four taxa show affinity with species to the east, implying faunal continuity between Jackson Hole and the Great Plains in the late Blancan. After the late Blancan, barriers to microtine dispersal developed or were accentuated near Jackson Hole, so that the modern microtine fauna of the region lacks a strong eastern affinity. Probable mechanisms that helped to develop barriers and redefine microtine distributions near Jackson Hole include: (1) climate change, which altered vegetation and glacial limits; (2) competition from immigrant taxa; and (3) tectonism, all of which took place during the Pleistocene.
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