The more complete fossil record of ochotonids in the Pale-arctic (versus that of the Nearctic) has allowed greater elucidation of their evolutionary history in Asia and Europe. The first record of Ochotona in North America seems to indicate the arrival of the small pika, O. spanglei, in the late Hemphillian, earliest Pliocene. The record of Ochotona for the Blancan is unknown, with only an undocumented record from South Dakota (Bjork, 1997). If the chronological assignment of Mark's Sink (lower levels) is correct, then these pika remains are the first documented Ochotona from the Blancan. Irving-tonian deposits in North America record both the large pika (O. whartoni) and the small species (O. princeps-like; Mead, 1987; Mead and Grady, 1996). If the Porcupine Cave record is representative of the greater intermountain West for the early and middle Pleistocene, then the large Ochotona whartoni was not in the region and may well have been restricted during this time to the western Arctic (see Mead and Grady, 1996; Morlan 1996). Given this overall sparse record of pikas for the North American Irvingtonian, the deposits in Porcupine Cave have provided the best opportunity so far to study the evolutionary history of Ochotona on this continent. Additional study of specimens from this site—especially if more specimens from well-stratified and well-dated sites become available— holds the promise of elucidating evolutionary and biogeo-graphic patterns during a critical time in the North American evolution of pikas.

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