Arvicoline Biodiversity and Biogeography

With no fewer than 13 species, the overall diversity of arvicoline rodents within Porcupine Cave exceeds that of any other North American locality. Moreover, arvicoline diversity within discrete stratigraphic horizons in some localities within the cave (e.g., Pit, level 4, with at least nine species) presents a unique example not only of taxonomic diversity, but also of species associations. Perhaps most notable in this regard is the stratigraphic co-occurrence of AHophaiomys with at least three of its purported descendants (Microtus sp. with five closed triangles, M. paroperarius, and M. meadensis). This stratigraphic co-occurrence is seen in the Pit (level 4) and in the CM Velvet Room excavation (level 3) and provides an interesting challenge to the purported ancestor-descendant relationship between these taxa. Perhaps the simplest explanation is that AHophaiomys persisted for a longer period of time in the higher elevations of Colorado than in the Great Plains, where most of the fossil occurrences are known. This refugium hypothesis should be tested in other relatively high-elevation localities (when they are identified and excavated). The recognition of significant taxonomic differences in contemporaneous faunas at high- and midlatitude localities through the Pliocene and Pleistocene clearly demands independent biochronologies for high-latitude sites (Bell et al., in press); in view of this, region-specific arvicoline biochronologies may well be required for high mountain sites as well (Bell and Barnosky, 2000).

Faunal provinciality along longitudinal gradients is difficult to document for early through middle Pleistocene faunas, in part because of the relative paucity of faunas of an appropriate age throughout much of North America. The Rocky Mountains have long been recognized to be a significant boundary between arvicoline rodent faunas in North America, and beginning in the 1980s Repenning and his colleagues began to discuss the separate arvicoline faunal histories east and west of the Rockies (e.g., Repenning, 1987, 1992; Repenning et al., 1995). Formal recognition of an Eastern United States Faunal Province and a Western United States Faunal Province was proposed by Fejfar and Repenning (1992); the proposal was based primarily on arvicoline rodent faunal histories. The Porcupine Cave faunas are uniquely situated in a high-elevation intermontane basin in the central Rockies and thus provide important insight into faunal dynamics across this significant boundary in the middle Pleistocene. Prior to their discovery in Porcupine Cave, Microtus paroperarius and AHophaiomyspliocaenicus were not known to occur west of the

Great Plains. Their discovery in Porcupine Cave was followed by additional discoveries of both taxa in Utah (Gillette et al., 1999) and Nevada (Bell, 1995; Bell and Barnosky, 2000), thus lessening the differences between eastern and western U.S. arvicoline faunas in the Irvingtonian. The recognition of Pliolemmus antiquus in the Mark's Sink locality of Porcupine Cave also represents a significant westward range extension (and a significant elevational extension) of an arvicoline previously known only from the Great Plains.

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