Squirrels (Rodentia, Sciuridae) are diverse taxonomically and ecologically in the extant North American fauna (Hall, 1981). On a broad scale, the roots of sciurid diversity extend deeply in time, with fossils known as far back as the early Oligocene (e.g., Sutton and Black, 1975; Emry and Thorington, 1984), but adaptive radiation leading to modern species and species groups was probably concentrated in the late Neogene. Details of this radiation remain sketchy. It is tempting to seek causation in Plio-Pleistocene environmental fluctuations associated with glacial-interglacial variation (e.g., Vrba, 1992), but testing this hypothesis is difficult. Although the latest Pleistocene record of sciurids is dense (see Kurten and Anderson, 1980; Harris, 1985), the earlier record (Blancan through early Rancholabrean) is relatively sparse, primarily documenting ground-dwelling species from the Great Plains (e.g., Hibbard, 1941a; Hazard, 1961; Paulson, 1961; Skinner and Hibbard, 1972; Eshelman, 1975; Goodwin and Hayes, 1994; Goodwin, 1995a, 1995b) and usually lacking sufficiently precise chronological control to reconstruct glacial-interglacial transitions.
The fossil assemblage from Porcupine Cave provides rich documentation of Irvingtonian faunal diversity at high elevations in the central Rocky Mountains (Barnosky and Ras-mussen, 1988). Samples from various sites in the cave probably span a significant period of time within the Irvingtonian, and at least one site (the Pit) may document one and perhaps more glacial-interglacial cycles. This record may allow investigation of faunal response to glacial-interglacial transitions prior to the Pleistocene-Holocene boundary (Wood and Barnosky, 1994; Barnosky et al., 1996). Prior to this report, information on Irvingtonian sciurids from the vicinity of Colorado and northern New Mexico was known only from Hansen Bluff (Rogers et al., 1985) and SAM Cave, with the latter locality producing at least seven species, including one species each of Cynomys, Tamias, and Sciurus, and four species of Spermophilus, according to the faunal list in Rogers et al. (2000).
Previous work on sciurids from Porcupine Cave demonstrated that they are diverse (more than seven species [Barnosky and Rasmussen, 1988; Anderson, 1996]); showed that some sciurids exhibited marked changes in relative abundance within the stratified Pit sequence (Wood and Barnosky, 1994; Barnosky et al., 1996); and permitted paleoclimatic interpretation based in part on the sciurid assemblage (Wood and Barnosky, 1994; Barnosky et al., 1996; Rouse, 1997). The primary purposes of the present study are twofold. First, I characterize more completely the taxonomic diversity and patterns of variation of the sciurid assemblage from various sites in Porcupine Cave. Second, I explore implications of the sciurid assemblage for biostratigraphic reconstructions, understanding temporal environmental variation, and working out the adaptive history of the Sciuridae.
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