bombifrons=Symbos cavifrons), and mountain deer (Navahoceros fricki) (Faunmap Working Group 1996). In addition, several large extinct carnivores—the short-faced bear (Arctodus simus), American lion (Panthera atrox), and American cheetah (Miracinonyx trumani)—were common to this area (Martin and Gilbert 1978, Chomko and Gilbert 1987, Walker 1987). Other ungulates (e.g., flat-headed peccary [Platygonus compressus]) and carnivores (e.g., saber-tooth cat [Smilodon floridanus]) may also have been present, but their remains are sparse and not easily extrapolated to the GYE.

These large mammals as well as at least 24 other genera became extinct in North America at the end of the Pleistocene (11,000 YBP). The two main hypotheses about this extinction involve overexploitation by human hunters and climatically driven environmental changes (Martin and Klein 1984). Whatever the cause of the extinction, it must have had broad ramifications, including alterations in biological interactions such as predation and competition; vegetational structure and composition created by seed dispersal, browsing, and grazing; and nutrient recycling. The disappearance of species assemblages for which there is no present-day representative is coincident with the extinction event (Graham and Lundelius 1984).

As the climate began to warm about 14,000 years ago and glaciers receded northward and higher, so did some of the boreal mammalian fauna. The collared lemming that today lives on the tundra in Alaska and Canada was one of the first mammals to be extirpated from the surrounding areas and probably the GYE, having vacated the contiguous northwestern United States by at least 10,000 YBP (Faunmap Working Group 1994). Other boreal species like the pika (O.princeps) and the heather vole (Phenacomys intermedium) remained at elevations lower than their current distribution until the middle Holocene, a time of maximum warmth and dryness (Grayson 1977, 1981; Mead et al. 1982; Mead 1987). As these species dispersed to higher elevations, species like the pygmy rabbit (Oryctolagus idahoensis) decreased in abundance and others, which were adapted to drier habitats (e.g., Lepus spp.), first appeared or increased in abundance (Grayson 1987). Little is known of the Holocene history of the Great Basin ungulates (Grayson 1982, 1993).

Similar changes took place in Wyoming east of Yellowstone as summarized by Walker (1987). At 10,300 to 9,300 YBP, some boreal mammal species were still present in the basin areas, but steppe forms were starting to be found in association with boreal habitat types. However, by 5,060 to 2,760 YBP, modern mammalian distributions are believed to have been established.

Lamar Cave has yielded an extensive fossil mammal record from the



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