Death Valley, California is today the hottest and driest area in the western Hemisphere, with temperatures of 134F (57C) recorded. During the late Quaternary, pluvial lake Manly covered much of the Valley floor and contributed to a much more moderate climate. The abrupt draining of Lake Manly in the mid-Holocene led to dramatic shifts in temperature and aridity and exerted substantial selection pressure on organisms living in this area. Our research investigates the adaptive response of Neotoma (woodrats) to temperature change over the late Quaternary along a steep elevational and environmental gradient (-84 to >3400m). By combining field-work, examination of museum specimens, and collection of paleomiddens, our project reconstructs the divergent evolutionary histories of animals from the Valley floor and nearby mountain gradients. We report on recent paleomidden work investigating the transition zone along the Grapevine Mountains for two species of woodrats differing significantly in size and habitat preferences (N. lepida, desert woodrat and N. cinerea, bushy-tailed woodrat). Here, at the limits of animal's thermal and ecological thresholds, we demonstrate dramatic fluctuations in the range boundaries between these species over the Holocene as climate shifted. Moreover, we find fundamental differences in the adaptive response of these two species related to the elevation of the site and local microclimate. Results indicate that although N. cinerea are currently extirpated in this area, they were ubiquitous throughout the late Pleistocene and were found as low as 400 m elevation.