The shasta ground sloth Nothrotherium shastense inhabited Rampart Cave in the Grand Canyon of the Colorado River, Arizona, for at least 25,000 years. During this interval occupation was probably discontinuous; radiocarbon dates of the dung from the surface of the cave are on the order of 10,000 years, from the 18-in. level 12,000 years, and from the 54-in. level older than 35,000 years. Dung samples collected at 6-in. intervals to a depth of 60 in. proved rich in well-preserved pollen. Within each level there is great variation in pollen content. In part this reflects seasonal change in flowering time of various plants growing adjacent to the cave. An increase in Artemisia, Pinus, Betula, Cupressaceae, and other montane trees and shrubs reveals that levels 18 through 48 represent a time of cooler or wetter climate with upper Sonoran vegetation displaced downward 2,000 to 4,000 ft. These samples were analyzed spectrographically for trace element content. On theoretical grounds the abundance of Mn proves a promising index of climatic change; when compared with the pollen evidence, there is a reasonably good fit between high Mn values and periods of greater moisture. These correspond to the Wisconsin glacial age. Co is very rare and Cu is abundant in dung from all levels. There is no indication that a change in abundance of these triggered ground sloth extinction, as claimed by Salmi. Nothrotherium excelled as a browser in the arid SW. Some of its preferred food plants, as creosote bush, yucca, snakeweed, and cactus, are not harvested systematically by existing large desert herbivores. The ecological niche of Nothrotherium remains unfilled. Its extinction defies an obvious environmental explanation either in terms of trace element deficiency, competition, or climatic change. The cause of extinction must lie elsewhere.