SOUTHERN GREAT BASIN LATE PLEISTOCENE SEASONALITY AND PALEOENVIRONMENT RECORDED in BISON LATIFRONS TOOTH ENAMEL

We analyzed stable carbon and oxygen isotopes in the enamel of a Bison latifrons maxillary m3 from Lincoln County, in east-central Nevada. A partial skull and horn core were collected from the Wilkin Quarry Site near Panaca, at a latitude of 38°N. The radiometric age is not yet known, but the genus Bison does not occur in North America prior to about 0.25 Ma, in the late Pleistocene. B. latifrons fossils are rare in the Great Basin; this is only the second known occurrence in Nevada.

We sampled the tooth serially. δ¹³C_enamel values range from -8.0‰ to -5. 4‰ (VPDB). The earliest mineralized sample has a value of -7.1‰; the values steadily rise to the peak value of -5.4‰, which is almost at the base of the tooth. However, the values then drop precipitously, with the latest mineralizing portions of the tooth having the lowest values. Bison bison third molars mineralize from the 9^th to the 24^th month of life. Assuming this animal was born in the spring, its m3 would have begun to grow in the winter, at an age of nine months, and it would have continued growing during the following spring and summer, finally reaching its full length in the late winter of its second year. The δ¹³C values, which reflect the relative amount of C₄ biomass consumed by the animal, are completely compatible with this life-history scenario. C₄ grasses are arid-adapted, so they thrive in the dry summer months. We interpret the δ¹³C pattern preserved in this tooth to record a progressive increase in C₄ biomass in this animal’s diet in the spring, summer, and fall of its second year of life. Furthermore, based on research with modern bison, we infer that the percentage of C₄ biomass consumed by this animal varied from a low of about 40% to a high of about 60%. Ninety percent of the diet of Bison bison consists of grasses and sedges. They are not highly selective in choosing between C₃ and C₄ grasses. These percentages can be used to approximate the relative abundance of C₄ grasses at this latitude in the Great Basin in the late Pleistocene. This compares to a relative abundance of about 25% today, so these data suggest that C₄ grasses represented a higher percentage of the flora when this animal was alive than they do today (in undisturbed habitat). The inferred late Pleistocene C₄G percentages correspond to a mean annual temperature in the range of 8 to 12°C., which is roughly the same as at Panaca today.