Paleoecology of late pleistocene megaherbivores: Stable isotope reconstruction of environment, climate, and response

Late Pleistocene megaherbivore communities of the Pacific and Mountain West states of California and Nevada are under-analyzed in regard to ecological function (diet, mobility, niche partitioning, and range of ecological tolerance). Stable isotope analysis is a powerful tool that is known to recover primary paleodiet and paleoenvironmental information from biogenic materials, such as enamel and dentin. This dissertation explores the use of carbon and oxygen stable isotopes in Late Pleistocene (40-10 Ka) megaherbivore teeth to gain a better understanding of inter- and intra-specific behavior and reconstruct Late Pleistocene ecosystems of California and Nevada. Radiocarbon dates exist for most of the assemblages included in this study (Potter Creek Cave, Samwel Cave, Devil Peak Cave, Gilcrease Site, and Tule Springs), allowing for the isotopic data to be integrated into a temporal framework, thereby making the results more meaningful. The exceptions are the Hawver Cave deposits from northern California and Wilkin Quarry deposits from southern Nevada, which were sans dates. The first efforts toward constraining the age of the Hawver Cave deposits were made as part of this study; one date, about 22 Ka, was retrieved which records conditions during the Last Glacial Maximum. Tooth enamel or dentin from several taxa of megaherbivores ( Odocoileus, Euceratherium, Equus, Bison, Mammuthus, Ovis, Nothrotheriops, Megalonyx) was analyzed from seven localities in northern California and southern Nevada. Most isotope paleoecology studies of mammals are conducted on enamel, due to its high resistivity properties against post-mortem alterations. However, ground sloths lack enamel, so dentin was examined for the ground sloth genera Nothrotheriops and Megalonyx. To identify the viability of primary isotopic signatures in dentin and enamel, X-ray diffraction (XRD) and scanning electron microscopy (SEM) was utilized. XRD and SEM results show that carbonate hydroxylapatite in fossil teeth underwent recrystallization, although chemical composition of fossil dentin samples are identical to modern dentin, while enamel samples were nearly identical. XRD and SEM validates the purity of the fossil material and supports the use of carbon and oxygen isotope analysis on dentin and enamel. This is the first isotopic study on Nothrotheriops dentin, and it adds to a very small number of studies on Megalonyx dentin. Results show that a13C and a18O data from ground sloth dentin are primary and allow for an interpretation of diet for each taxon; the data indicate ecological partitioning between co-occurring ground sloth genera. In addition to the ground sloth results, averaged a13C and a18O data for all the megaherbivores analyzed show that different species were able to tolerate a wide range of diets and habitats, while serial data show that individual animals exhibit less ecological flexibility. Serial a13C data reveal that individuals consumed a similar type of vegetation throughout the year, and serial a18O data suggest that individuals had limited mobility or occupied similar habitats seasonally. Paleoclimate models show that environmental conditions of northern California and southern Nevada during the Last Glacial Maximum were 5.5 to 7.5 OC cooler than modern temperatures and received up to 30% more precipitation. The models reveal that Late Pleistocene precipitation was distributed more abundantly through the year (i.e., less winter/summer extremes), which provides support for the serial isotopic results. Isotopic data acquired in this study were compared to two prior isotopic analyses of megaherbivore teeth from Nevada and California, in order to gain a larger-scale, regional interpretation of Late Pleistocene environments of the western United States. Northern California and northern Nevada had environments which hosted predominantly browsing species, while many of the same species in southern Nevada occupied an array of herbivorous niches (browsing, mixed feeding, and grazing). These data reveal a wide range of ecological plasticity for Late Pleistocene megaherbivore species. Fitting the isotopic information into the radiocarbon dates, for the assemblages, reveals that environments were becoming warmer and more arid toward the close of the Late Pleistocene. Data from Equus, Mammuthus, Bison, Camelops , and Nothrotheriops reveal that these taxa did not respond to this warming trend by increasing C4 consumption; rather, individuals expanded their dietary breadth to include increased browsing and increased grazing. This expansion in diet resulted in niche conservatism at the generic, if not at the specific, level for taxa through the Late Pleistocene.