Paleobiology of Late Pleistocene mastodonts and mammoths from southern Michigan and western New York.

Mastodonts, mammoths and elephants have laminae in the dentin of their tusks which occur at three spatial scales: 1$\\sp\\circ$--first order (millimeters), 2$\\sp\\circ$--second order (tenths of millimeters), and 3$\\sp\\circ$--third order (hundredths of millimeters). Laminae occur as dark/light couplets which are apparent in thin-section and also expressed as variations in luminance on transects across thin-sectioned tusks. Fourier analysis of luminance transects identifies laminae at these scales. In adults, there are typically 26 2$\\sp\\circ$ increments per 1$\\sp\\circ$ lamina and 14 3$\\sp\\circ$ increments per 2$\\sp\\circ$ lamina, suggesting that 1$\\sp\\circ$, 2$\\sp\\circ$, and 3$\\sp\\circ$ laminae represent annual, biweekly, and daily rhythms in dentinogenesis. Comparison with laminae in other mammals supports this assessment. Thus tusks offer a long record of a proboscidean's physiologic status and of paleoenvironmental conditions. Three factors central to different explanations of the late Pleistocene extinction of mastodonts and mammoths can be determined from analyses of tusks: season of death, seasonality and dietary flexibility. Season of death provides information about hunting versus scavenging of proboscideans and illuminates the role of human predation in the extinction. It is determined by (1) change in growth rate, where zones with thin 2$\\sp\\circ$ laminae are considered winter growth, and (2) oxygen isotope profiles across tusks. The oxygen isotope composition of precipitation is temperature dependent and thus varies seasonally. This signal is ingested through drinking water and preserved in tooth dentin. Isotope profiles and growth rate yield similar estimates of seasons of death for individual proboscideans. A change in seasonality (the annual range of temperature) underlies many environmental explanations for Pleistocene extinctions. The temperature dependence of the oxygen isotope composition of precipitation supplies a quantitative estimate of seasonality. To serve as a seasonality measure, complete annual shifts in precipitation composition must be preserved with fidelity in tusk dentin. For some proboscideans, diagenesis and homogenization in drinking water sources attenuate the signal. Analytical refinements and comparative studies with other taxa should alleviate these problems. Dietary rigidity is another proposed cause of proboscidean extinction. An animal's carbon isotope composition is controlled by diet and allows discrimination between browsing and grazing. Isotopic profiles of proboscidean tusks indicate that mastodonts and mammoths had seasonally variable diets. Also, mastodonts consumed a variety of different average diets. Thus mastodonts were dietary opportunists and should not have been susceptible to extinction through dietary stress.PhDPaleontologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/162478/1/9013945.pd