The Late Pleistocene Thermal Enclave: New Evidence From an American Mastodon Site in South Carolina, USA

During the Last Glacial Maximum, the area south of Virginia and east of the Appalachian Mountains was likely a thermal enclave, an area of warmer temperatures and higher biodiversity relative to northern and western regions. It has been proposed that megafaunal extinctions were more severe in this region, relative to the north, a hypothesis that can be tested by studying life histories of late Pleistocene megafauna from this region. A new American mastodon (Mammut americanum) site from North Charleston, South Carolina, provides an opportunity to investigate both the ecology of the region and the life history of a member of the thermal-enclave fauna. Palynological analysis of the organic-rich fossil matrix showed palynomorphs of northern (e.g., red pine), southeastern coastal plain (e.g., Spanish moss), and freshwater (e.g., water milfoil) plants, indicating the environment was a mixture of forest, prairie, and freshwater swamp. The most abundant species present in the matrix are Pinus, Carya andQuercus, some common pollen types of the Coastal Plain. The presence of Pseudoschizaea and Myriophyllum indicate the presence of standing water, while Pinus cf. resinosa and Tilia indicate a northern environment. This mosaic of environments is typical of those described for the thermal enclave. The site has produced fossil elements from a single mastodon, including its tusk, both mandibles with three molars, a humerus, four vertebrae, and several ribs. This individual is a large male, based on femur size and tusk circumference, and died between 43 and 47 years of age, based on molar wear and stage of molar eruption. Preliminary analysis of tusk growth rate shows that only about 0.12 mm of dentin was added per two-week period. This rate is lower than expected based on tusk growth rates of male mastodons from the Great Lakes region, suggesting that the North Charleston mastodon experienced nutritional or environmental stress. Stable isotope analysis of this mastodon’s tooth enamel is underway and will allow for clarification of diet and habitat in relation to conditions suggested by the palynological and growth rate analyses. Ultimately, the results from this and other studies of mastodon paleoecology can be used to investigate what effect, if any, this stress had on the rate and severity of megafauna extinctions in each region