14C-Chronostratigraphy of Late Pleistocene Megafauna Extinctions in Relation to Human Presence in the New World Thomas W. Stafford, Jr., Stafford Research, Inc. Russell Graham, Earth and Mineral Sciences Museum, Penn State University Ernest Lundelius, Department of Geological Geosciences, University of Texas Holmes Semken, Department of Geology, The University of Iowa Greg McDonald, National Park Service, Fort Collins, Colorado John Southon, Earth System Science Department, University of California, Irvine Abstract: Direct radiocarbon dates on extinct New World megafauna are evidence that the extinction occurred as two distinct events. Non-proboscidean megafauna species went extinct ca. 11,400-11,300 RC yr. BP, whereas Mammuthus and Mammut survived until ca. 10,900 RC yr. BP. Stratigraphically, Clovis artifacts post date sediments containing non-proboscidean species. Our opinion is that Clovis peoples encountered only Mammuthus and Mammut. After the proboscideans' extinction, bison filled many niches left by herbivorous megafauna, thereafter beginning the population explosion of Bison spp. If humans were present a few hundred to several thousand years before Clovis peoples, these humans' impact on the megafauna was negligible to not measurable. Our conclusion based on stratigraphy and 14C geochronology is that the late Pleistocene megafauna extinction occurred as two events, each 100 to 200 calendar years in duration, and that if humans had any effect on the extinction, it was restricted to remnant proboscidean populations. Thomas W. Stafford, Jr. Dr. Thomas W. Stafford, Jr. is a geochronologist and biogeochemist studying the stratigraphic and chronological evidence worldwide for climate and faunal and human population changes during the last 50,000 years. He uses physical stratigraphy, AMS 14C dating, and archaeological and paleontological evidence to discern the time relationships between and the causes for these natural and human events. The goal is to distinguish human from natural factors and to determine if biological changes in the last 20,000 years are fundamentally different or the same as those over the last several millions of years