BIOSTRATIGRAPHY AND NORTH AMERICAN PLEISTOCENE MEGAFAUNAL EXTINCTIONS

Several scenarios have been proposed to explain the terminal Pleistocene large mammal extinction event in North America, including climate warming and/or cooling, overhunting by early humans, disease, and bolide detonation or impact. Each of these premises seeks factors making the end-Pleistocene deglaciation uniquely different from earlier such periods of climate change, then infers that the observed difference is responsible for the extinction. Common to all of these scenarios is the assumption that large mammals present in North America at the end of the Pleistocene were also present in similar abundance, with similar geographic distributions, during earlier, equally severe periods of climate change (e.g., ~125 ka BP). Consideration of biostratigraphic data negates this assumption. The genus Bison, index taxon for the Rancholabrean North American Land Mammal Age (≤ 240 ka - ~10 ka BP), was numerically profuse and geographically widespread in the later Pleistocene, particularly in the American West. Living bison are massive grazing ruminants that dramatically impact biological communities; their Pleistocene forebears were still larger. The presence of these animals constitutes a significant difference between later Pleistocene and earlier large mammal faunas in North America. Further, the latest Pleistocene species Bison antiquus was more widely distributed and present in greater profusion than earlier species such as the larger B. latifrons south of the glacial ice. The increased abundance and distribution of these large, aggressive, herd-dwelling ruminants through time in late Pleistocene North America constitutes a critical distinction between end-Pleistocene communities and those of all earlier, similarly intense glacial-interglacial transitions, particularly with respect to the potential of increased competition for available resources. Responses of the megafauna to changing climatic conditions at the end of the Pleistocene were therefore unique. Extinction scenarios for Pleistocene North America should avoid assuming a relatively static long-term faunal component, and consider the potential impacts of non-human immigrant species on natives – particularly when immigration and subsequent evolutionary trajectories are relatively close in time and space with climate changes.