BIOGEOGRAPHY OF THE NORTH AMERICAN PLEISTOCENE MEGAFAUNAL EXTINCTION

Changes in the geographic distributions of taxa confronting extinction provide insight into the mechanisms and velocities of these biologically important events. Here, we test the spatial structure of Pleistocene megafauna for changes in the distribution of genera across North America prior to, and synchronous with, their end-Pleistocene extinction. Analyses focus on genera with the most prolific records at the terminus of the Pleistocene (Nothrotheriops, Camelops, Equus, Mammut, and Mammuthus). Stratigraphic and radiocarbon-dated occurrences are obtained from the FAUNMAP, Canadian Archaeological Radiocarbon, and Paleobiology Databases. Radiocarbon-dated occurrences from the Terminal Pleistocene are collected from the literature and vetted for chronometric reliability. Occurrence data are separated into Early/Middle Pleistocene, Preglacial, Full Glacial, and Terminal Pleistocene stages and spatially standardized to remove collection biases. We use Monte Carlo simulations to test the spatial distributions of genera between periods. Simulations develop null expectations for changes in range size (minimum convex polygons and estimates of total spatial domains), geographic centroids, and range overlap between taxa; thus, permitting the discrimination of biological processes from sampling effects. Ecological Niche Models (MAXENT) provide independent tests of the ecological (environmental) significance of range divisions and/or overlap among taxa. Large-scale spatial reorganizations at the Terminal Pleistocene are evident, including a significant collapse in geographic area supporting Nothrotheriops, an eastward shift by Mammut, and the Pleistocene’s first significant geographic division between Mammuthus and Mammut. Niche models support ecological divisions among Proboscideans. For other taxa, geographic responses were largely non-significant, suggesting that many megafauna experienced spatially uniform extinction pressures across their geographic ranges.