THE LAST MARCH OF THE PROBOSCIDEANS: CHANGES IN LANDSCAPE USE AND MOBILITY PRECEDING THE PLEISTOCENE MEGAFAUNAL EXTINCTION

During extinction events, changes in species landscape use and mobility provide insight into the mechanisms and velocities of these biologically important periods. The terminal Pleistocene offers a fossil record with high spatial and temporal resolutions for testing millennial-scale changes in species landscape use and mobility prior to, and synchronous with, extinction. Two sources of data are useful for establishing past mobility: (1) geographic locality data, and (2) strontium isotope ratios (⁸⁷Sr/⁸⁶Sr). As rocks weather, Sr is released into overlying soils and surface waters, and subsequent incorporation of Sr into plants and animals occurs with little fractionation. Thus, organismal Sr ratios reflect the resources consumed during tissue formation. Matching organismal Sr ratios to local and regional geochemical signals historically required detailed sampling of local plants, animals, and water. Recently developed continental-scale Sr models offer new opportunities to reconstruct movement patterns of highly mobile organisms. Here, using a combination of spatially explicit analysis and a continental-scale Sr model, we develop an analytical framework to test for changes in megafaunal mobility prior to extinction. Our analyses focus on American mastodons (Mammut americanum) and Columbian mammoths (Mammuthus columbi). We assessed mobility using differences between measured Sr ratios for skeletal material and modeled ratios for each specimen’s recovery location. To estimate the magnitude of movement, we identified regions of North America that matched skeletal Sr ratios and calculated both great circle distances to those regions and least-cost paths using circuit theory. Our results indicate mastodons and mammoths had significantly different geographic responses prior to extinction. Mammoths underwent a protracted and significant reduction in mobility following the Last Glacial Maximum (p << 0.01) with a rapid and unprecedented increase in mobility within the millennium preceding their extinction. In contrast, mastodons exhibit more consistent mobility patterns across the Pleistocene. These results suggest that extinction pressures were geographically and/or taxonomically heterogeneous. Our analytical framework is conducive to a variety of geographic analyses of fossil data.