THE PAST 4.5 MILLION YEARS OF SMALL MAMMAL EVOLUTION IN THE GREAT PLAINS: USING MULTIPLE PROXIES TO TEST THE RELATIVE ROLES OF BIOTIC AND ENVIRONMENTAL FACTORS

The Miocene-Holocene deposits of Meade Basin in southwestern Kansas contain abundant small mammal fossils, with excellent sampling of rodents, and numerous superposed paleosols interbedded with local faunas. These materials provide archives of ancient biologic and climatic change. Integrating these records provides a unique opportunity to test hypotheses of the relative roles of environmental versus biotic factors on the evolution of small mammal communities over long timescales. Environmental factors can include tectonics and climatic events, whereas biotic factors may include species interactions and adaptations, as well as immigration and extinction. Specifically of note in the Meade Basin is that rodent species richness was relatively invariant during the last 4.5 million years, despite a number of major environmental changes that apparently affected the region, including the transition to the modern, C₄-dominated grassland ecosystem; local cooling and/or increased available moisture coincident with global cooling during this time period; the onset of Northern Hemisphere glaciation and variation in extent of the Laurentide ice-sheet; and numerous ashfall events (Huckleberry Ridge, Cerro Toledo B, Lava Creek).

In this study, we aim to combine the high-resolution small mammal fossil record with paleoenvironmental records from carbonate clumped isotope (Δ₄₇) paleothermometry, δD and δ¹³C values of leaf wax n-alkanes in sediments, lignin phenols, δ¹³C of bulk sedimentary organic matter, environmental rock magnetism, paleosol elemental geochemistry, and plant phytoliths. Here, we present preliminary results from our initial analytical efforts generating paleotemperature and δ¹⁸O_{soil water} records from Δ₄₇ analyses of paleosol carbonate, δD and δ¹³C records from leaf wax n-alkanes, and iron-oxide and rock magnetic properties for the paleosols. Additionally, we utilize extensive isotopic datasets from the modern Meade Basin ecosystem to investigate existing interactions among small mammal fauna, as well as to calibrate the faunal response to environmental factors today. We will discuss the patterns of change from these records, how they relate to global and regional climate change events, and how these records integrate with the record of biologic change.