Rodent community change at the Pliocene–Pleistocene transition in southwestern Kansas and identification of the Microtus immigration event on the Central Great Plains

A dense fossil rodent database and new paleomagnetic data from the Meade Basin of southwestern Kansas document the transition from a Pliocene community characterized by cricetids of southern aspect and archaic arvicolids with rooted molars to a Pleistocene community with cricetids of temperate aspect and arvicolids with ever-growing molars. This new information suggests a revised date for the Blancan–Irvingtonian North American Land Mammal Age boundary between 2.06 and 1.95 Ma, coincident with the extinction of the Sigmodon minor lineage and the immigration of Microtus. Carbon and oxygen isotope compositions of paleosol carbonates indicate the faunal changes occurred during the final expansion of C4 grasses in the Great Plains to modern abundance and a strong cooling trend reflecting the end of the early Pliocene warm interval and onset of northern hemisphere glaciation. Although extinctions balance originations through the study period, pulses of enhanced extinction and origination occurring near the Pliocene–Pleistocene boundary strongly suggest environmental change as the likely forcing mechanism for rodent community compositional shifts. The Meade Basin rodent sequence provides the most complete history of any terrestrial vertebrate group in North America for this period of time.