ECOSYSTEM STRUCTURE DURING THE TRANSITION TO THE MODERN GRASSLAND IN THE GREAT PLAINS

Fossiliferous strata in the Meade Basin (southwest Kansas) preserve numerous superposed mammalian faunas and calcareous paleosols that range in age from late Miocene to late Pleistocene. The faunas provide a well-resolved biostratigraphy that is constrained with several volcanic ashes and document the development of the modern grassland fauna in the region; the stable isotope composition of the paleosols provides the environmental and climatic context of the faunal evolution. Carbon isotope ratios of paleosol carbonate reflects the proportion of C₃ (trees, shrubs, cool-climate grasses) and C₄ (warm-climate grasses) plants that grew in an ancient soil and provides a means to reconstruct past mammalian habitats; ratios in mammal teeth reflect the dietary proportion of C₄ biomass. Late Miocene paleosol carbonates in the Meade Basin have mean δ¹³C values (-7.1±0.91‰) that are statistically indistinguishable from Miocene sections elsewhere in the Great Plains (-6.8±0.83‰). Early Pliocene values in the Meade Basin are significantly higher but scarcely more variable than Miocene values (-4.9±0.94‰). Late Pliocene to early Pleistocene values are higher still and somewhat more variable than Miocene and early Pliocene values (-2.6±1.12‰), with most of the variability reflecting a temporal trend towards higher values. Carbon isotope compositions of Meade Basin carbonates indicate that the modern C₄-dominated grassland developed in two phases: an initial increase from typical Miocene C₄ abundance (about 20%) to a consistent early Pliocene level (about 40%) and a subsequent increase to modern levels (>60%) during the late Pliocene to early Pleistocene. Detailed lateral sampling along the same stratigraphic level and the distribution of diets of small mammals from two early Pliocene faunas suggest that the early Pliocene landscape had patchy distribution of C₃- and C₄-dominated habitats that migrated laterally on timescales short relative to carbonate accumulation and long relative to single faunal samples. The environmental reconstructions from the carbon isotope data suggest an important role for habitat change as a driver of faunal change.