REGIONAL VARIABILITY OF MICROWEAR ON THE MOLARS OF LEPTOMERYX FROM EOCENE-OLIGOCENE STRATA OF WYOMING AND NEBRASKA

Climate change across the terrestrial Eocene-Oligocene boundary of the Great Plains is recorded by shifts in sedimentology, facies, paleopedology, and isotopic records, and is interpreted as a shift to overall cooler and drier conditions. As an independent test of paleoenvironmental shifts caused by climatic change, we compared microwear on M₂ molars of Leptomeryx from the White River Group (WRG) at Toadstool Park, Nebraska (n = 9) and Flagstaff Rim, Wyoming (n = 11). Comparisons of microwear were made within defined isochronous intervals and through time at each section. Numbers of pits and scratches and the scratch/pit ratio were quantified on original, uncoated specimens using environmental scanning electron microscopy and Microware 4.0 software, and evaluated with the Kruskal-Wallis statistical tests. The Leptomeryx M₂ scratch densities (p = 0.03668) and scratch/pit ratios (p = 0.008765) are significantly greater at Flagstaff Rim, Wyoming than Toadstool Park, Nebraska. However, pit densities were not significantly different (p = 0.3423) between locations. The microwear patterns suggest paleoecological differences between the two locations, possibly related to differences in Leptomeryx diet or in amount or character of sediment adhering to ingested vegetation. There is little fossil evidence of vegetation type at either locality, other than root traces. However, sediments of the WRG are a mixture of volcaniclastic enriched mudstone, siltstone, and sandstone, with generally coarser overall particle sizes in Wyoming that reflect proximity to siliciclastic sources. The degree of overall volcaniclastic enrichment and number of airfall tuffs is also higher at Flagstaff Rim. Paleosols suggest a shift from closed canopy forest to progressively open conditions at each locality and, although microwear differences could result from differences in vegetation, no correlations between microwear features and stratigraphic level were detected at either locality, indicating that any changes in paleoecology over time did not significantly alter the diets of Leptomeryx.