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Selected fossil bison rib bones from the Hudson-Meng bonebed (early Holocene age) were studied in order to assess possible sources of isotopic variability along a north-south transect which spans large variations in paleotopography and resulting hydrological conditions. Analyses performed on the specimens included microscopic examination in thin-section, determination of ä13C values in collagen, and determination of ä13C and ä18O values in bone carbonate. Explanations for spatial trends in isotopic data centre around the eVects of pre-burial topography upon deposition and preservation of skeletal samples, arguing for enhanced analysis and consideration to be given to site paleosurface characteristics during the selection of specimens for paleodietary and paleoclimate analysis. All enamel and bone carbonate data suggest a diet ä13C value between 2"19 and 2"20 ‰ for these early Holocene animals, while most collagen values predict dietary ä13C values ranging from 2"21 to 224·5‰. Two interpretations exist for the diVerence between the ä13C value of diet predicted from bone carbonate and the apparent ä13C value of the modern grass biomass available to grazers in a nearby location: first, that the grass biomass available to grazers in this area 9500 bp had a higher ä13C value; and second, that the bison diet consisted of a selective subset of species available in the environment.? 1998 Academic Press Limite