ISOTOPIC RECORDS OF RODENT PALEOECOLOGY NEAR THE EDGE OF THE LAURENTIDE ICE IN THE LATE PLEISTOCENE: INSIGHTS AND LESSONS FROM THE MODERN ECOSYSTEM OF THE PRYOR MOUNTAINS, MONTANA

Last Canyon Cave (LCC) is a rock shelter in the Pryor Mountains of southcentral Montana that has yielded a ~35,000 year vertebrate fossil record. The majority of the record is comprised of small mammal bone fragments and fecal pellets. LCC is an interesting late Pleistocene and Holocene site because of its close proximity to the southern edge of the Laurentide ice sheet during the Last Glacial Maximum. Paleoenvironmental and paleoecological reconstructions of this region can now be informed by δ13C chronologies derived from LCC fossil materials, including small mammal bone collagen. In particular, these isotopic records provide insight into the presence of C4 grasses, and the response of small mammal paleodiets to glacial-interglacial climate change.

Our interpretations of LCC fossil isotopic records were informed by sampling of the modern Pryor Mountains ecosystem, including small mammals (all rodents), arthropods and vegetation. We collected hair samples from abundant rodents at two sites near LCC. These sites are separated by only a few kilometers, but experience large differences in precipitation. By analyzing rodent ecological differences between these modern environments we gained insight into how late Pleistocene climatic and environmental variability may be reflected in fossil isotopic records. Using modern rodent hair δ13C values alone we were able to identify patterns of C4 plant consumption, and insectivory in diet. At the wetter modern site the herbivorous resources in rodent diet were purely-C3 plant, and insectivory was common. At the drier site both C3 and C4 plants were consumed, and insectivory was less common. Estimates of C4 grass abundance calculated from community-level averages of rodent hair δ13C values at the dry site were similar to modeled estimates for the region. In contrast, estimates based upon wet site δ13C values dramatically underestimated regional C4 grass abundance. This type of fine scale spatial variability in small mammal isotopic datasets highlights the degree of ecological averaging that can be associated with analysis of fossil materials.