Carbon and nitrogen isotopic analysis of Pleistocene mammals from the Saltville Quarry (Virginia, USA): Implications for trophic relationships

The late Rancholabrean (> 10,000 years ago) Saltville Quarry in Virginia, USA preserves a community of basal herbivores, including several families of giant mammals, representing a range of potential feeding strategies (browsers versus grazers) and digestive mechanisms (ruminants versus non-ruminants). In this study, carbon and nitrogen isotopic analysis of well-preserved bone collagen from a variety of body elements was used to determine the trophic relationships among these herbivores. A range of δ15N values was observed with ruminants and non-ruminants clustering into two distinct groups. Analysis of a young mammoth revealed a relatively high δ15N composition, which is consistent with a milk-dominated diet and supports the previously noted “juvenile effect” observed in the isotopic analyses of sub-adult mammals. In addition, the isotopic evidence supports the view that a giant ground sloth was primarily herbivorous, as opposed to competing hypotheses for an omnivorous or carnivorous nature. The measured 13C abundances across the range of mammals, and of fossil organic matter, indicate a lack of C-4 grasses in the regional environment and a reliance on C-3 vegetation by all herbivores included in this study. These findings may record a regional geographic transition between C-3 and C-4 dominated grasslands. However, the overlapping isotopic signatures may also be considered as a potential indicator of extreme competition amongst the basal herbivores in this ecosystem, which has broader implications for the subsequent mass extinction of mega-herbivore clades at the end of the Pleistocene epoch.