FIVE MILLION YEARS OF RODENT PALEODIETS IN THE MEADE BASIN, GREAT PLAINS, USA

Faunas in the Meade Basin (MB) record evolutionary changes in the local small mammal community over the last 5 Myr, spanning the local evolution of modern C₄–dominated grasslands and long-term climatic changes. We use carbon and oxygen isotope compositions of these faunas to reconstruct resource use by small mammals. Specifically, we characterize how food resources were partitioned within communities, and how diets varied through time in response to changing community membership, and environmental change.

Using LA-IRMS, we measured δ¹³C and δ¹⁸O values of tooth enamel from 70 taxa representing 6 small mammal lineages (N= 326). We compared the fossil data to isotopic values of modern MB specimens, which includes δ¹³C and δ¹⁵N values in hair (N = 196, 11 spp.) and collagen (N = 28, 4 spp.), and δ¹³C and δ¹⁸O values in bone carbonate (N = 30, 4 spp.).

Fossil δ¹⁸O values have no major temporal trends, but late Pleistocene fauna have lower values than Pliocene and modern faunas, which may reflect cooler Pleistocene climates. Small mammal δ¹³C values indicate they utilized C₃ and C₄ resources over the last 5 Myr as local C₄ biomass increased from 40% to >70%. C₄-dominated diets are not recorded in faunas until 3 MA, which may indicate lower C₄ consumption or absence of C₄-consuming taxa 5-3 MA. The δ¹³C values of both 3-2 MA and modern faunas span the full C₃ plant to C₄grass range.

The highest δ¹³C values prior to 3 MA are from an archaic vole, but Pleistocene and modern voles almost exclusively eat C₃ derived resources. This suggests a dietary change between older voles with rooted molars and younger voles with ever-growing molars. Cricetids have the largest range in δ¹³C values (12‰), and their mixed C₃:C₄ diets likely reflect herbivory and invertivory (Onychomys and Peromyscus). Most Pliocene sciurid δ¹³C values vary between -28 and -19‰ and become more negative in the late Pleistocene. Modern Cynomys differ from all other sciurids with some δ¹³C values indicating C₄-dominated diets. Heteromyids, (Dipodomys and Prodipodomys), have the most positive δ¹³C values throughout the record and differ from other heteromyids with C₃-dominated diets. Our study documents differences in small mammal diets over the past 5 Myr, and provides insight into variations in resource partitioning among taxa during intervals of faunal and environmental changes.