STABLE ISOTOPE PALEOECOLOGY OF CARNIVORES DURING THE EOCENE-OLIGOCENE TRANSITION OF NEBRASKA

Stable isotope ratios of carbon and oxygen, δ¹³C and δ¹⁸O, have been used to elucidate the paleoecology of mammalian herbivores, but are underutilized in the study of predator-prey relationships in the fossil record. This is especially true in the White River Group (WRG) of the Great Plains across the Eocene-Oligocene climatic transition, where several studies investigate stable isotope paleoecology of herbivores, but not of carnivores. This study tests the hypothesis that common WRG carnivore taxa can be differentiated isotopically in accordance with δ¹³C values of preferred prey taxa. Thirty-one individuals of four common EOT carnivore families (eight nimravids, seven canids, eight hyaenodontids, and eight amphicyonids) were sampled for δ¹³C and δ¹⁸O values. Isotopic data from the carnivores were compared to all previously published herbivore data (245 samples of 14 genera). All samples included in this study are from Sioux or Dawes County, Nebraska, and represent individuals from both the Chadronian (late Eocene) and Orellan (early Oligocene) North American Land Mammal Ages. Statistically, canids have significantly higher δ¹³C (mean δ¹³C= -8.9‰) when compared to nimravids (Welch’s t-test, t(12.3) = 2.9, p = 0.014) and hyaenodontids (Welch’s t-test, t(12.7) = 3.8, p = 0.002). Nimravids have the lowest δ¹³C (mean δ¹³C= -10.7‰). Amphicyonids have the largest interquartile range of δ¹³C values from -11.8‰ to -7.9‰, averaging -9.8‰. Hyaenodontids have the smallest interquartile range of δ¹³C values, from -10.4‰ to -9.7‰, averaging -10.1‰. Overall, across the Chardonian and Orellan, there is an increase in both δ¹³C and δ¹⁸O values. High δ¹³C values in canids suggest omnivory, as seen in modern canids and supported by microwear evidence. The low δ¹³C values in nimravids support a hyper-carnivorous diet and consumption of larger herbivores, a hypothesis speculated in previous publications. The large range of δ¹³C for amphicyonids implies a generalist diet, while the narrow δ¹³C values for hyaenodontids imply dietary specialization. Using δ¹³C analysis as a method to understand ecological relationships between carnivores and their preferred prey allows an understanding of how the changing climate of the EOT affected the large mammals of Nebraska.