OXYGEN ISOTOPIC ANALYSIS OF FOSSIL ENAMEL: PRELIMINARY INVESTIGATION OF EOCENE MAMMALIAN TAXA

Macro and microvertebrate fossils were collected from the late Eocene in the Oglala National Grasslands of Nebraska. Teeth were separated from sediment, identified, and processed for stable isotopic analysis. Using oxygen isotopic ratios from bone phosphate, this project will compare mammalian taxa and make inferences about environmental conditions. According to petrographic analysis of fossil bone collected from the same area, no diagenesis had occurred. Therefore, we assume the original isotopic composition of phosphate in teeth is preserved, and allows us to evaluate parameters like the isotopic composition of meteoric water and paleotemperature. Preliminary isotopic data for three taxa (Megacerops, Mesohippus, and Archaeotherium) show δ¹⁸O values (reported relative to SMOW) that range between +15.8 and +19.0‰, and average +17.7‰. Megacerops, Mesohippus, and Archaeotherium average 18.3, 18.8, and 16.8‰, respectively. Megacerops has the least variation of δ¹⁸O values with a standard deviation (σ) of 0.4, followed by Archaeotherium (σ = 0.9) and Mesohippus (σ = 1.4). These variations could represent differences in diet and stratigraphic position. Megacerops and Mesohippus are herbivorous grazers, whereas Archaeotherium is an omnivore. Furthermore, Archaeotherium specimens were collected from different stratigraphic intervals and likely represent temporal changes in isotopic compositions of variable environmental conditions. To our knowledge, little is known about the isotopic composition of fossil mammals from the late Eocene of northwest Nebraska. Thus, the current study, if successful, will attempt to establish the foundations of isotopic compositions from bone phosphate of late Eocene taxa and interpret paleoenvironmental conditions.