EARLY EOCENE PALEOENVIRONMENTS IN WYOMING BASED ON STABLE ISOTOPE ECOLOGY OF FOSSIL MAMMALS

Ecological upheaval amidst climate change during the early Eocene favored the success of the Mammals, with 40 new mammalian families appearing within 1.5 million years of the Paleocene-Eocene Thermal Maximum (PETM). Two periods of mammalian turnover are recognized in the early Eocene Willwood Formation, central Bighorn Basin, Wyoming: biohorizon A (55.1–54.8 Ma) and biohorizon B (~54.2–54.0 Ma). Although mammalian evolution is well studied during this interval, new types of data are needed to bridge the gap between early Eocene climate change and faunal turnover. Carbon and oxygen isotopes of tooth enamel relate to behavior and dietary preferences, and may provide insight into changes in aridity and vegetation structure.

Here we present δ¹³C and δ¹⁸O values of tooth enamel from four mammalian taxa (Hyracotherium, Hyopsodus, Cantius, and Diacodexis) representing a range of ecological niches. We use laser ablation GC-IRMS to overcome the analytical challenge of small tooth size, and our approach is to track these four species at several localities spanning the early Eocene of the Willwood formation, sampling many individuals of each species at each locality. Carbon isotopes across all individuals, taxa and levels range between ‑16.7 and ‑10.7‰ (PDB), consistent with pure C₃ diets. Of note, we find no clear evidence of closed-canopy habitats, even for the presumably arboreal primate Cantius, in agreement with previous isotope work and consistent with inferences based on fossil leaf assemblages. Cantius has the highest δ¹⁸O values at each of the five localities studied so far, and appears to be an ‘evaporation sensitive’ taxon obtaining most or all of its water through feeding, like many extant arboreal primates. An ‘aridity index’ based on the isotopic difference between Cantius and Hyracotherium suggests aridification during the mid-early Eocene, coincident with cooling indicated by leaf margin analysis and oxygen isotopes in hematite. However, this pattern is based on five levels spanning ~2 Ma, and must be regarded as preliminary. As part of our goal to better understand the environmental context of biohorizons A & B, and peak global warmth in the late early Eocene, we plan to generate similar data from ~20 additional localities, and to develop a parallel carbonate clumped isotope soil temperature record.