In pure C3 ecosystems, enamel d13C reflects habitat preference (i.e., degree of vegetation openness) and/or plant water-stress. Similarly, bone d13C tracks trends in vegetation and aridity. Enamel d18O mainly depends on rainwater composition with a minor humidity effect. Both soil water composition and soil temperature determine instead bone CO3 d18O. Hence, combined enamel and bone CO3 d18O values may be used to distinguish changes in water composition from changes in temperature.
Bone d13C shows no change between 35.7 and 35.0 Ma, with an average (±1 SE) value of -6.97±0.05‰ (V-PDB). In contrast, average enamel d13C increases from -9.08±0.06‰ to -8.54±0.10‰ at ~35.4 Ma; this could reflect an increase in aridity or simply differences in dietary and habitat preferences between Subhyracodon and Leptomeryx. Values for CO3 d18O abruptly increase by 3.9‰ for enamel (from 21.04±0.12‰ to 24.95±0.41‰, V-SMOW) and 1.7‰ for bone (from 20.47±0.10‰ to 22.19±0.11‰), at ~35.4 Ma, coincident with a lithologic change from claystones and conglomeratic sandstones to tuffaceous siltstones. These data possibly indicate an increase in aridity and/or mean annual temperature, and might reflect the climatic effect of the late Eocene extraterrestrial impacts or the opening of the Drake Passage and consequent strengthening of the Antarctic Circumpolar Current.