Biologic and climatic signals in the oxygen isotopic composition of Eocene-Oligocene equid enamel phosphate

Oxygen isotope results for tooth enamel-phosphate (δP) from late Eocene-early Oligocene fossil horses are presented to determine if paleobiologic and paleoclimatologic information is recorded in fossil tooth enamel chemistry. Teeth from jaws of Mesohippus and Miohippus from the White River Formation (or Group) in the western Great Plains are well preserved and have excellent geochronologic control. Although there is clear evidence for post-depositional alteration of the enamel, a hydroxylapatite mineralogy is preserved and isotopic exchange of oxygen does not appear to be significant. There are distinctive patterns of δP variation among individual teeth from the same jaw. These patterns reflect season of birth and timing of enamel mineralization. Most of the horses were born in the spring, and mineralization of the enamel is complete after 1–1.5 years. These results show that paleoclimate reconstruction from enamel δP must account for tooth position and timing of mineralization. The observation that there are different intrajaw patterns of δP variation among Mesohippus and Miohippus may provide a basis to reconstruct changes in climate seasonality in ancient environments.