Miocene Continental Paleoclimate and Fossil Horse Evolution:Trace Element and Stable Isotope Geochemistry of Enamel- Phosphate

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We propose to explore the potential for linking mid-Miocene paleoclimate and horse evolution by establishing the systematics of trace elemental and isotopic preservation in the bone and tooth phosphate of North American fossil horses. We will test whether enamel phosphate provides quantitative information about changes in paleoenvironment (temperature, hydrology), paleodiet (graze vs. browse), and evolution (the mid-Miocene radiation of horses), or if bone and tooth phosphate is diagenetically opaque. This preliminary work could catalyze new avenues for unraveling Miocene continental paleoclimate changes, an important period of global climatic deterioration with scant quantitative paleogeochemical information from continental records. Fossil horses are particularly suitable for study because they are geographically widespread and common in fossil assemblages from the Eocene to late Pleistocene and have modern analogues. Horses underwent an explosive adaptive radiation and presumed dietary shift in the middle Miocene (16-12 Ma) perhaps responding to paleoclimatic changes and the spread of savannah grasslands. This climatic shift is inferred from paleoceanographic data; the dietary shift, from studies of the functional morphology of teeth. We wish to determine if a paleodietary shift is evident in the chemistry of the fossil teeth; if the continental paleoclimate record corroborates the marine record in magnitude, direction, and timing; and if paleoclimatic change occurred concurrently with paleoenvironmental and evolutionary changes. In this preliminary venture, we will explore the effects of diagenesis, inter- and intraspecific variations, and characterize the del 18Opo4 of fossil horse teeth and bones from the Burge Quarry, one of the richest and best-preserved middle Miocene (-12 Ma) horse faunas. Diagenesis will be evaluated with trace element, SEM, and XRD analyses of enamel phosphate and compared to modern horses. If diagenesis proves to be negligible, del 18Opo4 of tooth-enamel in modern and fossil teeth will establish the suitability of reconstructing the continental paleoclimate record.