A tale of two quarries: Biologic and taphonomic signatures in the oxygen isotope composition of tooth enamel phosphate from modern and miocene equids

The oxygen, isotope composition of tooth enamel phosphate (delta(P)) from cheek teeth in jaws of modern equids is compared with that of fossil equids from Thomson and Burge quarries (Miocene, Nebraska) to determine if paleobiologic and taphonomic signatures are preserved in the delta(P) of fossil teeth. Three distinct patterns of delta(P) variation are found in modern and fossil jaws. Each pattern cart be related to the season of birth. An oxygen isotope mass balance model that incorporates seasonality and nursing of foals during tooth enamel mineralization is used to interpret the delta(P) pattern, along the toothrow. The range of delta(P) for the same tooth position. among ''Merychippus'' primus jaws from Thomson Quarry is relatively small (2.2 parts per thousand), and the cheek tooth delta(P) pattern is similar among individual jaws. This is comparable to a modern living population and consistent with a taphonomic interpretation of catastrophic accumulation for the Thomson ''M.'' primus population. The range of delta(P) for the same tooth position. among Pseudhipparion retrusum jaws from Burge Quarry is Larger (4 parts per thousand), and the cheek tooth delta(P) patterns are not similar among individual jaws. This is consistent with an attritional accumulation over a long period of time. Such wide ranges in delta(P) limit application to continental paleoclimate reconstruction because of the low signal to noise ratio.