A Tale of Two Quarries: Biologic and Taphonomic Signatures in the Oxygen Isotope Composition of Tooth Enamel Phosphate from Modern and Miocene Equids

Abstract

The oxygen isotope composition of tooth enamel phosphate (68) 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 8p offossil teeth. Three distinct patterns of 8p variation are found in modern and fossil jaws. Each pattern can be related to the season of birth. An oxygen isotope mass balance model that incorporates seasonality and nursing offoals during tooth enamel mineralization is used to interpret the 8p pattern along the toothrow. The range of 8p for the same tooth position among Merychippus primus jaws from Thomson Quarry is relatively small (2.2%o), and the cheek tooth 8p 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 8p for the same tooth position among Pseudhipparion retrusum jaws from Burge Quarry is larger (4%o), and the cheek tooth 8p patterns are not similar among individual jaws. This is consistent with an attritional accumulation over a long period of time. Such wide ranges in 8p limit application to continental paleoclimate reconstruction because of the low signal to noise ratio. INTRODUCTION