SERIAL SAMPLING OF MAMMALIAN ENAMEL DOES NOT EQUAL SEASONAL SAMPLING

The isotopic record preserved in mammalian enamel is a key source of data for reconstructing paleoenvironments and animal behavior. Serial sampling (i.e., drilling samples of enamel at increments perpendicular to the height axis of the tooth and parallel to the occlusal plane) is a prevalent method for assessing environmental seasonality or seasonality in the feeding behavior of fossil mammals. Most such studies assume comparatively minimal time averaging; few studies have attempted to empirically determine how much time is represented in these serial samples. Here, we employ a novel method to determine how much time is represented by serial samples from a published data set of late Pleistocene Equus and Bison teeth from Rancho La Brea, California. In common with other such studies, the original study estimated that each sample represented approximately one month of prism formation. To test this hypothesis, specimens were microCT scanned, sampling tracks or troughs were reconstructed, and dimensions of the enamel removed were measured for 157 samples. On average, each trough was found to have been sampled to a depth that removed approximately 114 days of enamel growth, and no sample represented fewer than 57 days. The depth of the troughs was always uneven, and the deepest portions of the trough sampled > 6 months of enamel prism formation. Given this amount of time averaging, the variability observed in the published values cannot necessarily be interpreted simply as seasonal variation. The high degree of time averaging in samples can also explain the lack of variation observed in some teeth. Thus, both claims of seasonal signal or lack of a seasonal signal in the data may be a function of sampling rather than climatic or behavioral processes affecting isotopic values. Our data highlight the necessity of considering taxon-specific rates of enamel prism formation and geometry of prism growth in designing and implementing sampling schemes for isotopic studies. Enamel growth rates and prism geometry should not be inferred from isotopic values or simply assumed from overall crown growth times, when they can be measured with greater precision using standard histologic methods.