Analysis of differential X-ray attenuation as a method for correlating annual dentin increments in proboscidean tusks

Abstract

The tusks of woolly mammoths (Mammuthus primigenius) are composed of increments of dentin that form annually throughout an individual’s entire life. Analyses of tusks, however, are frequently limited to study of discrete samples, often in the form of cores that each contain only a subset of the total number of years recorded in a tusk. These cores should be taken at locations that are close enough together to ensure that adjacent cores share some of the same years, and for a comprehensive analysis, a series of such cores should sample all the years in a tusk. A method for correlating annual increments between cores therefore plays a critical role in reconstructing the entire series of years contained in a tusk. The density of tusk dentin is known to fluctuate annually. In this study, X-ray computed microtomography (microCT) scans of cores from the right tusk of the Yukagir mammoth show dramatic fluctuations of radiodensity associated with each year boundary. Mathematical correlation of these fluctuations (or density features) based on their unique pattern (or signature) of X-ray attenuation, expressed as luminance in three-dimensional reconstructions of microCT scans, is an effective way to correlate years between adjacent cores. The data described here demonstrate that correlation of density features between cores using attenuation signatures judged to be most similar (because they yielded the highest R-value when compared to candidate years in a neighboring core) supports the results of prior thin section analysis using daily and weekly increments to correlate years between cores. This study is a stringent test of this novel method of correlation as the Yukagir mammoth’s tusk was previously sampled at locations relatively far apart, and its success suggests that this method can be used on many tusks for which coring is the only option for study.