Ontogenetic variations in Sr/Ca and Ba/Ca ratios of dental bioapatites from Bos taurus and Odocoileus virginianus

Sr/Ca and Ba/Ca ratios of bone are commonly used as biochemical indicators of trophic level in modern and fossil mammals. Concerns over the effects of diagenesis on Sr/Ca and Ba/Ca ratios of bone led archaeologists and paleontologists to favor tooth enamel, which is less prone to alteration. Sr/Ca and Ba/Ca ratios of bone, enamel, and dentin from three farm-raised steers (Bos taurus) and five wild white-tailed deer (Odocoileus virginianus) from central Missouri were compared. Our results show that changes in diet, discrimination, and growth rate during ontogeny can lead to significant differences in Sr/Ca and Ba/Ca ratios of different bioapatite types as well as significant differences within the same bioapatite forming at different times. Early- and late-forming tooth enamel can have significant differences in Sr/Ca and Ba/Ca ratios equivalent to almost one full trophic step. Although differences between early- and late-forming dentin are typically not significant, dentin Sr/Ca and Ba/Ca ratios are significantly greater than enamel values. This difference in Sr/Ca or Ba/Ca ratios between enamel and dentin from the same tooth can be greater than one full trophic step. These results have profound implications for the use of dental bioapatites in trophic level reconstructions. They highlight the importance of consistency in bioapatite selection, tooth selection, and relative location of sampling within the enamel cap. Furthermore, this expected difference in Sr/Ca and Ba/Ca ratios could be used as another means of checking for diagenetic alteration in ancient samples.