Stable carbon isotope composition of bone hydroxylapatite: significance in paleodietary analysis

Abstract

The stable carbon isotope composition of the structural carbonate derived from animal bone hydroxylapatite (δ13CB-HA) could record an animal’s diet. These records provide critical evidence for different paleontological disciplines, e.g., paleodiet analyses, and paleoclimate reconstructions. Compared to those of other body tissues, such as bone collagen or teeth enamel hydroxylapatite, δ13CB-HA values record information on the whole diet of an animal in its last years. δ13CB-HA can be applied to fossil animals of various body sizes. The δ13C analytical instruments available only require that prepared bone samples be approximately 2–5 mg for precise measurement, allowing δ13CB-HA analysis to be feasible on most vertebrate fossils without destructive sampling, especially on small mammals or birds whose teeth are not large enough for sampling or are lost. Moreover, δ13CB-HA can be used from different times or under less than ideal burial environments. For fossils dating back to Devonian or buried in hot and humid regions, dietary information has been completely lost in bone collagen during post-depositional processes but still remained in the δ13CB-HA values because hydroxylapatite is less influenced by diagenetic effects after deposition. In addition, systematic methods such as X-ray diffraction and Fourier transform infrared spectroscopy have been developed to qualitatively or semiquantitatively assess the influence of diagenesis on bone hydroxylapatite to ensure the credibility of the δ13CB-HA values. With the above merits, δ13CB-HA analysis is therefore becoming an increasingly important method in paleodiet-related research. Currently, applications of the δ13CB-HA method on fossil animals are primarily focused on two aspects, namely, paleodietary reconstruction of fossil animals with uncertain diets and paleoenvironmental reconstruction based on the δ13CB-HA values of fossil herbivores. The published researches, combined with our new results from early birds, demonstrate the considerable significance of the δ13CB-HA method in paleontological and paleoenvironmental research. Notably, the δ13CB-HA-based paleodietary analysis of early vertebrates, especially the large number of small birds or mammals discovered in the past decades would be an important work in the near future.