Combined Ca, Sr isotope and trace element analyses of Late Cretaceous dinosaur teeth: assessing diet versus diagenesis

The Sr and Ca isotope composition, along with trace element content in fossil teeth, provides valuable insights into biogenic and diagenetic processes. Identifying pristine biological signals is crucial for reconstructing the diet and trophic levels of extinct taxa. We present novel geochemical data from Tyrannosauridae and Ceratopsidae teeth of the Late Cretaceous, using radiogenic Sr (⁸⁷Sr/⁸⁶Sr), stable Sr (δ^88/86Sr), and Ca (δ^44/42Ca) isotopes, along with trace elements abundances to differentiate biogenic signals from diagenetic alteration.

Our results reveal potential taxon-specific diagenetic effects, likely influenced by enamel microstructure. Tyrannosaurid enamel contains lower concentrations of rare earth elements (REE) and uranium (U) than dentine, whereas ceratopsid teeth typically exhibit higher REE and U compared to both the enamel and dentine of tyrannosaurids. Enamel δ^44/42Ca values differ significantly between herbivorous ceratopsids and carnivorous tyrannosaurids, reflecting trophic level effects seen in modern mammals and reptiles. A positive correlation between δ^44/42Ca and δ^88/86Sr suggests partial preservation of biological fractionation along the trophic chain. Yet, the lack of negative δ^88/86Sr values in our dataset – typically expected in biologic tissues – suggests alteration by diagenetic processes of both stable and radiogenic Sr. While δ^44/42Ca in enamel likely remains a reliable dietary proxy, Sr isotope composition of our samples appears then to be significantly altered. The presence of high δ^88/86Sr in terrestrial fossil teeth could serve as a novel diagenetic proxy to assess habitat related ⁸⁷Sr/⁸⁶Sr values, aiding provenance and mobility studies in fossil ecosystems.