Mercury concentration: A new window into ancient diets and trophic levels in vertebrate paleontology

Direct evidence about feeding habits and trophic levels of extinct species is scarce in the fossil record. Typically, such information is based on geochemistry, stomach contents, anatomy, and, whenever possible, comparison with living counterparts. Despite the challenges involved, such reconstructions are vital for understanding the structure of past ecosystems. Here, we propose a novel geochemical approach in paleontology that uses total mercury concentration (THg) in fossil vertebrate tissues (bones and tooth enamel) and its bioaccumulation and biomagnification properties as geochemical indicators for the inference of feeding habits and trophic levels. Thin section and X-ray diffractometry (XRD) revealed that THg values found in bioapatite from the diverse and well-preserved fossil vertebrates of the Upper Cretaceous Bauru Group (Brazil) are not altered by diagenetic processes. The results show that each vertebrate group analyzed (e.g., fishes, turtles, amphibians, crocodylomorphs, avian, and non-avian dinosaurs) are characterized by different THg concentrations (ranging from 1.66 to 15 ng.g −1 Hg). Notably, similar values were found in fossils of the same taxonomic group regardless of their geological unit of origin. More importantly, taxa with different feeding habits were positioned at different trophic levels based on the bioaccumulation and biomagnification of Hg. Reliable Hg analyses in fossils require confirming that bones are free from diagenetic contamination that may artificially elevate their Hg concentrations. • Hg levels in Bauru Group vertebrate fossils indicate their trophic positions. • Some sphagesaurids show low Hg levels equivalent to herbivores. • Baurusuchids and abelisaurids were Bauru Group top predators.