Revealing enameloid shark chemistry at the nanoscale

Shark teeth are considered excellent bio-archives because of their high abundance and preservation potential. Chemical proxies recorded by the teeth enameloid layers are used to interpret ecological and environmental parameters throughout the geological record. The use of these proxies relies on the assumption that biomineralization processes for enameloid formation have remained constant during shark evolution. Here, we test such an assumption by comparing the chemical composition at the nanoscale, using the technique of atom probe tomography (APT), of enameloid in modern and fossil shark teeth. Results indicate that there are clear differences in the chemistry at the core and inter-crystalline grain boundaries of fluorapatite crystals. These boundaries are enriched in strontium in all shark teeth, whereas there are differences in the distribution of magnesium, sodium, and iron. Teeth of the modern shark Isurus oxyrinchus have magnesium and sodium distributed at the inter-crystalline grain boundaries. Teeth of Eocene fossil sharks, Striatolamia macrota and Macrorhizodus praecursor, have a unique distribution of iron, at the inter-crystalline boundaries, and sodium, at the core of the crystals. This observation may indicate that biomineralization processes resulting in enameloid formation are not constant across the phylogeny of sharks. Overall, our findings strongly suggest that the enameloid content and distribution of magnesium, iron, strontium, and sodium are highly controlled by biomineralization processes. The role of magnesium and sodium seems to be similar in mammalian enamel and shark enameloid formation. Yet, nanoscale chemical differences, such as the presence of strontium in tooth enameloid, are likely associated to functional morphology.