Decoding Carbonate Concentrations and Substitutions in Tooth Enamel Across the Tree of Life Using Paired Infrared Spectroscopy and X-Ray Crystallography

Calcium phosphate constitutes the crystalline component of tooth enamel. Carbonate ions (CO3) are present in these crystals which alters their physical and chemical properties. We do not know whether carbonate concentrations vary across different organisms, or whether this variation is systematic based on evolutionary lineage. Using Infrared spectroscopy and X-ray crystallography on powdered enamel samples of different modern animals (mammals, reptiles, fishes) across the tree of life, we find correlations among carbonate content, fluorine content, and shape of the crystal lattice. Fluorine rich enamel in marine fish has lower carbonate contents and shorter lattices, while hydroxyl (OH) rich enamel in mammals, reptiles, and freshwater fish, has higher carbonate contents and longer lattices. Paleontologists use the chemistry of tooth enamel carbonate to reconstruct past environments and climates. Understanding an organism's original chemistry and crystal structure can inform the degree to which ancient tooth enamel has been altered during fossilization, and consequently potential biases to interpretations of past environments and climate. Paired Infrared and X-ray crystal structure data show that carbonate substitution in the crystal structure clusters across different regions of the evolutionary tree of life, and provides a chemical and structural baseline against which fossil enamel can be compared.