Deep-time preservation of amino acids in mammalian fossil tooth enamel

Tooth enamel, primarily composed of bioapatite, is a promising archive of endogenous organic matter for studying ancient fauna. Despite its low organic content (~1%), protein residues have been identified in teeth up to 24 million years old. This study investigates the preservation of total hydrolysable amino acids (THAAs) in fossil enamel dating back as far as 48 million years. Modern and fossil enamel from large herbivorous mammals (Equidae, Rhinocerotidae, Proboscidea) across various taphonomic settings and Cenozoic periods reveal that AAs persist at least to the Eocene. The "intra-crystalline" organic fraction stabilizes after an initial rapid decline within the first 0.10 million years. Preservation appears independent of taphonomic context, and the relative abundance of amino acids is similarly variable in both modern and fossil samples. These findings demonstrate that enamel is a diagenetically robust substrate for long-term organic preservation, with significant potential for phylogenetic and ecological reconstructions in the fossil record.