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The advent of biomineralization represented a key evolutionary innovation, enabling vertebrates to form skeletons and teeth made of bioapatite, a nanocrystalline apatite phase with variable chemical composition. This adaptability has driven vertebrate morphological diversity and ecological success. Bioapatite structure is shaped by metabolic processes, with ionic substitutions and vacancies influencing its crystallographic parameters. Major elements alter the a and c parameters of the bioapatite cell, while trace elements have minimal impact. Rare Earth Element (REE) signatures in fossil bioapatite, once considered direct proxies for paleo-seawater, may reflect complex diagenetic histories. This study examines bioapatite crystallinity and REE patterns in Recent and fossil catsharks (Scyliorhinidae s.l. ) using μ-XRD and LA-ICP-MS. Recent teeth and scales of Galeus melastomus and Scyliorhinus canicula from the Italian coasts were tested and compared with fossil Eocene to Pliocene counterparts from strata across Europe. Our results underscore the value of crystallographic parameters in tracing both biological and post-mortem transformations in bioapatite. In particular, diagenetic processes led to organic matter loss and increased carbon incorporation, especially in tooth regions that were originally rich in organics. Fossil samples showed reduced a lattice parameters and unit cell volumes, increased c parameters, and enhanced crystallinity. Tooth crowns had higher crystallinity and volume than roots, reflecting lower organic contents. Crystallographic differences between scales and teeth suggest functional adaptations, with teeth exhibiting variable crystallite sizes for biomechanical efficiency. Diagenetic effects are seemingly independent of geological age, being rather influenced by depositional conditions and, possibly, by original physiological traits.
Medici et al. (Thu,) studied this question.