New insights into the enameloid microstructure of batoid fishes (Chondrichthyes)

Abstract

Chondrichthyan teeth are capped with a hypermineralized tissue known as enameloid. Its microstructure displays a hierarchical organization that has increased in structural complexity from a homogenous single-crystallite enameloid (SCE) in early Chondricthyans to the complex multilayered enameloid found in modern sharks (consisting of bundles of crystallites arranged in intriguing patterns). Recent analyses of the enameloid microstructure in batoid fishes, focused on Myliobatiformes and fossil taxa, point to the presence of a bundled (or fibred) multilayered enameloid, a condition proposed as plesiomorphic for Batoidea. In this work, we provide further enameloid analysis for a selection of taxa covering the phylogeny of batoids. Our SEM analysis shows a superficial layer of SCE, where individualized crystallites are clearly discernable, capping the teeth in most of the species studied. A bundled double-layered enameloid was found only in a Rhinoidei, Rhina ancylostoma Bloch & Schneider, 1801. We conclude that the most widespread condition among extant batoids is a monolayer SCE lacking microstructural differentiation, probably plesiomorphic at least for crown batoidea. We suggest that the complex bundled enameloid present in other batoids is a convergent character that has appeared repeatedly during the evolution of batoids, probably as a mechanical adaptation towards moderate durophagous diets.