Paleo-evo-devo implications of a revised conceptualization of enameloids and enamels

Understanding the origin and evolution of the mineralized skeleton is crucial for unravelling vertebrate history. However, several limitations hamper our progress. The first obstacle is the lack of uniformity and clarity in the literature for the definition of the tissues of concern, especially of enameloid(s) and enamel(s), resulting in ambiguous terminology and inconsistencies among studies. Moreover, the identification criteria currently employed to characterize hypermineralized tissues in extinct taxa, such as the presence or absence of tubules for enameloids, may lead to unsupported conclusions. We suggest that comparative developmental studies may be key to unambiguous terminology, truly diagnostic identification criteria and developmentally informed evolutionary hypotheses. We exemplify this approach by: (i) introducing a new conceptual framework for enameloid(s) and enamel(s), with clear terminologies, definitions and interactions between concepts; (ii) suggesting more rigorous ways to identify tissues, based on the observation of defining or additional properties, as well as on the comparison of developmental scenarios when possible; (iii) constructing a clear phylogenetic framework to discuss their homologies and highlighting possible transitions between these tissues; and by (iv) proposing developmental models that explain both enamel and enameloid formation, and suggest possible transitions between them.