Tooth histotype and enameloid microstructure of Priohybodus arambourgi d'Erasmo , 1960 ( Elasmobranchii : Hybodontiformes ) from the Late Jurassic of northeastern Brazil

Sharks possess varied diets and different prey capture mechanisms. As a result, the group exhibits a significant diversity in dental morphology, also reflecting the internal structures of the teeth. Internal dental anatomy points to the existence of three histotypes in elasmobranch phylogeny: orthodont, osteodont, and pseudoosteodont. Priohybodus arambourgi, the oldest known hybodontiform to exhibit fully serrated edges on a multicuspidated tooth crown, a pattern observed in numerous modern sharks, was recently recorded in Upper Jurassic strata of the Tucano Basin, Northeastern Brazil. Understanding that tooth microstructural variations reveal a relevant tool for ecological and evolutionary investigations of extinct Elasmobranchii, this study presents the internal dental pattern of this hybodontiform through paleohistological analyses and scanning electron microscopy (SEM). As a function of derived tooth morphology, it was expected that P. arambourgi would share a dental pattern rather similar to some pseudoosteodont Neoselachii, with a pronounced osteodont condition sensu stricto and a complex enameloid. However, the results show the presence in P. arambourgi of pseudoosteodont dental layers and compact monocrystalline enameloid, with crystallites oriented perpendicular to the tooth surface in the outer regions and randomly arranged in the innermost regions of the tissue. The slender and faint enameloid layers seem to be ineffective to support such morphology, explaining the presence of extensive micro-scratches on enameloid and singular pattern of tooth wear. This study corroborates that the pseudoosteodont pattern remained conservative throughout the evolution of Hybodontiformes, even found in derived cutting teeth morphology, also indicating that P. arambourgi could have had a more generalist diet.