Paleohistologia em dentes de hybodontiformes (Euselachii) : uma abordagem evolutiva

Abstract

Teeth constitute the main source of information about basal-divergent elasmobranchs, such as Hybodontiform sharks. In general, these teeth exhibit microstructural variations in their tissues (layer distribution), being classified into three histotypes: orthodont, osteodont, and pseudoosteodont. Among Hybodontiformes, such variations are phylogenetically broad, distributed throughout geological periods, and they remain still scarcely explored. Thus, this study aimed to investigate the microstructural changes in the teeth of two Hybodontiformes sharks: Priohybodus arambourgi, from the Aliança Formation (Tucano Basin, Upper Jurassic), and a hybodontiform of the Serra do Cadeado, from the Rio do Rasto Formation (Paraná Basin, Middle/Upper Permian). For this purpose, paleohistological description and comparison were carried out with Scanning Electron Microscopy (SEM) analyses. It was observed that both species exhibit a pseudoosteodont histotype, with significant variations in dentin layer thickness. P. arambourgi showed a higher concentration of osteodentin in the core, in addition to wear and micro-scratches on dental enameloid, suggesting a more generalist diet. The Serra do Cadeado taxon, such as P. arambourgi, also presents predominance of osteodentin, but with abundant and relatively prominent vascular canals, thicker orthodentin, and pronounced wear on the dental enameloid. The microstructural characteristics bring the Serra do Cadeado hybodontiform closer to certain genera, such as Hybodus, Acrodus, Crassodus, and Polyacrodus. The organization and orientation of fluorapatite crystallites in both taxa reflect functional adaptations related to diet, while the observed wear probably indicates functional teeth, with slower tooth replacement. These results reinforce the conservation of the pseudoosteodont histological pattern within Hybodontiformes and expand the evolutionary and ecological understanding of the group through dental microstructure.