First occurrence of Ischyodus sp. (Holocephali) in the locality of Les Ginestes (Occitania, France)

Holocephalans are cartilaginous fishes that are present in the fossil record since the Early Devonian. Most of the preserved remains consist of dental plates; i.e. mineralized elements that evolved from the fusion of separate teeth in the early chimaeroids during the Paleozoic. This transformation would strengthen the dental structure and therefore has been interpreted as an adaptation to durophagy. In both Mesozoic and Cenozoic sediments, the discovery of a high diversity of dental plates has been used to name a large number of taxa within a worldwide record. Here we describe both the morphology and the histology of a new Late-Jurassic specimen that we attribute to the genus Ischyodus, the first occurrence of this taxon in the locality of Les Ginestes (Occitania, South-Western France). Our observations revealed the presence of vascular pleromine both above the tritors (i.e. the surface that has been in contact with the preys) and within the larger internal vascular cavities that cross the dental plate. The expression of such a hypermineralized tissue indicates that they have a role in: 1) reinforcing the apical parts of the dental plate that are in direct contact with the hard prey 2) absorbing the consequent mechanical stress that is driven through the dental plate internal canals.