Widespread complex enameloid microstructure in teeth and rostral denticles of Cretaceous sclerorhynchoid skates (Rajiformes: †Sclerorhynchoidei)

Enameloid is a type of hard mineralised tissue covering teeth and dermal denticles of extant and extinct chondrichthyans. Understanding the diversity of enameloid microstructures across chondrichthyan clades is key to highlight potential phylogenetic or adaptive signals provided by histological characters. Among chondrichthyans, the enameloid microstructure of teeth and odontodes of batomorphs (skates and rays) has been understudied despite the high extant and past taxonomic and morphological diversities of this clade. Here, we focus on the sclerorhynchoid rays, a speciose extinct (Cretaceous) clade of skates that developed rostra bearing enlarged denticles in most taxa. We explore the enameloid microstructure of both teeth and rostral denticles of 13 species representing a wide range of morphologies. Our results show that the tooth enameloid microstructure of sclerorhynchoid skates achieved a complex arrangement, with a high diversity of bundled components. This microstructure is furthermore complexified by a strong degree of apico-basal variation in the organisation of the enameloid components. This organisation is the most complex known so far among batomorph teeth and probably elasmobranchs in general. Our results further confirm the presence of a similarly complex enameloid microstructure in the dermal denticles covering the elongated rostrum of these extinct skates. Most importantly, we provide evidence for the pervasiveness of this complex enameloid across rostral denticles of all studied taxa, despite the wide range of taxa and morphologies represented. The common enameloid organisation reported in both teeth and rostral denticles is, from base to the apex: a Single-Crystallite Enameloid (SCE) layer passing into Radial Bundled Enameloid (RBE) sometimes through a phase of protobundles as the enameloid thickens slightly higher in the crown, with apico-basal Parallel-Bundled Enameloid (PBE) appearing mixed with RBE bundles higher up at the base of the cusp, followed by the classical three-layered enameloid including SCE, PBE and Tangled-Bundled Enameloid (TBE) in the apex, crossed by rare Thick Radial Bundles (TRB). The presence of protobundles and RBE forming the entire enameloid thickness in the basalmost crown enameloid of teeth and rostral denticles analysed here was so far restricted to placoid scales and teeth of some hybodonts and batomorphs. Our study identified protobundles as transitional organisation between SCE and structured bundles and the RBE as an organisational stage preceding the PBE. We demonstrate that TRB are isolated RBE bundles and that both RBE and PBE bundles can participate in circumferential bundles whose development is linked with the size of the cutting edges. We found no evidence for a clear phylogenetic signal provided by the distribution of enameloid microstructure characters neither within teeth, nor among rostral denticles sampled here. Instead, we propose a possible link with geometric constraints (height, width), with SCE and RBE (with or without protobundles) being restricted to thin enameloid layers. Taken together, these results have strong implications for the understanding of enameloid character distribution in elasmobranch odontodes.