Enameloid microstructure of some Cretaceous Hexanchiformes and Synechodontiformes (Chondrichthyes, Neoselachii): new structures and systematic implications

Modern neoselachian sharks may be separated from more basal relatives by the presence of tooth enameloid comprising three layers. Although enameloid microstructure studies were mostly used in the aim of differentiating supposed basal neoselachians from hybodonts, differences in the enameloid organization among neoselachians have been recognized suggesting the potential for use of enameloid microstructure as a phylogenetic tool within the neoselachian sharks. The enameloid microstructure of five taxa of neoselachian sharks belonging to two orders, the Hexanchiformes and Synechodontiformes, has been studied. The Hexanchiformes are a monophyletic order with extant representatives, whereas the extinct Synechodontiformes have been considered as monophyletic, paraphyletic, or polyphyletic by different authors. This study has revealed numerous new enameloid microstructures such as amalgamated crystallites in the internal units [parallel-bundled enameloid (PBE) and tangled-bundled enameloid], cavities at the shiny-layered enameloid (SLE)/PBE boundary, radial furrows in the PBE, as well as different original organizations at the base of the crown and in the PBE at the level of the cutting edges. Pachyhexanchus pockrandti (Hexanchiforme) and Welcommia bodeuri have the most of the features in common and they share more characters with those of Paraorthacodus sp. and Sphenodus sp. than with Synechodus sp.