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

Abstract

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.

Chondrichthyan tooth enameloid: past, present, and future

“…However, the number of taxa analyzed in the literature remains very low compared to the overall taxonomic diversity of this group (including modern and fossil taxa) and the tooth enameloid microstructure of the vast majority of shark families remains to be explored. Although the ‘triple‐layered’ enameloid is characteristic of shark teeth, this apparent conservative structure appears to be absent in basal selachimorphs (Andreev & Cuny, ) and the ‘triple‐layered’ enameloid of derived shark teeth is complexified by a number of additional structures (Cuny & Risnes, ; Guinot & Cappetta, ), which are summarized below.…”

“…Methods for etched section studies include embedding the teeth in transparent polyester or epoxy resin (Cuny & Risnes, ; Gillis & Donoghue, ; Guinot & Cappetta, ; Enault et al ., ). Depending on the size of the teeth, the resin blocks can then be cut (transversely or longitudinally) using a diamond saw and/or ground using a lapidary and abrasive paper until the desired plane of section is reached.…”

“…Published enameloid microstructure analyses are frequently based on either sections or surfaces, yet a combination of both seldom occurs in the literature; but see also Guinot & Cappetta (). However, we argue that both are actually necessary to gain a comprehensive understanding of the enameloid microstructure of a given taxon.…”

The enameloid microstructure of euselachian (Chondrichthyes) scales

“…The enameloid microstructure of chondrichthyan teeth has repeatedly been studied since the early work of Reif (1973), focusing principally on euselachian sharks (Gillis and Donoghue, 2007 and references therein), and emerged as a useful tool for taxonomic studies of the group (e.g., Reif, 1977Reif, , 1978Duffin, 1980;Cuny et al, 1998;Cuny and Risnes, 2005;Botella et al, 2009a;Guinot and Cappetta, 2011). In fact, changes in the organization of tooth enameloid can be related to the evolutionary history of Chon drichthyes.…”

New Triassic stem selachimorphs (Chondrichthyes, Elasmobranchii) and their bearing on the evolution of dental enameloid in Neoselachii

“…Here we present a modified list of dental characters that are considered to apply to Synechodus, although only full revision of the genus can resolve the affinities of preJurassic forms affiliated with it. These include: crown cusps not distinctly separated from each other, crown base overhanging the crown-root junction, presence of baso-labial root concavity, pseudo-polyaulacorhize type of root vascularization occasionally changing to anaulacorhize in lateral or posterior tooth files, clutching-type dentition, gradual decrease of tooth height toward the commissure, enameloid layer containing shiny-layered enameloid (SLE), and well-structured parallel-bundled enameloid (PBE) with developed thick radial bundles (sensu Guinot and Cappetta, 2011).…”