The homology and phylogeny of chondrichthyan tooth enameloid

Abstract

A systematic SEM survey of tooth microstructure in (primarily) fossil taxa spanning chondrichthyan phylogeny demonstrates the presence of a superficial cap of single crystallite enameloid (SCE) on the teeth of several basal elasmobranchs, as well as on the tooth plates of Helodus (a basal holocephalan). This suggests that the epithelial-mesenchymal interactions required for the development of enameloid during odontogenesis are plesiomorphic in chondrichthyans, and most likely in toothed gnathostomes, and provides phylogenetic support for the homology of chondrichthyan and actinopterygian enameloid. Along the neoselachian stem, we see a crownward progression, possibly modulated by heterochrony, from a monolayer of SCE lacking microstructural differentiation to the complex triple-layered tooth enameloid fabric of neoselachians. Finally, the occurrence of fully-differentiated neoselachian enameloid microstructure (including compression-resistant tangle fibered enameloid and bending-resistant parallel fibered enameloid) in Chlamydoselachus anguineus, a basal Squalean with teeth that are functionally ''cladodont,'' is evidence that triple-layered enameloid microstructure was a preadaption to the cutting and gouging function of many neoselachian teeth, and thus may have played an integral role in the Mesozoic radiation of the neoselachian crown group.

Structure, composition, and mechanical properties of shark teeth

“…Studies combining different analytical methods were reported on shark teeth, sometimes on fossilized shark teeth (Daculsi and Kerebel, 1980;Dahm and Risnes, 1999;Frazzetta, 1988;Gillis and Donoghue, 2007;Glas, 1962;Kesmez et al, 2004;LeGeros and Suga, 1980;Moeller et al, 1975;Powlik, 1995;Preuschoft et al, 1974;Reif, 1974;Whitenack et al, 2011. Here we report on a comprehensive study of the chemical composition, the (ultra)structure, and the micromechanical properties, separately for dentin and enameloid, of two shark species (Isurus oxyrinchus, shortfin mako shark; and Galeocerdo cuvier, tiger shark).…”

Mechanical properties and adaptations of some less familiar bony tissues

“…They often have a tissue type called enameloid which seems to consist of elements derived from both dentinal (collagenbased) and enamel (enamelin-based) organic tissues. Indeed, there seems to be a spectrum from 'pure' dentine to 'pure' enamel (see Gillis and Donoghue, 2007, for a review) and the situation is, like much of biology, a bit of a mess. Petrodentine, found in lungfish (Dipnoi) is, however, undoubtedly dentine, but it is most unusual.…”

Histology of the heterostracan dermal skeleton: Insight into the origin of the vertebrate mineralised skeleton

“…There are no odontoblast lacunae. The enameloid is up to 50 lm thick and aside from the permeating ondontoblast canaliculi, is structureless, compatible with its interpretation as single crystallite enameloid (SCE) sensu Gillis and Donoghue (2007). The superficial layer of both the scales and the cephalothoracic shield is expanded peripherally by marginal accretion of new tubercles, producing lines of arrested growth.…”