Enameloid microstructure of the serrated cutting edges in certain fossil carcharhiniform and lamniform sharks

Abstract

The triple-layered enameloid organization of neoselachian teeth has proven to be a reliable systematic character of the group. This study uses scanning electron microscopy to investigate the orientation of the parallel enameloid bundles in the area of the serrated cutting edges in certain fossil elasmobranchs. The examined teeth come from two Upper Cretaceous Squalicorax species and the Upper Miocene carcharhiniforms Galeocerdo sp., Carcharhinus sp., and Hemipristis serra. The parallel bundles are revealed by surface etching, which removes the superficial shiny-layered enameloid. In the teeth of Squalicorax, the bundles around the cutting edge bend once, before they reach the serrations. The studied carcharhiniform species show a more complicated pattern with a change of parallel bundle course inside the serrations. H. serra teeth do not display the first bending of the bundles, whereas it was present in the other two carcharhiniforms. The course of the crystalline bundles in both Squalicorax species is not affected by the presence of the serrations, regardless of the twofold difference in tooth size between them. In the carcharhiniform species, the bended bundles occur within the primary and secondary serrations and are always associated with them. This feature might have functional significance by strengthening the cutting edge or could simply develop as a consequence of the enameloid mineralization around the individual serrae.

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

“…Nevertheless, some species like W. bodeuri and P. pockrandti have an RCEL of which the R1 unit is much more developed and can reach up to 15 μm in cross section. This feature has already been reported by Cuny and Risnes (2005) from sections of teeth of the Upper Triasic Synechodontiforme Romphaiodon minor (Agassiz, 1837) and by Andreev (2010) from Squalicorax (Palaeoanacorax?) sp., whereas the classical RCEL has been described by Cuny and Risnes (2005) in P. eocaenus (Palaeocene), Striatolamia macrota (Agassiz, 1843; Lamniforme, Eocene), and by Gillis and Donoghue (2007) in the Cretaceous Carcharias sp.…”

Development and microstructure of tooth histotypes in the blue shark, Prionace glauca (Carcharhiniformes: Carcharhinidae) and the great white shark, Carcharodon carcharias (Lamniformes: Lamnidae)

“…1) consider that a triplelayered enameloid is synapomorphic only for Selachiomorpha. The structure and composition of the three layers of enameloid in carcharhiniform and lamniform sharks have been documented (e.g., Reif 1973;Andreev 2010;Enax et al 2012), but developmental regulatory pathways leading to their differentiation have yet to be described in detail. Reif (1973 p. 234) was the first to define three layers of enameloid, identifying the layers as the shiny or "single crystal 'enamel,'" "parallel-fibered 'enamel,'" and the "haphazardly fibered 'enamel.'"…”

Microvertebrates from multiple bone beds in the Rhaetian of the M4–M5 motorway junction, South Gloucestershire, U.K.

“…Hybodontiformes in their serrations (Andreev, 2010). Teeth of this species were classified 435 under Selachii as Pseudodalatias according to their single-crystalline enameloid (Reif, 1978).…”