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

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.