Department of Geology, Paleontology and Fossil Fuels, Sofia University St. Kliment Ohridski, Sofia, Bulgaria.
Microsc Res Tech. 2010 Jul;73(7):704-13. doi: 10.1002/jemt.20811.
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.
软骨鱼类牙齿的三层釉质组织结构已被证明是该类群的一个可靠系统特征。本研究采用扫描电子显微镜观察了两种上白垩统尖棘鲨属物种以及上中新世的真鲨目物种姥鲨、沙虎鲨和半锯鲨的锯齿状切缘区域平行釉质束的取向。研究的牙齿来自于两种上白垩统尖棘鲨属物种以及上中新世的真鲨目物种姥鲨、沙虎鲨和半锯鲨。通过表面蚀刻揭示了平行束，表面蚀刻去除了表面闪亮的釉质层。在尖棘鲨的牙齿中，在到达锯齿之前，围绕切缘的束弯曲一次。研究的真鲨目物种显示出更复杂的模式，在锯齿内部平行束的方向发生变化。H. serra 的牙齿没有显示束的第一次弯曲，而其他两种真鲨目物种则有。两种尖棘鲨属物种的晶体束的方向不受锯齿的影响，尽管它们的牙齿大小相差两倍。在真鲨目物种中，弯曲的束发生在初级和次级锯齿内，并始终与它们相关。这个特征可能通过增强切缘的强度而具有功能意义，或者可能仅仅是由于单个锯齿周围的釉质矿化而发展起来的。
