Carcharopsis prototypus and the adaptations of single crystallite enameloid in cutting dentitions

Carcharopsis is a Palaeozoic shark comprising an enameloid-covered cutting dentition comprising serrated teeth. The enameloid ultrastructure of the teeth of C. prototypus is examined for the first time and consists of a 35 μm thick layer of tightly compacted apatite crystallites with a maximum individual length of 0.1 μm. The crystallites are randomly-oriented toward the base of the enameloid layer, but assume a roughly parallel orientation higher up in the enameloid, with their long axes arranged normal to the tooth surface. The enameloid of typical hybodonts comprises a compact outer crystallite layer and a looser, bundled inner layer. The Cretaceous hybodonts ,  Priohybodus arambourgi and Thaiodus ruchae , by contrast, have a compact, less differentiated enameloid very similar to that of C. prototypus . This suggests that crystallite compaction is a corollary of the evolution of serrated dentitions possessing single crystallite enameloids.