Abstract
The enameloid microstucture of chondrichthyan teeth has been studied for decades and it has proven to be a useful taxonomic tool. Changes in enameloid organization have been related to the emergence of new trophic strategies and Mesozoic radiation of the neoselachian crown group. However, in contrast to the abundance of these data on tooth enameloid, descriptions of chondrichthyan scale enameloid are almost nonexistent. The topology and microstructure of scale enameloid in particular euselachian groups: fossil Mesozoic Hybodontiformes and living neoselachians, including batoids and sharks, are described. It is shown that a thick layer of single crystallite enameloid (SCE) covers all studied scales. Although the enameloid of scales clearly does not reach high levels of microstructural differentiation present in the dental enameloid of some neoselachians, we found some degree of organization, such as oriented crystallites, differentiation into sublayers, and the presence of poorly structured sets of densely arranged parallel crystallites. As scales lack feeding functions of teeth, we suggest that the emergence of microstructural organization/differentiation of chondrichthyan enameloid can be understood as consequence of a self-organizing process rather than adaptive pressure.
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