Diverse Forms of Denticles Facilitate the Evolutionary Success of Sharks

Abstract The ground sharks Carcharhiniformes are the most specious order of modern sharks, and although occupying ecologically diverse habitats, they share a common feature of having a variety of ridges on the crown of their skin teeth (dermal denticles). For the first time, we objectively quantify dermal denticle disparity using deep learning on scanning electron microscopy images, concluding ten morphogroups across modern sharks (Selachii). Seven of the ten morphogroups display these ridged denticles. Through nanoindentation, electron probe micro-analysis, and computational fluid dynamics, we establish mechanical properties, elemental composition, and swimming performance, and find that denticle ridges are highly variable and present a spectrum of functional properties beyond drag reduction. Further, we found a radiation event within the ground sharks around 170 MYA, resulting in the enormous diversity of the clade, for which ridged dermal denticles may have been key.