Diverse mechanical properties, composition, and performance of shark dermal denticles.

Sharks have roamed the oceans since the Devonian, surviving multiple mass extinctions. Today, sharks are agile, streamlined swimmers, ambush predators hiding beneath the sand, and giants cruising in slow motion. Impressively, they are all covered in an armour of tiny skin teeth (dermal denticles). We quantify dermal denticle disparity using deep learning on scanning electron microscopy images, discerning ten morphogroups across modern sharks (Selachii). Seven of the ten morphogroups display 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 identify a potential correlation between a radiation event within the ground sharks Carcharhiniformes around 170 MYA, resulting in the enormous diversity of the clade, and ridged dermal denticles.