Placoid scales aka dermal denticles are found in cartilaginous fishes such as sharks, rays, and chimaeras.
They are also structurally homologous with vertebrate teeth (“denticle” translates to “small tooth”), having a central pulp cavity supplied with blood vessels, surrounded by a conical layer of dentine, all of which sits on top of a rectangular basal plate that rests on the dermis.
The shape of denticles is specific to individual species and cannot grow in size, but rather more scales are added as the fish increases in size.
- SEM image of Banded Wobbegong Shark (Orectolobus ornatus), Brier Shark (Deania calcea), Tiger Shark (Galeocerdo cuvier) and Gummy Shark (Mustelus antarctius) denticles.
- all photographs by Sue Lindsay/ Australian Museum