Placoid scales

The skin of sharks and rays is rough and scaly. It is dried and used as an abrasive, like sandpaper. In some Asian cultures, it has been used to decorate the handles of weapons and tools, and in making small decorative objects like snuff bottles and opium cases. If tanned like leather, sharkskin can be pliable and tough, making it suitable for shoes, boots, and other objects. Shagreen is shark or ray skin that is bumpy with placoid scales. This material is often dyed and used to make coin purses, game pieces, and other small objects. 

The natural skin is covered with denticles or bumps called placoid scales, which are unique to sharks, rays and skates. These consist largely of caldum carbonate and calcium phosphate and are mote mineralised than normal fish scales (which have a higher collagen content). The denticles appear as spots, some tiny, and some lai r and spiky. 

Elasmobranch fishes (sharks, rays, and skates) have a unique skin structure which has adapted for different species to serve various purposes. Their skin is covered with miniscule scales that are extremely similar to teeth (hard enamel outside with a pulpy inside) in which the base of each scale is embedded in the surface of the skin. These scales are known as placoid scales or denticles. Sharks (Fig. 1), in particular, have a chain of evolutionary development dating back 400 million years, with some of the youngest species of highly adapted fast-swimming sharks (e.g. the shortfin mako shark hums oxyrinchus) dating back approximately 8 million years. To meet the needs of various species, the microstructure of the denticles has adapted to serve four essential functions. The primary purpose for this bony skin is protection from predators and or prey as well as ectoparasites. A secondary purpose for most species is incorporation... 

Raschi W, Tabit C. (1992) Functional aspects of placoid scales A review and update. Aust ]Mar Freshwater Res Ay. 123—147. 

These scales show a number of similarities to later chondrichthyan scales in their differentiation into polyodontode crowns and clearly defined bases with well-developed vascular canals penetrating the neck region. All of these pre-date the widely accepted earliest chondrichthyan scales, which are simple placoid scales referred to the genus Elegestolepis from the Llandovery of Siberia (KaratujuteTalimaa 1973 1992). 

Nelson, G.J. (1970) Pharyngeal denticules (placoid scales) of sharks, with notes on the dermal skeleton of vertehrates , American Museum Novitates, 2415, 1-26. 

Williamson, W.C. (1849) On the microscopic structure of the scales and the dermal teeth of some ganoid and placoid fish. Philosophical Transactions of the Royal Society of London, 139 435-475. 

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