placoid

Placoid refers to a type of dermal scale found in cartilaginous fishes, including sharks, rays, skates, and chimaeras. Also called placoid scales or dermal denticles, these tooth-like structures cover the skin and are homologous with teeth, sharing similar tissues and developmental origins.

Each placoid scale consists of a dentine core capped by an enamel-like outer layer called enameloid, with a central pulp cavity connected to underlying dermal blood vessels. The scales are embedded in the dermis and anchored by a network of collagen fibers. The structure provides rigidity and helps reduce skin-surface friction, contributing to streamlined movement through water.

Functionally, placoid scales modify skin texture, reduce hydrodynamic drag, and protect against abrasions. The pattern, size, and orientation of denticles vary among species and can reflect habitat and swimming style. Evolutionarily, placoid scales are a primitive characteristic of cartilaginous fishes and contrast with the various scale types found in bony fishes, such as cycloid and ctenoid scales.

In paleontology, the enamel-like enameloid and dentine in placoid scales fossilize fairly well, allowing researchers to identify early sharks and related fishes from fossilized remains even when soft tissue is not preserved. Placoid scales thus provide key insights into the biology, ecology, and evolution of cartilaginous fishes.