placoideos

Placoid oideos, commonly referred to as placoid scales or dermal denticles, are the tooth‑like skin scales of cartilaginous fishes (Chondrichthyes), including sharks, rays, and chimaeras. They cover the body and contribute to the streamlined surface that reduces drag as the animal moves through water.

Structure and composition: Each placoid scale has a cusp that protrudes through the epidermis. The outermost layer is enameloid (an enamel‑like substance), beneath which lies dentine with tubules, surrounding a pulp cavity. The scale is anchored to the dermis by a basal connective tissue plate, forming a compact, overlapping mosaic that remains attached to the animal as it grows.

Development and growth: Placoid scales develop in the dermis and erupt through the epidermis. They grow by deposition of dentine and enameloid over time, keeping the skin protected while maintaining a smooth surface that minimizes hydrodynamic resistance.

Function and significance: The denticles reduce turbulence and surface friction, enhancing swimming efficiency and protection against abrasion. The microstructure of placoid scales has been a focus of biomimetic studies, inspiring designs that mimic shark skin to reduce drag and biofouling.

Evolution and classification: Placoid scales are a primitive feature of cartilaginous fishes and help distinguish Chondrichthyes from bony fishes, which possess elasmoid, cosmoid, or ganoid scales. Placoid scales should not be confused with placoderms, an extinct group of armored jawed fishes.

Note: Placoid scales are characteristic of modern sharks, rays, and related species, and their fossilized denticles provide information about ancient marine environments and the evolution of vertebrate skin.