Ganoid

Ganoid refers to a type of fish scale and, in historical usage, to a broad, now largely obsolete grouping of early ray-finned fishes. In ichthyology, ganoid scales are thick, bony, and covered with ganoine, an enamel-like tissue that gives a glossy surface. The scales are typically rhomboid or diamond-shaped and interlock tightly through peg-and-socket joints, producing a durable armor-like cover.

Extant fishes with ganoid scales include the bichirs (Polypteriformes) and gars (Lepisosteiformes). These groups retain the primitive ganoid scale type, though the scales can be reduced or modified in some species. The term ganoid is also used, especially in older literature, to describe a wide assemblage of primitive ray-finned fishes possessing ganoine-covered scales. That broader grouping, often called ganoids, is now considered paraphyletic and is not recognized as a formal taxonomic unit.

In the fossil record, many Paleozoic and Mesozoic actinopterygians exhibit ganoid scales, indicating that ganoine-covered scales were common in early ray-finned fishes. Over evolutionary time, many lineages developed lighter, more flexible scale types such as cycloid or ctenoid scales, making ganoid scales less widespread among later fishes.

The ganoine layer and the distinctive scale shape have made ganoid scales a key feature in discussions of early actinopterygian biology and the evolution of fish integument. They also contribute to the historical distinction between more primitive and more derived scale morphologies in the lineage leading to modern teleosts.