ganoine

Ganoine is a hard, enamel-like tissue that coats the outer surface of the scales of certain ray-finned fishes, most notably the gar (Lepisosteiformes). This glossy layer is part of what is called a ganoid scale, one of the oldest and most distinctive scale types in living fishes. The ganoine surface gives the scale a rhomboid shape and a bright, iridescent appearance when preserved.

Ganoine is an acellular, highly mineralized tissue that sits on top of a bony base. In many Paleozoic forms, the ganoine layer lies over cosmine, a dentine-like tissue, with the interior of the scale composed of bone. The exact histology varies among lineages, but ganoine is consistently enamel-like in its hardness and gloss.

In biology and paleontology, ganoine serves to armor the fish, reduce wear on the scale, and aid in streamlined movement. Ganoid scales, with ganoine on the surface, fossilize well, making them useful for taxonomic and evolutionary studies of early ray-finned fishes. Among living fishes, gars retain ganoine-covered scales, whereas most other extant taxa have other scale types. The study of ganoine contributes to understanding the evolution of tooth- and scale-producing tissues in vertebrates.