Ganoid fish

In most biological nomenclature, a scale (Greek lepis, Latin squama) is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran (butterfly and moth) species, scales are plates on the surface of the insect wing, and provide coloration. Scales are quite common and have evolved multiple times through convergent evolution, with varying structure and function.

Scales are generally classified as part of an organism's integumentary system. There are various types of scales according to shape and to class of animal.

Fish scales are dermally derived, specifically in the mesoderm. This fact distinguishes them from reptile scales paleontologically. Genetically, the same genes involved in tooth and hair development in mammals are also involved in scale development.

True cosmoid scales can only be found on the extinct Crossopterygians. The inner layer of the scale is made of lamellar bone. On top of this lies a layer of spongy or vascular bone and then a layer of dentine-like material called cosmine. The upper surface is keratin. The coelacanth has modified cosmoid scales that lack cosmine and are thinner than true cosmoid scales.

Ganoid scales can be found on gars (family Lepisosteidae), bichirs, and reedfishes (family Polypteridae). Ganoid scales are similar to cosmoid scales, but a layer of ganoin lies over the cosmine layer and under the enamel. Ganoin scales are diamond shaped, shiny, and hard. Within the ganoin are guanine compounds, iridescent derivatives of guanine found in a DNA molecule. The iridescent property of these chemicals provide the ganoin its shine.

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