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Cosmine was first described in the Osteolepiform Megalichthys hibberti by Williamson in 1849, in a purely descriptive, pre-Darwinian, non-evolutionary framework. [4] Goodrich [5] expanded on Williamson's descriptions, hypothesizing a transition from a monoodontode scale (like a chondryicthian placoid scale) to a complex polyodontode scale through fusion of discrete units.
A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as well as possible hydrodynamic advantages.
Fish scales are dermally derived, specifically in the mesoderm. This fact distinguishes them from reptile scales paleontologically. 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.
Also part of the fish's integumentary system are the scales that cover the outer body of many jawed fish. The commonly known scales are the ones that originate from the dermis or mesoderm, and may be similar in structure to teeth. Some species are covered by scutes instead. Others may have no scales covering the outer body.
The fish has broad scales and a ... adding plant material and ... it is a major threat to some species of Tui chub due to it erasing the distinct genetic diversity ...
Existing fish groups featuring ganoin are bichirs and gars, but ganoin is also characteristic of several extinct taxa. [4] It is a characteristic component of ganoid scales . Ganoine is an ancient feature of ray-finned fishes, being found for example on the scales of stem group actinopteryigian Cheirolepis . [ 4 ]
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The fish have "flexible, armor-like scales" made up of "a hard, mineralized outer layer" and "a tough-but-flexible inner layer" that help protect it from attacks by piranhas. [12] The scales, which are typically 5–7cm but can reach 10cm in larger individuals, are built from collagen fibers in a layered structure.