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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.
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.
Derived from placoid scales, they have a thick coat of enamel, but without the underlying layer of dentin. These scales cover the fish's body with little overlapping. They are typical of gar and bichirs. Cycloid scales are small, oval-shaped scales with growth rings like the rings of a tree. They lack enamel, dentin, and a vascular bone layer.
For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons. [10] [11] Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other apes.
Aristotle (ca. 340 B.C.) may have been the first scientist to speculate on the use of hard parts of fishes to determine age, stating in Historica Animalium that “the age of a scaly fish may be told by the size and hardness of its scales.” [4] However, it was not until the development of the microscope that more detailed studies were performed on the structure of scales. [5]
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 ]
The scales of fish provide protection from predators at the cost of adding stiffness and weight. [48] Fish scales are often highly reflective; this silvering provides camouflage in the open ocean. Because the water all around is the same colour, reflecting an image of the water offers near-invisibility. [49]