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Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size. Similar scales can also be found under the head of the denticle herring . The amount of scale coverage is much less in rays.
These scales, also called denticles, are similar in structure to teeth, and have one median spine and two lateral spines. The modern jawed fish ancestors, the jawless ostracoderms and later jawed placoderms, may have had scales with the properties of both placoid and ganoid scales.
Placoid Scale. Super Smooth scales (dermal denticles) coat the skin of sharks, rays, and cartilaginous fishes due to the absence of dermal bone. These scales are present in the dermis, which has fibrous connective tissue components, and project through the epidermis, which contains secretory cells and stratified epidermal cells, to the surface.
Ganoid scales are flat, basal-looking scales. 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 ...
A large portion of the skate's dorsal body is covered by rough skin made of placoid scales. Placoid scales have a pointed tip that is oriented caudally and are homologous to teeth. Their mouths are located on the underside of the body, with a jaw suspension common to Batoids known as euhyostyly. [11]
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.
Bony fish do not have placoid scales like cartilaginous fish, instead they consist of three types of scales that do not penetrate the epidermis in the process. The three categories of scales for Osteichthyes which are cosmoid scales, ganoid scales, teleost scales.
The trunk and tail are laterally compressed, while the head region is dorsoventrally compressed. The entire body is covered by an exoskeleton of placoid scales. The mouth is located on the ventral side and is bound on both sides by jaws. It has two rows of homodont or polyphyodont teeth, which are homologous to the placoid scales covering
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