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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.
Apart from electric rays, which have a thick and flabby body, with soft, loose skin, chondrichthyans have tough skin covered with dermal teeth (again, Holocephali is an exception, as the teeth are lost in adults, only kept on the clasping organ seen on the caudal ventral surface of the male), also called placoid scales (or dermal denticles ...
Placoid scales as viewed through an electron microscope. Also called dermal denticles, these are structurally homologous with vertebrate teeth. Placoid (pointed, tooth-shaped) scales are found in the cartilaginous fishes : sharks , rays .
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 ...
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.
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]
Placoid scales (denticles) were present in Cladoselache, but they are tiny and bluntly conical structures which are absent from much of the body. Larger denticles occur on fin margins and adjacent to the lateral line , [ 11 ] [ 10 ] while three-cusped scales are found along the margin of the jaw. [ 10 ]
placoid scales Teeth-like scales found in sharks and rays. posterior Towards the hind end of the fish. postorbital Behind the eye. precaudal Anterior to the tail portion. premaxillaries Two bones forming the front portion of the upper jaw. preocular spine A spine positioned above and in front of the eye. preopercle, preoperculum