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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 .
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 ...
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 ...
Members of the elasmobranchii subclass have no swim bladders, five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins, and small placoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper.
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.
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
The oldest of these scales have been dated back to the Ludlow epoch (427.4 Ma to 423 Ma), [2] [3] making Elegestolepis the oldest known shark. [4] Elegestolepis dates back to about 420 years ago, but some scales that may yet represent another shark ancestor are known from 450 million years ago.