Chondrichthyes(redirected from Placoid fish)
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A class of vertebrates comprising the cartilaginous, jawed fishes. The Chondrichthyes have traditionally included the subclasses Elasmobranchii (sharks, skates, and rays) and Holocephali (ratfishes). A classification scheme for the Chondrichthyes follows.
- Class Chondrichthyes
- Subclass Elasmobranchii
- Order: Cladoselachii
- Subclass Holocephali
- Order Chimaeriformes
A group of Devonian armored fishes, the Placodermi, has usually been regarded as ancestral to the Chondrichthyes, but this derivation is not certain. Another group of primitive jawed fishes called acanthodians, which are considered by many as ancestral to the higher bony fishes, exhibit certain primitive elasmobranch-like features. In any case it is probable that the elasmobranchs and ratfishes arose independently of each other sometime during the Silurian or Early Devonian. See Acanthodii, Placodermi
The most distinctive feature shared by the elasmobranchs and ratfishes is the absence of true bone. In both groups the endoskeleton is cartilaginous; in some cases it may be extensively calcified. Because even calcified cartilage is rarely preserved, the fossil record of the Chondrichthyes is represented mainly by teeth and spines, with only occasional associated skeletons.
Other characteristics of the Chondrichthyes include placoid scales, clasper organs on the pelvic fins of males for internal fertilization, a urea-retention mechanism, and the absence of an air (swim) bladder. Both groups have primarily always been marine predators, although they have repeatedly invaded fresh water throughout their long history. The elasmobranchs have probably always fed as they do today, on other fishes as well as on soft and hard-bodied invertebrates. The ratfishes have most likely concentrated on invertebrates, although modern forms occasionally also feed on smaller fishes. See Ray, Swim bladder
a class of fishes that includes two subclasses: Elasmobranchii and Holocephali. The fish have a cartilaginous skeleton fortified by calcification. The scales are placoid. There is a spiral valve in the intestine and an arterial cone in the heart. Many Chondrichthyes have a cloaca. In Elasmobranchii the skull is hyostylic (with a single articulation of the jaw with the skull) or amphistylic (with two such articulations). In Holocephali the skull is autostylic (with the jaw concresced with the skull). In almost all Chondrichthyes insemination is apparently internal. Males have a special copulatory organ, the pterygopodium, which is formed from rays of the ventral fins.
The Chondrichthyes are viviparous or oviparous. The majority are predators. They are predominantly marine fish; only a few Elasmobranchii have adapted to freshwater life. Fossil Chondrichthyes are known from the Devonian.
G. U. LINDBERG