Boschung, Herbert T. Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama.
Last reviewed:October 2019
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A class of vertebrates comprising the cartilaginous, jawed fishes. Members of the Chondrichthyes (chondrichthyans, or the cartilaginous fishes) are characterized by well-developed jaws and bony teeth; pectoral and pelvic girdles, with each supporting paired fins; a cartilaginous skeleton that lacks true bone; toothlike scales of ectodermal and mesodermal origin; paired nostrils with blind olfactory sacs (not opening into the mouth); a vertebral column having either a straight or heterocercal posterior end; the absence of a swim bladder; a cranium without sutures; and internal fertilization. Traditionally, the class Chondrichthyes includes the subclasses Holocephali (chimaeras) and Elasmobranchii [sharks (see illustration), rays, and skates]. A group of Devonian armored fishes, the Placodermi, has been regarded as ancestral to the Chondrichthyes, but this derivation is not certain. Another group of primitive jawed fishes called acanthodians, 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 chimaeras arose independently of each other during the Silurian or Early Devonian. 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. See also: Acanthodii; Cartilage; Gnathostomata; Placodermi; Vertebrata
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