Actinopterygii(redirected from Chondrost)
Also found in: Medical.
A group of bony fishes, also known as actinops or ray-finned fishes, containing about half of all vertebrate species and about 96% of all living “fishes” (a nonmonophyletic group derived from more than one lineage when tetrapods are excluded). Living Actinopterygii comprise Polypteriformes (bichirs and reedfish), Acipenseriformes (sturgeons and paddlefishes), Lepisosteiformes (gars), Amiiformes (bowfins), and Teleostei (teleosts). Actinops are characterized by the presence of a single dorsal fin, an enclosed sensory canal in the dentary bone, a specialized tissue called ganoin, and several other anatomical characters. About 40% of living actinopterygian species live exclusively or almost exclusively in fresh water. The rest inhabit mostly marine, brackish, or combination environments.
The fossil record indicates that actinopterygians are at least as old as the Late Silurian (about 420 million years before present). Fossil actinopterygians are speciose and extremely abundant, making up the majority of vertebrate fossils that are known by complete skeletons. Many major radiations of early actinopterygians, such as pycnodonts, semionotiforms, and palaeonisciforms, have been extinct for tens of millions of years. Other early actinopterygian groups, such as the Cheirolepiformes, have been extinct for hundreds of millions of years. Based on the fossil record, the most major differentiation of the group began in the late Mesozoic.
(rayed fins), a group of Osteichthyes (bony fishes) of the subclass Teleostomi. It includes the vast majority (95 percent) of fishes now in existence. Actinopterygii differ from the other group of Teleostomi fishes, the Crossopterygii, by the absence of the central axis of basal elements of the skeleton in the paired fins. The scales are ganoid or bony, the skull hyostylic.
There are five superorders: Palaeonisci, Polypteri, Chondrostei, Holostei, and Teleostei. The most ancient are the Palaeonisci, which appeared in the middle Devonian; their tail is usually heterocercal and the scales ganoid. Most marine and freshwater fishes of the Carboniferous and Permian belonged to this superorder, which became extinct in the Cretaceous. The descendants of the Palaeonisci were fishes of the superorder Holostei, which appeared in the Triassic and predominated during the Jurassic and Cretaceous. Of this superorder the bowfin and several species of the order Lepisosteiformes have survived to the present. The Holostei have a homocercal tail and an ossified internal skeleton; the scales are ganoid or bony (cycloid). Probably as early as the end of the Triassic the Teleostei became distinct from the Holostei; subsequently the Teleostei gave rise to all the various modern fishes.
Another group of descendants of the Palaeonisci, the Chondrostei, also appeared at the end of the Triassic and have survived to the present day. Polypteri are known as fossils from the Eocene; apparently they also developed from the Palaeonisci, which thus are the ancestors of all the presently living Actinopterygii.
REFERENCENikol’skii, G. V. Chastnaia ikhtiologiia, 3rd ed. Moscow 1971.
G. V. NIKOL’SKII