CHONDROSTEAN

any member of the subclass Chondrostei, a group of primitive, ray-finned bony fishes, represented by the sturgeons and paddlefishes and many fossil species. The extant bichir and reedfish of Africa are often included in this group. The sturgeon, a food fish, is widely distributed in the Northern Hemisphere. Marine species appear along both the American and European coasts of the Atlantic Ocean, while other species appear in the Black Sea, the Caspian Sea, and southern rivers of that region. Long and somewhat tapered, sturgeons generally grow to a length of 2 m (7 feet), but one species (Huso huso, or hausen) can reach 8.5 m in length. The sturgeon is protected by two narrow rows of bony plates on its side and one row along its top. With a mouth at the base of its pointed snout, the sturgeon is a bottom-feeding fish. In front of the toothless mouth are four fleshy barbels (whiskers) that the sturgeon drags across river and ocean bottoms in search of small fish or invertebrates on which to feed. Ocean species of sturgeon migrate up rivers to spawn; freshwater species may breed near lakeshores. Their numerous small eggs (prized as caviar) hatch and grow rapidly. Like the sturgeon, the paddlefish is highly specialized. Its most distinguishing feature is a long paddle-shaped snout, made of cartilage and often half the paddlefish's length . The Chinese paddlefish may reach lengths of 6.3 m, while the Mississippi paddlefish only grows to 1.8 m. Paddlefish feed entirely on plankton. They reach maturity at seven or eight years and breed during spring floods. The skull and skeletons of both the paddlefish and sturgeon are more cartilaginous than those of ancestral forms of Chondrostei. Both have rayed fins (i.e., fins supported with bony rods), but while the sturgeon is covered with scales, the paddlefish has only a few vestigial scales. The bichir and reedfish inhabit the freshwaters of central Africa. The bichir grows no longer than 70 cm (28 inches), while the reedfish grows to 90 cm. Both are elongated in shape, with the bichir almost cylindrical, or eellike. They have numerous dorsal finlets and are covered with hard rhomboid (diamond-shaped) scales. Their upper bodies tend to be dark and their lower bodies pale. Living near stream edges, bichir and reedfish forage at night for insects, worms, crustaceans, and small fish. Extinct orders of Chondrostei include the Palaeonisciformes, Tarrasiiformes, Haplolepiformes, and 13 other orders collectively known as the subholosteans. The rise of the holosteans and teleosts during the Mesozoic Era is believed to have led to the decline of these fossil orders. The palaeonisciforms, the earliest known chondrosteans, lived in both marine and freshwater habitats throughout the world, beginning in the Carboniferous Period (360 to 286 million years ago). They grew to lengths of one metre, and most were covered with hard rhomboid scales. Their caudal fins were heterocercal, as are the sturgeon's, and their dorsal and anal fins were almost exactly opposite. The jaws and jaw musculature of palaeonisciforms were more primitive and less powerful than those of the holoceans. Among the tarrasiiforms and haplolepiforms, which flourished in the Carboniferous Period, the skeletal structure diverged from that of the palaeonisciforms and their jaws and jaw musculature were more efficient. The surviving chondrosteans share with their ancestors a similar skull structure. Fossil specimens of bichir and reedfish date from 65 million years ago and those of sturgeons and paddlefish from 100 to 65 million years ago. any member of the subclass, or infraclass, Chondrostei, a group of primitive, ray-finned bony fishes represented by the sturgeons and paddlefishes and many fossil species. They comprise one of the three major subdivisions of the class Actinopterygii, the other two being the holosteans and the teleosts. Fossilized chondrosteans first appear in rocks of the Middle Devonian Period (about 375,000,000 years ago). The only living representatives are the sturgeons and paddlefishes; the living bichirs (polypterids and the closely related reedfish) of Africa are also considered to be chondrosteans by some ichthyologists. The chondrosteans were most numerous and diversified during the last part of the Paleozoic Era (ending 225,000,000 years ago) and the beginning of the Mesozoic Era (beginning 225,000,000 years ago). With the rise of the holosteans and teleosts (the other two major subdivisions of the Actinopterygii) during the Mesozoic, the chondrosteans declined, until by the end of the Cretaceous Period (65,000,000 years ago) they had been reduced to a few genera. The few living chondrosteans are highly specialized and aberrant forms. Their evolutionary history has not been clearly documented. Except for the sturgeon, which is a food fish for man and the source of caviar, they have no economic importance. A study of the living sturgeons, paddlefishes, and bichirs, however, provides some understanding of extinct forms. Additional reading S.M. Andrews et al., "Pisces," in W.B. Harland et al. (eds.), The Fossil Record: A Symposium with Documentation, ch. 26 (1967), a classification of fish, with first and last occurrences for each family; E.S. Goodrich, "Vertebrata craniata," fasc. 1, "Cyclostomes and Fishes," in E.R. Lankester (ed.), A Treatise on Zoology, 9 vol. (1909, reprinted 1964), a classic work on the anatomy of fish that is still useful; D. Heyler, Vertbrs de l'autunien de France (1969), a detailed account of some palaeonisciform fish from a classic locality in France; J.P. Lehman, "Etude complementaire des poissons de l'Eotrias de Madagascar," K. Svenska Vetensk-Akad. Handl., 2:1-201 (1952), a detailed study of early Triassic fish from Madagascar; "Super-ordre des Chondrostens (Chondrostei): Formes fossiles," in P.P. Grass (ed.), Trait de zoologie, vol. 13 (1958), a comprehensive reference work on the organization and classification of the chondrosteans, and "Actinopterygii," in J. Piveteau (ed.), Trait de palontologie, vol. 4 (1966); D.V. Obruchev (ed.), Fundamentals of Paleontology, vol. 11, Agnatha, Pisces (1967); and A.S. Romer, Vertebrate Paleontology, 3rd ed. (1966), two comprehensive reference works.