Diagnostic Features:
Medium to large-sized (0.15 to 21.1 m) jawed vertebrates with cartilaginous skeletons which are often calcified but never ossified. The two pairs of appendages (fins) are supported by pectoral and pelvic girdles. The skull lacks sutures in modern forms; the upper jaw is formed by the palatoquadrate; the teeth are attached to subepithelial tissue, and not fused to the jaws. The branchial arches are >shaped. The labyrinth of the inner ear opens to the outside by means of an endolymphatic duct. The posterior end of the vertebral column is straight or upturned (heterocercal), or rarely downturned (hypocercal). The fin membranes generally are supported by unsegmented rays (ceratotrichia). The nostrils are ventral, and partially divided by a flap into incurrent and excurrent openings; the snout usually is produced into a rostrum, and the mouth is ventral. A swimbladder is lacking. The intestine has a spiral valve. A sympathetic nervous system, pancreas, spleen. and rectal salt gland are present. The heart has two or more valves. The body usually is uniformly covered with tooth-like placoid cales, which are of epidermal and dermal origin. The males of living species have the pelvic fins modified into intromittentorgans (claspers). Fertilization is internal, species range from oviparous to viviparous; embryos develop within leatherlike cases, larval development extends from 2 months to 2 years. This class of vertebrates is typically marine, but some penetrate or live exclusively in freshwater. Within the marine environment they occur from the shoreline to depths of 4000 m. and from tropical to polar seas. Marine species maintain high levels of urea and trimethylamine oxide for osmoregulatory purposes, and are slightly hyperosmotic to the environment. There are 797-843 species of living chondrichthians in 50 families and 14 orders. All species are carnivores; their prey ranges from zooplankton, to benthic invertebrates and fishes, to large pelagic fishes and cetaceans.
John D. McEachran