Chondrichthyes is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opercula and swim bladders. The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class). Extant chondrichthyans range in size from the 10 cm (3.9 in) finless sleeper ray to the over 10 m (33 ft) whale shark.
Predators. Can be pelagic (in which case they must keep swimming to get water through their gills) or demersal (in which case they can pump water in through the spiracles behind their eyes and out through their gills). Due to their lack of a swim bladder, pelagic species must continuously swim to avoid sinking (buoyancy is given by large amounts of liver oil). Most are Marine. Only 5% are restricted to freshwater (e.g. the freshwater stingray). Only 5% swim through the open ocean (e.g. the great white shark). Some give birth to eggs surrounded by egg cases/capsules while others give live birth.
Among the first vertebrates to evolve jaws and bony teeth. Evolved from spiny sharks (Acanthodii). Very diverse group.
The earliest unequivocal fossils of acanthodian-grade cartilaginous fishes are Qianodus and Fanjingshania from the early Silurian (Aeronian) of Guizhou, China around 439 million years ago, which are also the oldest unambiguous remains of any jawed vertebrates. Shenacanthus vermiformis, which lived 436 million years ago, had thoracic armour plates resembling those of placoderms.
By the start of the Early Devonian, 419 million years ago, jawed fishes had divided into three distinct groups: the now extinct placoderms (a paraphyletic assemblage of ancient armoured fishes), the bony fishes, and the clade that includes spiny sharks and early cartilaginous fish. The modern bony fishes, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. The first abundant genus of shark, Cladoselache, appeared in the oceans during the Devonian Period.
