Although all fishes are primarily aquatic, cold-blooded, backboned animals, there are fundamental differences among them.
One class of fishes—the class Cyclostomi—is best known from the fossils of its extinct members that lived from Ordovician to Devonian times, some 500 to 400 million years ago. The few living cyclostomes—lampreys and hagfishes—are mostly marine.
Cyclostomes differ from other fishes in many ways but particularly in lacking jaws and true teeth and in having the gills (the chief respiratory organ of most fishes) contained in separate pouches rather than supported on gill-arches surrounding the pharynx. In all respects, the anatomical and physiological organization of cyclostomes is on a primitive level. The principal axial support for the cyclostome body is provided by a notochord (the evolutionary and embryonic forerunner of a backbone) and not by a column of separate vertebrae. In addition, no true paired fins are developed in cyclostomes. Extant cyclostomes are slender eellike creatures with scaleless bodies, and most are either parasitic, or they feed on dead animal matter.
Five distinct jawed fish classes —Crossopterygii, Dipnoi, Actinopterygii, Selachii, and Holocephali—still survive in the seas and freshwaters of the world. Two other classes—Acanthodii and Placodermi—are known only from fossils.
In addition to possessing jaws, the jawed fishes differ from the cyclostomes in many other ways. In jawed fishes, for example, the gills are carried on jointed arches flanking the gill slits (openings on the anterior sides of the fish body), true paired fins, as well as unpaired median fins supported by both internal and external skeletal elements, are developed, and there is a complex cranial skeleton of bone and cartilage. In most jawed fishes the notochord is greatly reduced, and its supporting function is taken over by vertebrae that develop around the notochord during embryonic and larval growth. (In some living fishes and in many groups now extinct, complex vertebrae are not developed and a reinforced notochord persists.) The brain and sense organs of jawed fishes are also well-developed and more complex than in cyclostomes.
The five extant classes of jawed fishes may conveniently be brought together in two groups: the cartilaginous fishes, and the bony fishes. The cartilaginous fishes, or Chondrichthyes, include the sharks, rays, skates, and chimaeras (classes Selachii and Holocephali). The bony fishes, or Osteichthyes, include the vast majority of the world’s fishes (classes Crossopterygii, Dipnoi, and Actinopterygii). Together these two groups are usually considered “true fishes.” They are, however, markedly different and any common ancestor they may have had is certainly a distant one.
Both groups are known from fossils in Devonian strata. The bony fishes have proved to be more successful, with between 30,000 and 40,000 living species as compared with the 500 or 600 species of living cartilaginous fishes.