About the same time the first of the class Osteichthyes (“bony fishes”) appeared on the earth. This group, which now contains the vast majority of fish species, differs from the others in having true bone in at least some part of the skeleton. Because of the various highly specialized members of the group, it is hard to find characteristics that are common to all species. All have jaws and at least one pair of nostrils, but some lack fins or scales; in fact, practically every fish characteristic is lacking in one species or another, or is so modified as to be hardly recognizable.
One very old group of bony fishes – that comprising the crossopterygians (a name meaning “fringed fins”) – is of especial interest, because it was from them that the first backboned land animals, the amphibians, evolved. Although at one time the fringe-fins were the chief animals of prey in fresh water, only two salt-water species are known to be alive today. These “living fossils” are the East London coelacanth and the Anjouan Island coelacanth. With their close relatives the lungfishes, the fringe-fins constitute one of the major divisions of the bony fishes, the subclass Choanichthyes (“nostril fishes”) – so called from the fact that they, or their ancestors, had a nose that connected with the mouth.
With the exception of five species of lungfishes and two fringe-fins, all of the living bony fishes are included in the subclass Teleostomi (“perfect mouth”); these are the ray-finned fishes. The scientific name refers to the presence of true bone in the structure of the jaws and skull; the popular name to the structure of the fins, by which these fishes can be distinguished from the fringe-fins. Although a few of the ray-fins have fins with a fleshy base like those of the fringe-fins, the arrangement of the bones inside is fundamentally different.
One point concerning the evolutionary status of fishes should be emphasized. Although they belong to the most ancient group of back-boned animals, and a number of them which are still alive today can be called “living fossils” because of their similarity to ancient types, the vast majority of species are as up-to-date, geologically speaking or in an evolutionary sense, as any bird or mammal. Most fishes of today represent highly specialized lines of descent that were undergoing evolutionary change at the same time that the amphibians, reptiles, birds, and mammals were evolving to their present state. Therefore, fishes, in their own way, are just as “modern” or “advanced” as any of these so-called “higher creatures.”