Osteichthyes Fossils: Bony Fishes

            Osteichthyes are not only the most plentiful class of fishes on the planet, but are also the most plentiful vertebrates of all time, numbering over 23,500 known species. The word “Osteichthyes” means “bony fish,” and refers to how their skeletons are made entirely of bone, as opposed to the chondrichthyes fishes, whose skeletons are cartilaginous. Other characteristics that unite the bony fishes are the pairs of gill arches that are protected by an operculum (a hard bony flap on the sides of the face), and a swim bladder. The bony fishes are comprised of two different sub-classes: the Actinopterygians (“ray-finned fishes”), and the Sarcopterygians (lobe-finned fishes”). Ray-finned fishes are so named because their fins are made up of a web of skin that is structurally supported by bony spines that are called “rays,” whereas the fins of lobe-finned fishes are fleshy stalks that are connected to the body by a single bone. Though the ray-finned fishes are far more diverse and plentiful, it would be the lobe-finned fishes (of which the famous coelacanths are members) that would play a much larger role in defining life on earth. As millions of years passed from the original lobe-finned fishes, one variety would eventually evolve the very limbs that would take fish from out of the water and onto land. These land-based descendants, known as tetrapods, evolved in the Devonian period approximately 395 million years ago. Indeed the genetic relationship between the two groups is so close, that lobe-finned fishes are actually more distantly related to other particular groups of fish than they are tetrapods.