Fish or more precisely their ancestors, armored fish were the first living creatures that had two-piece jaws and thus form the origin of land vertebrates. The development of bony fish and cartilaginous simultaneously about 400 million years ago.
Until recently rays were considered a subspecies of sharks,but recent findings show them to be an independent parallel development. Sharks , rays and chimaeras comprise the group of cartilaginous fish. they all have breathing holes on top of the head, five to seven open gill slits on each side and placoid scales made with dentine.
Bony fish, however, have full-length swim bladders, two gill slits covered with operculum, one on each side, scales made of bone material and the eponymous bony skeleton (“bones”). Some fish still in existence today such as the sturgeon are relatively early developments and still have features such as bone plates as skin coverage and a partially bony skeleton.