Osteichthyes, or bony fishes, includes two major groups: Sarcopterygii, or lobe-finned fishes, and Actinopterygii, or ray-finned fishes. The characteristics that unite this diverse group include lungs or a gas-filled swim bladder derived from lungs, segmented fin rays, bone, and bony scales. Although the tetrapods (including birds, reptiles, mammals, and amphibians) are formally within the Sarcopterygii, they are discussed within their own entries; only animals commonly thought of as "fishes" are discussed here.

Sarcopterygii (Lobe-Finned Fishes)

Although only eight sarcopterygian fish species exist today, they are interesting because scientists believe they are the likely descendants of the fishes that gave rise to the terrestrial vertebrates, or tetrapods. The defining feature supporting this notion is a limblike fin with supporting bones that attach to the pelvic and pectoral girdles. There are six species of lungfish (Dipnoi or Dipneusti) found in Africa, South America, and Australia. Lungfish have true lungs, which...