Sarcopterygii
Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes whose members are known as lobe-finned fishes. The group Tetrapoda, a superclass including amphibians, reptiles (including dinosaurs and therefore birds), and mammals, evolved from certain sarcopterygians; under a cladistic view, tetrapods are themselves considered a group within Sarcopterygii. The known living non-tetrapod sarcopterygians include two species of coelacanths and six species of lungfishes. Characteristics. Early lobe-finned fishes are bony fish with fleshy, lobed, paired fins, which are joined to the body by a single bone. The fins of  lobe-finned fishes differ from those of all other fish in that each is borne on a fleshy, lobelike, scaly stalk extending from the body. The scales of sarcopterygians are true scaloids, consisting of lamellar bone surrounded by layers of vascular bone, dentine-like cosmine, and external keratin. The morphology of tetrapodomorphs, fish that are similar-looking to tetrapods, give indications of the transition from water to terrestrial life. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs. The first tetrapod land vertebrates, basal amphibian organisms, possessed legs derived from these fins. Sarcopterygians also possess two dorsal fins  with separate bases, as opposed to the single dorsal fin of actinopterygians (ray-finned fish). The braincase of sarcopterygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early sarcopterygians have a symmetrical tail. All sarcopterygians possess teeth covered with true enamel. Most species of lobe-finned fishes are extinct. The largest known lobe-finned fish was "Rhizodus hibberti" from the Carboniferous period of Scotland which may have exceeded 7 meters in length. Among the two groups of extant (living) species, the coelacanths and the lungfishes, the largest species is the West Indian Ocean coelacanth , reaching in length and weighing up . The largest lungfish is the African lungfish which can reach 2 m (6.6 ft) in length and weigh up to . Classification. Taxonomists who subscribe to the cladistic approach include the grouping Tetrapoda within this group, which in turn consists of all species of four-limbed vertebrates. The fin-limbs of lobe-finned fishes such as the coelacanths show a strong similarity to the expected ancestral form of tetrapod limbs. The lobe-finned fishes apparently followed two different lines of development and are accordingly separated into two subclasses, the Rhipidistia (including the Dipnoi, the lungfish , and the Tetrapodomorpha which include the Tetrapoda) and the Actinistia (coelacanths). Taxonomy. The classification below follows Benton (2004), and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank. Phylogeny. The cladogram presented below is based on studies compiled by Janvier "et al". (1997) for the "Tree of Life Web Project", Mikko's Phylogeny Archive and Swartz (2012). Evolution. Lobe-finned fishes (sarcopterygians) and their relatives  the ray-finned fishes (actinopterygians) comprise the superclass of bony fishes (Osteichthyes) characterized by their bony skeleton rather than cartilage. There are otherwise vast differences in fin, respiratory, and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence of cosmoid layers in the scales of sarcopterygians. The earliest fossils of sarcopterygians were found in the uppermost Silurian, about 418 Ma (million years ago). They closely resembled the acanthodians (the "spiny fish", a taxon that became extinct at the end of the Paleozoic). In the early–middle Devonian (416–385 Ma), while the predatory placoderms dominated the seas, some  sarcopterygians came into freshwater habitats. In the Early Devonian (416–397 Ma), the sarcopterygians split into two main lineages: the coelacanths and the rhipidistians. Coelacanths never left the oceans and their heyday was the late Devonian and Carboniferous, from 385 to 299 Ma, as they were more common during those periods than in any other period in the Phanerozoic. Coelacanths of the genus "Latimeria" still live today in the open (pelagic) oceans. The Rhipidistians, whose ancestors probably lived in the oceans near the river mouths (estuaries), left the ocean world and migrated into freshwater habitats. In turn, they split into two  major groups: lungfish and the tetrapodomorphs. Lungfish radiated into their greatest diversity during the Triassic period; today fewer than a dozen genera remain. They evolved the first proto-lungs and proto-limbs, adapting to living outside a submerged water environment by the middle Devonian (397–385 Ma). Hypotheses for means of pre-adaption. There are three major hypotheses as to how lungfish evolved their stubby fins (proto-limbs). History through to the end-Permian extinction. The first tetrapodomorphs, which included the gigantic rhizodonts, had the same general anatomy as the lungfish, who were their closest kin, but they appear not to  have left their water habitat until the late Devonian epoch (385–359 Ma), with the appearance of tetrapods (four-legged vertebrates). Tetrapods are the only tetrapodomorphs which survived after the Devonian. Non-tetrapod sarcopterygians continued until towards the end of Paleozoic era, suffering heavy losses during the Permian–Triassic extinction event (251 Ma).