Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fish known as the lobe-finned fish or sarcopterygians, characterised by prominent muscular buds (lobes) within the fins. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only bony spines supporting the fins.
The group Tetrapoda, a mostly terrestrial superclass of limbed vertebrates including amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (with mammals being the only extant group), evolved from sarcopterygian ancestors; under a cladistic view, tetrapods are themselves considered a subgroup (or rather, the dominant crown group) within Sarcopterygii.
The known extant 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 that resembles a limb bud. 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.
Subclass Sarcopterygii
†Order Onychodontida
Order Actinistia
Infraclass Dipnomorpha
†Order Porolepiformes
Subclass Dipnoi
Order Ceratodontiformes
Order Lepidosireniformes
Infraclass Tetrapodomorpha
†Order Rhizodontida
Superorder Osteolepidida
†Order Osteolepiformes
†Family Tristichopteridae
†Order Panderichthyida
Superclass Tetrapoda
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).