Cladistic classification of Sarcopterygii

Cladistic classification of Sarcopterygii is the classication of Sarcopterygii as a clade containing not only the lobe-finned fishes (coelacanths and lungfish) but also the tetrapods, which are closely related to lungfish. The taxon Sarcopterygii was traditionally classified as a paraphyletic group considered either a class or a subclass of Osteichthyes (bony fish). Identification of the group is based on several characteristics, such as the presence of fleshy, lobed, paired fins, which are joined to the body by a single bone.

Taxonomic and fossil history
The properties defining the sarcopterygians are in contrast to the other group of bony fish, the Actinopterygii, which have ray-fins made of bony rods, called lepidotrichia. These two bony fish groups were classified together as Osteichthyes at one time, the whole combined group was seen as parallel to the tetrapods (mammals, birds, reptiles, and amphibians).

The extensive fossil record and numerous morphological and molecular studies have shown, however, that lungfish and some fossil lobe-finned fish ("rhipidistians") are more closely related to tetrapods than they are to coelacanths; as a result tetrapods are nested within Sarcopterygii. This abides to cladistics in that in order for a group to be valid, it must have an ancestral species and all descendants of that common ancestor based on shared characteristics. As such mammals, sauropsids (birds and "reptiles"), and amphibians are highly derived sarcopterygians despite superficially looking nothing like the standard lobe-finned fish anatomically speaking. However, similarities can be noticed in their limb bones and tooth enamel. Additionally, lungfish and tetrapods share a divided atrium.

Classification
Multiple Linnean classifications have been proposed with the explicit intent to incorporate Sarcopterygii as a monophyletic taxon instead of maintaining its traditional paraphyletic definition.

Ahlberg (1991)
Class Osteichthyes


 * Subclass Actinopterygii
 * Subclass Sarcopterygii
 * Plesion Onychodontia
 * Infraclass Actinistia
 * Infraclass Rhipidistia
 * Superdivision Tetrapodomorpha
 * Plesion Rhizodontida
 * Plesion Osteolepiformes
 * Plesion Panderichthyidae
 * Division Tetrapoda
 * Superdivision Dipnomorpha
 * Plesion Porolepiformes
 * Plesion Powichthys
 * Plesion Youngolepis
 * Plesion Diabolepis
 * Division Dipnoi

Nelson et al. (2016)
Class Osteichthyes


 * Subclass Actinopterygii
 * Subclass Sarcopterygii
 * Infraclass Actinistia
 * †Infraclass Onychodontida
 * Infraclass Dipnomorpha
 * †Infraclass Rhizodontida
 * †Infraclass Osteolepidida
 * †Infraclass Elpistostegalia
 * Infraclass Tetrapoda

Betancur-Rodrigues et al. (2017)
Superclass Sarcopterygii


 * Class Coelacanthimorpha
 * Class Dipnotetrapodomorpha
 * Subclass Dipnomorpha
 * Subclass Tetrapodomorpha

Other classifications do not use Sarcopterygii as a ranked taxon but still nonetheless still reject traditional paraphyletic assemblages. In the scheme below, sarcopterygian groups are marked in bold letters.

Tedersoo (2017)
Phylum Craniata


 * Class Cyclostomi
 * Class Chondrichthyes
 * Class Cladistia
 * Class Actinopteri
 * Class unspecified
 * Class Ceratodontimorpha
 * Class Amphibia
 * Class Mammalia
 * Class Sphenodontea
 * Class Squamatea
 * Class Testudinea
 * Class Crocodylea
 * Class Aves