Non-tetrapod sarcopterygians were once the dominant predators of freshwater ecosystems during the Carboniferous and Permian periods, but suffered significant decline after the Great Dying.

[6] The physical structure of tetrapodomorphs, fish bearing resemblance to tetrapods, provides valuable insights into the evolutionary shift from aquatic to terrestrial existence.

[10] The fin-limbs found in lobe-finned fishes like the coelacanths display a strong resemblance to the presumed ancestral form of tetrapod limbs.

Lobe-finned fishes seemingly underwent two distinct evolutionary paths, leading to their classification into two subclasses: the Rhipidistia (comprising the Dipnoi, or lungfish, and the Tetrapodomorpha, which includes the Tetrapoda) and the Actinistia (represented by coelacanths).

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.

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 rhipidistians, whose ancestors probably lived in the oceans near river mouths and estuaries, left the marine world and migrated into freshwater habitats.

Lungfish radiated into their greatest diversity during the Triassic period; today, fewer than a dozen genera remain, having evolved the first proto-lungs and proto-limbs, adapting to living outside a submerged water environment by the middle Devonian (397–385 Ma).

[17][18] The surviving tetrapods then underwent adaptive radiation on dry land and become the dominant terrestrial animals during the Carboniferous and the Permian periods.

The cladogram presented below is based on studies compiled by Janvier et al. (1997) for the Tree of Life Web Project,[32] Mikko's Phylogeny Archive[33] and Swartz (2012).