sarcopterygian

Sarcopterygian refers to the lobe-finned fishes, a clade of bony fishes whose pectoral and pelvic fins are fleshy and lobed, with a robust internal skeleton that bears homology to tetrapod limbs. The group includes living coelacanths (Actinistia) and lungfishes (Dipnoi), as well as many extinct forms known from fossils.

The defining feature is the fin skeleton in which the bones within the fin resemble a limb: a proximal major bone (analogous to a humerus or femur) and a series of progressively smaller bones that support the fin lobe, surrounded by muscular tissue. This arrangement is linked to the evolution of limbs and girdles in the transition from water to land.

In the fossil record, sarcopterygian lineages first appear in the Silurian and diversify in the Devonian, giving rise to early tetrapods. Transitional taxa such as Eusthenopteron, Panderichthyes, Tiktaalik, and Ichthyostega illustrate progressive changes in the pectoral girdle, fin skeleton, and digits that underpin the fin-to-limb transition.

Living sarcopterygians are represented by two lineages: coelacanths, which inhabit deep marine waters and possess a distinctive lobed tail and fin arrangement; and lungfishes, which reside in freshwater habitats in Africa, South America, and Australia and can breathe air with lungs. Some lungfishes also retain gills and aquatic habits.

In taxonomy, Sarcopterygii is used for lobe-finned fishes and their descendants; in several classifications, Tetrapoda is included within Sarcopterygii, reflecting their evolutionary origin from these fishes.