sarcoptergians

Sarcopterygii, or lobe-finned fishes, are a major group of bony fishes distinguished by fleshy, lobed paired fins that are thick and muscular, with a bony internal skeleton that can articulate with the limbs. This structural arrangement is considered a key precursor to the evolution of terrestrial limbs in tetrapods. The clade includes both living and extinct lineages and is one of the two main divisions of bony fishes, alongside the ray-finned fishes (Actinopterygii).

Today, only two living groups of sarcopterygian fishes survive: the coelacanths (Actinistia) and the lungfishes (Dipnoi). Coelacanths are marine, typically deep-water residents known for their distinctive lobed fins and ancient lineage. Lungfishes inhabit freshwater environments and possess lungs; many species can survive drought by estivating in mud. In the fossil record, sarcopterygii were once highly diverse, with numerous extinct lineages such as the rhizodonts and osteolepiforms. These groups show a progressive modification of the fins toward weight-bearing, leg-like structures.

The most significant evolutionary importance of sarcopterygii lies in their relationship to tetrapods. A sequence of fossil forms, including rhipidistians and later transitional species such as Tiktaalik and early tetrapods like Acanthostega and Ichthyostega, documents the transition from aquatic lobe-finned fish to terrestrial vertebrates. This transition underpins the origin of land-dwelling vertebrates and the subsequent diversification of amphibians, reptiles, birds, and mammals.

Classification within Osteichthyes traditionally treats Sarcopterygii as a subclass or clade containing Actinistia and Dipnoi, with most other lobe-finned lineages now extinct.