lobefinned

Lobe-finned fishes are a clade of bony fishes known as Sarcopterygii, distinguished by paired fins that are fleshy and lobed, containing a central bone structure connected to the body by a single base. These lobed fins differ from the slender, ray-supported fins of most other fishes. The fin bones include elements homologous to the limbs of tetrapods, such as proximal girdles and distal segment bones, a condition that has made lobe-finned fishes central to studies of vertebrate evolution. They first appear in the fossil record in the Silurian period, and their fin architecture evolved toward limb-like structures in several lineages.

Two living lineages survive today: coelacanths and lungfishes. Coelacanths (Latimeria and related species) are deep-water fishes with distinctive lobed fins and a long evolutionary history; modern representatives inhabit parts of the western Indian Ocean and nearby seas. Lungfishes (Dipnoi) inhabit freshwater environments in Africa, South America, and Australia, and possess lungs in addition to gills, enabling them to breathe air and survive in seasonally drying habitats.

Fossil lobe-finned fishes include many forms known as stem-tetrapods, such as Eusthenopteron, Osteolepis, and the transitional taxa Tiktaalik and Acanthostega. These fossils reveal changes in fin structure and girdles that precede the appearance of true digits and terrestrial limbs, marking the fin-to-limb transition and the origin of tetrapods in the late Devonian.

Today, lobe-finned fishes occupy varied ecological niches; their surviving lineages provide critical insight into vertebrate evolution. Ongoing research examines their morphology, genetics, and fossil record to illuminate how fins evolved into legs.