Fleshy-finned fish that invented the road to land
Sarcopterygii—from Greek flesh and fin—are lobe-finned vertebrates whose fins hold muscular limb buds and internal bones, unlike ray-fins. Appearing in Silurian–Devonian seas, most living species are now land animals: the tetrapods nested inside this clade. Non-tetrapod lobefins ruled Carboniferous–Permian waters before dwindling.
The name joins Greek words for flesh and fin. Each paired fin sits on a scaly, muscular stalk joined to the body by a single bone, rather like a limb bud, unlike the bony spines covered in skin that support the fins of ray-finned fishes. Together the two groups form the bony fishes, marked by swim bladders related to lungs and skeletons of bone instead of cartilage. Because lungfish and fossil rhipidistians sit closer to four-legged animals than coelacanths do, cladists place all tetrapods, humans included, inside the lobe-finned group; Michael Benton's 2004 scheme reflects this despite ranking tetrapods higher.
In the Early Devonian the lineage split into coelacanths and rhipidistians. Coelacanths stayed in the sea, peaked between 385 and 299 million years ago, and after the end-Permian catastrophe had their greatest burst of diversity in the Early Triassic; 121 species in 47 genera are known. The living genus Latimeria keeps so many ancient traits it is called a living fossil, and the western Indian Ocean species reaches 2 metres. Rhipidistians moved from estuaries into fresh water and divided into lungfish and tetrapodomorphs, both turning swim bladders into lungs.
Why fins became legs is debated. Alfred Romer proposed that drying waterholes forced fish overland. A 2010 team argued for tidal flats, citing 395-million-year-old trackways at Zachelmie in Poland, the oldest tetrapod evidence. Gregory Retallack's 2011 woodland idea points to transitional fossils in humid, wooded floodplains, where limbs could push through roots and plants. A minority view stresses escape from predators and richer oxygen on land.
Non-tetrapod lobe-fins were top freshwater predators in the Carboniferous and Permian; the biggest, Rhizodus hibberti from Britain, reached 6 metres. Tetrapods emerged in the late Devonian, and the rest of the group declined heavily after the Great Dying about 251 million years ago.
Source: Sarcopterygii
