Lobe-finned fish

Lobe-finned fish, or sarcopterygians, are bony fish whose paired fins sit on fleshy lobes supported by a central bone and muscle, unlike the fan of thin rays in most fish. The oldest well-known members appear around 419 million years ago in the Late Silurian of China, including Guiyu oneiros. They soon split into groups: coelacanths, lungfish, and a line of fish with tough, limb-like fins. That last line led to the first four-legged animals.

Their fins held a pattern of bones that foreshadows the arm: one bone, then two, then a cluster of smaller ones. Some lobe-finned fish lived in shallow, weedy water and could gulp air with lungs, a useful trick in oxygen-poor pools. That mix of sturdy fins and air-breathing made it possible, in the Devonian, to prop up the body and move through shallows. Only eight species survive, two coelacanths and six lungfish. Coelacanths were thought to have died out with the dinosaurs until a living one was brought up off South Africa in 1938.