What Is a Lobe-Finned Fish? The Group That Walked Out of the Water

A ray-finned fish supports its fins with slender bony rays attached more or less directly to the body wall, which produces a flexible fan controlled by muscles inside the body. A lobe-finned fish has a fleshy muscular projection from the body containing a sequence of internal bones, with a single large bone at the base, two bones beyond it and further elements after that, and the muscles moving the fin are located within the fin itself. That arrangement is the one found in every tetrapod limb, with the upper arm bone followed by two forearm bones followed by wrist and digits, and the correspondence is exact enough that the bones are given the same names in fossil fish and in mammals. The structure did not evolve for walking, since it appears in fully aquatic animals and served for manoeuvring and for propping against the bottom, and it was subsequently available when other uses arose.

The living representatives are few and the fossil record is extensive:

• •Coelacanths, known from fossils and assumed extinct until a living specimen was identified in 1938, with two species known today
• •Lungfish, with six living species across Africa, South America and Australia, breathing air and surviving drought in burrows
• •Tetrapods, which is to say every amphibian, reptile, bird and mammal, since land vertebrates are a branch within this group rather than a separate one
• •Rhizodonts, osteolepiforms and other extinct lineages known only as fossils
• •Several forms sitting close to the transition, including specimens with limb-like fins, ribs and necks that combine features of fish and tetrapods
• •Early tetrapods with more than five digits, which shows that the familiar count settled later than the limb did

The move from water to land is one of the best documented transitions in the fossil record and the sequence is not what the popular account suggests. The features generally regarded as adaptations to land appeared while the animals were still aquatic, which is the key point. Limbs with digits evolved in animals living in water, probably for moving through dense vegetation in shallow, and the earliest known digit-bearing animals show no evidence of terrestrial capability. Air breathing preceded the move, being present in many fish including living lungfish, and is an adaptation to oxygen-poor warm water rather than to land. A neck, allowing the head to move independently of the body, appeared in aquatic forms. Robust ribs and a stronger shoulder girdle likewise. What changed at the transition was the combination and the context rather than the invention of the features, which is the general pattern in major evolutionary shifts.

The transition occupied a long stretch of the Devonian period and the constraints explain the pace. Weight is the obvious one, since water supports a body and air does not, requiring a stronger spine, a load-bearing limb girdle and ribs that do not collapse the chest. Breathing changes entirely, since gills collapse without water to hold them open, and lungs had to take over the whole job rather than supplement it. Sensory systems developed in water work badly in air, since the lateral line detecting water movement is useless and hearing requires a new mechanism to transfer airborne vibration, which explains why ear structures appear late. Water loss through skin and eggs constrains where an animal can be. Feeding is harder without water to draw prey into the mouth, so the suction method used by fish had to be replaced. Each of these was solved partially and separately, which is why the earliest land-capable animals were still tied to water.

The two surviving groups are informative and are routinely misdescribed. Coelacanths are frequently called living fossils, which misleads in two ways, since the living species are not the same as the fossil ones and the lineage has continued evolving, and since the group was never as static as the label implies. They live at depth, give birth to live young, have a peculiar joint in the skull and an oil-filled organ where other fish have a swim bladder. Lungfish are the closer relatives of land vertebrates, which was established by anatomy and confirmed genetically, and they possess the largest genomes of any animal by a considerable margin, which is a curiosity with no obvious explanation. Both groups are relicts of a formerly diverse radiation, and both are threatened, with coelacanths taken accidentally in deep fisheries and lungfish affected by habitat change in the seasonal waters they depend on.

A fleshy fin containing one bone, then two, then more, with the muscles inside the fin, is the arrangement every tetrapod limb inherited, and the bones carry the same names in fossil fish and in mammals. Limbs with digits, air breathing and necks all appeared in animals still living in water. Coelacanths and lungfish are the surviving relicts, and land vertebrates are a branch inside the group.