Why Is the Coelacanth Called a Living Fossil? Lazarus Fish

Known only from fossils until 1938

The Swiss-born naturalist Louis Agassiz named the first coelacanth genus, Coelacanthus, from a fossil in 1839. The name means “hollow spine” and refers to the hollow fin rays seen in that fossil. Through the 19th and early 20th centuries, paleontologists described many more species from rocks around the world.

The oldest coelacanth fossils are about 410 million years old, from the Early Devonian period, and more than 100 fossil species are now known. Coelacanths reached their greatest diversity in the Early Triassic, about 250 million years ago. Their fossils then become rarer and stop near the end of the Cretaceous. With no younger remains, scientists assumed the whole group died out with the dinosaurs.

So when a living coelacanth was hauled up by a trawler off South Africa in December 1938, it caused a sensation. A group “missing” for about 66 million years had reappeared. Paleontologists call such a comeback a Lazarus taxon, and the coelacanth is the textbook example. The story of the find is told in how was the coelacanth discovered?

What makes the coelacanth look ancient?

Several features of the living coelacanth are rarely or never seen in other living fish:

• Lobed fins: the paired fins and two of the unpaired fins grow from fleshy, muscular stalks with bones inside, rather than being thin fans of rays.
• A notochord instead of a backbone: adults keep a thick, oil-filled tube along the back, a structure that most vertebrates replace with solid vertebrae early in development.
• A three-lobed tail: the tail has an extra small lobe in the middle.
• A hinged skull: a joint across the braincase lets the front of the head tilt up, a feature known in many fossil lobe-finned fishes but unique among living vertebrates.
• A leftover lung: a small, nonworking lung sits inside a fat-filled organ, while many fossil coelacanths had a large lung protected by bony plates.

Fossil coelacanths from the Jurassic and Cretaceous, such as Holophagus and Macropoma, already show the same general body plan. That resemblance is the main reason the living fish earned its nickname.

Is the coelacanth the ancestor of land animals?

No, but it is a close cousin of the fish that were. Coelacanths belong to the lobe-finned fishes, or sarcopterygians, a group that also includes lungfish and the ancestors of all four-legged animals, called tetrapods. The bones inside a coelacanth’s paired fins correspond to the upper bones of our arms and legs.

When the 1938 fish was found, many scientists hoped it would show how fish first crawled onto land. A coelacanth even moves its paired fins in a diagonal rhythm, the left front fin with the right rear fin, like the legs of a trotting animal. But coelacanths live in the deep sea and never come out of the water.

In 2013, researchers published the full genome of Latimeria chalumnae. Their analysis showed that lungfish, not coelacanths, are the closest living fish relatives of tetrapods. The coelacanth genome is still valuable for studying how genes for limbs, the immune system and the removal of nitrogen waste changed when vertebrates moved onto land.

Has the coelacanth stopped evolving?

No. The 2013 genome study found that the coelacanth’s protein-coding genes have changed more slowly than those of tetrapods, which fits its conservative body plan. Other parts of the genome, however, have changed at ordinary rates, including stretches of “jumping genes” that differ even between the two living species.

The two living species themselves show that change continues. The West Indian Ocean coelacanth and the Indonesian coelacanth look very similar, but genetic studies suggest they split roughly 30–40 million years ago. Neither species is known as a fossil, and the closest known fossil relative of Latimeria lived in the Late Jurassic, about 150 million years ago, leaving a very long gap with no fossils at all.

Why do scientists dislike the term living fossil?

The phrase “living fossil” was first used by Charles Darwin in 1859, and it is still popular. But in a 2013 review titled “Why coelacanths are not ’living fossils’”, the biologists Didier Casane and Patrick Laurenti argued that it gives the wrong impression, and several lines of evidence back that view.

First, ancient coelacanths were far more varied than the living fish suggests. The Early Triassic Rebellatrix had a deeply forked tail like a tuna’s and was probably a fast-swimming hunter. The mawsoniids lived in rivers, lakes and estuaries as well as the sea, and some reached 5 m (16 ft) or more, among the largest bony fish of their time. Second, the living species are not identical to any known fossil. Third, as the genome shows, coelacanths have kept evolving like any other animal.

Most scientists today use “living fossil” loosely, for a living species from an old lineage whose body shape has changed relatively little. In that limited sense, the coelacanth still fits, alongside the nautilus and the tuatara.

Why are there no coelacanth fossils from the last 66 million years?

The long gap in the fossil record does not mean coelacanths were absent. Fossils form best where dead animals are quickly buried in fine sediment, such as quiet lagoons, lakes and shallow seas, and many Mesozoic coelacanths lived in exactly those places. Living coelacanths shelter in caves on steep volcanic slopes, often 100–250 m (330–820 ft) deep, where bodies are rarely buried and rocks are seldom exposed for fossil hunters. Their deep-water ancestors probably left very few remains for the same reason.