Lungfish, coelacanths and rhipidistians

The lungfish are freshwater fishes which feed on molluscs. Only three groups still survive. Protopterus is found in Africa, Lepidosiren in South America, and Neoceratodus in Australia. The lungfish can survive dry seasons by burrowing into the mud and forming a cocoon. They then use their lungs to breathe. Fossil lungfish of this type, and also their burrows, have been found from rocks of the Permian period.

Lungfish occur on three continents

The fact that these three kinds of living lungfish are found on three widely separated continents is evidence of an important theory concerning the structure of the earth. It is believed that once, long ago, the continents were joined together. Slowly they began to drift away from each other, floating on the vast layer of molten rock which occurs beneath the earth's crust. It is interesting to look at a world map and see that in many places the continents would fit into each other very neatly, like pieces of a huge jigsaw puzzle. If this theory is correct, then presumably the continents drifted apart after the lungfishes had evolved.

How the continents fit together

The coelacanth - a living fossil

Most of the second group of lobe-finned fishes, the coelacanths, lived in the sea. Their rear fins were jointed, so that these fishes could spin them around much more freely than other fishes can. Until 1938 it was thought from their fossils that coelacanths lived from the Devonian period to the Cretaceous, and had been extinct for 100 million years.

Then, in 1938, Miss M.C. Latimer, curator of a local museum, discovered a coelacanth in a pile of fish brought ashore in South Africa by a fishing boat. A scientist, Professor Smith, examined this blue-scaled fish and gave it a name, Latimeria chalumnae: Latimeria after Miss Latimer and chalumnae because it was caught at the mouth of the Chalumna river. Since then several more coelacanths have been caught, although no one has succeeded in keeping one of these fish alive in captivity.

It is strange to think that the coelacanths have been living on the Earth for over 300 million years, whereas people have been around for only four million years or so.

coelacanth

The rhipidistians

The third group of lobe-finned fishes, the rhipidistians, is interesting because its members are most closely related to land vertebrates. One of the best known examples is Eusthenopteron, which was 30 to 60 centimetres long. As with the other members of its group, its teeth and the skeleton of its fins were very like those of the early amphibians, the group of animals which live on land but lay eggs in water and includes present-day frogs, toads and newts.

The rhipidistians were freshwater fishes, which preyed on other aquatic creatures. They became extinct early in Permian times, roughly when most other lobe-finned fishes were dying out. This may have been due, in part, to competition from their descendants, the amphibians, a group of vertebrates we shall be looking at on the next pages.