Although there are many different species of cartilaginous fish living today, they are far outnumbered by the bony fish. There are two main groups of bony fish, the lobe-fin fish, and the ray-fin fish. I will look at the lobe-finned fish in this article, and will cover the ray-finned fish in my next article.
Most of the lobe-fin fish have gone extinct, but they are still very interesting animals, because the first land-living vertebrate animals, the amphibians, arose from the lobe-finned fish. The lobe-fin fish get their name from the fact that their fins are carried on muscular, scale-covered lobes. These lobes look like short, stubby legs, and it is from these that the legs of the land-living vertebrates arose.
There are three main groups of lobe-finned fish. The Rhipidiastians were common in the Devonian Period (395 to 345 million years ago), and a few survived until the end of the Paleozoic Era (230 million years ago). They could reach over three meters (ten feet) in length, and were carnivorous (meat eating). Like many other Devonian fish, the Rhipidistians could breath air if the water became poor in oxygen. The Rhipidistians are the group that gave rise to the amphbians.
The next group of lobe-finned fish are the Coelacanths. See my article "Coelacanth: The Living Fossil" for the scoop on Coelacanths.
The third group of lobe-finned fish are the lungfishes or Dipnoi. The lungfish are probably the group of fish best known for their ability to breath air, and to estivate. Estivation is like hibernation; however, instead of sleeping for a long period during the winter, estivating animals will "sleep" for shorter periods at times other than winter, such as during a drought that could endanger fish survival. The lungfishes became rare in the fossil record towards the end of the Paleozoic Era (230 mya), but there are three genera still extant (still living today).
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