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The Fish, Part IV

© Beverly Eschberger

This is my last article (for now) in my series about the fish. In my last article, I began talking about the osteichthyan fish, also known as the bony fish. The osteichthyan fish are divided into two groups, the lobe-finned fish and the ray-finned fish. I discussed the lobe-finned fish in my last article and will discuss the ray-finned fish in this article.

The ray-finned fish, also called the actinopterygians, are the group of fish that dominate both the fresh water and marine (salt water) environments today. The ray-finned fish are also the largest group of extant (still living) vertebrates, with more than 20,000 species.

The ray-finned fish first appear in the fossil record during the Devonian Period (395 to 345 million years ago). The name "ray-finned" comes from the fact that, unlike the lobe-finned fish, their fins do not have lobes but are supported by bony rays that grow straight out from the body.

The earliest group of ray-finned fish were the chondrosteans. They were mostly small fish, and throughout the Palaeozoic Era (570 to 230 million years ago) they were rather insignificant freshwater fishes. Towards the end of the Palaeozoic Era, however, as the placoderms and other ancient groups of fish were disappearing, the chondrosteans began to take over their niches.

Like the placoderm fish, the chondrosteans had armored heads. Although this provided protection, it restricted the movements of their jaws. By losing their body armor, the chondrosteans were able to develop mouths that could be extended to snap at prey and suck in small animals.

They also reduced the size of their scales and developed primitive swim bladders. The swim bladder allows fish to control their bouyancy by altering the amount of gas in the bladder. This meant that the chondrosteans no longer needed the movements of their tails to provide lift in the water, allowing their heterocercal ("unequal lobes") tails to gradually become homocercal, or symmetrical, a shape that we see in extant ray-finned fish today.

The next group of ray-finned fish, the holosteans, appeared in the Permian Period (280 to 230 million years ago). During the Triassic Period (230 to 195 million years ago), the holosteans replaced the chondrosteans almost completely, although a few chondrostean species still survive today (the sturgeon, the paddle-fish (Polyodon), and the bichir (Polypterus) of Africa). These three species have survived only in restricted areas of fresh water that have relatively little competition.

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