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Through the Ages Scientists learn how fish developed by studying the fossils of fish that are now extinct. The fossils show the changes that occurred in the anatomy of fish down through the ages. |
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First Fish The first fish appeared on the earth about 500 million years ago. These fish are called ostracoderms. They were slow, bottom-dwelling animals that were covered from head to tail with heavy armour of thick bony plates and scales. Like today's lampreys and hagfish, ostracoderms had no jaws and had poorly formed fins. For this reason, scientists group lampreys, hagfish, and ostracoderms together. Ostracoderms were not only the first fish, but also they were the first animals to have a backbone. Most scientists believe that the history of all other vertebrates can be traced back to the ostracoderms. The ostracoderms gave rise to jawed fish with backbones, and they in turn gave rise to amphibians (vertebrates that have legs and live both on land and in water). The amphibians became the ancestors of all land vertebrates. Ostracoderms probably reached the peak of their development about 400 million years ago. About the same time, two other groups of fish were developing - acanthodians and placoderms. The acanthodians became the first known jawed fish. The placoderms were the largest fish up to that time. Some members of the placoderm group called Dinichthys grew up to 30 feet (9 meters) long and had powerful jaws and sharp bony plates that served as teeth. |
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Age of Fishes The Age of Fishes was a period in the earth's history when fish developed remarkably. Scientists call this age the Devonian Period. It began about 410 million years ago and lasted about 50 million years. During much of this time, dinichthys and other large placoderms ruled the seas. |
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First Bony Fish The first bony fish appeared early in the Devonian period. They were mostly small or medium-sized and, like all fish of that time, were heavily armoured. These early bony fish belonged to two main groups - sarcopterygians and actinopterygians. The sarcopterygians had fleshy or lobed fins. Few fish today are even distantly related to this group. The coelacanth and the lungfish are the only surviving sarcopterygians. In addition, certain scientists include the African bichir in this group. Some scientists believe that among fish, lungfish are the nearest living relatives of land vertebrates. The actinopterygians had rayed fins without fleshy lobes at the base. Among the first actinopterygians were the chondrosteans, which differed in many ways from modern ray-finned fish. The chondrosteans were the ancestors of today's ray-finned fish, which make up about 95 per cent of all fish species. The paddlefish and sturgeons are the only surviving chondrosteans, and most scientists believe the bichirs are their nearest relatives. The first sharks appeared during the Devonian Period. They looked much like certain sharks that exist today. The first rays appeared about 200 million years after the first sharks. By the end of the Devonian Period, nearly all jawless fish had become extinct. The only exceptions were the ancestors of today's lampreys and hagfish. Some acanthodians and placoderms remained through the Devonian Period, but these fish also died out in time. The first modern fish, or teleosts, appeared during the Triassic Period, which began about 240 million years ago. The chondrosteans of the Devonian Period had given rise to another group of primitive bony fish, the holosteans. The holosteans, in turn, became the ancestors of the teleosts. The only surviving holosteans are the bowfin and freshwater gars. The teleosts lost the heavy armour that covered the bodies of most earlier fish. At first, all teleosts had soft-rayed fins. These fish gave rise to present-day catfish, minnows, and other soft-finned fish. The first spiny-finned fish appeared during the Cretaceous Period, which began about 138 million years ago. These fish were the ancestors of such highly developed present-day fish as perch and tuna. Since the Cretaceous Period, teleosts have been by far the most important group of fish. |
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Fish
Through the Ages
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