Fish and amphibians

Fish predate dinosaurs by several million years. The first fish appeared on 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 a heavy armor 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 they were also 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 Dunkleosteus grew up to 23 feet (7 meters) long and had powerful jaws and sharp bony plates that served as teeth.

The Age of Fishes

The Age of Fishes was a period in 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.

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 armored. 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. 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 percent of all the world's 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

The first modern fish appeared during the Mesozoic Era, which began about 248 million years ago. The chondrosteans of the Devonian Period had given rise to the first neopterygians, the group from which most of today's fish species developed. Modern bowfin and freshwater gars resemble the earliest forms of neopterygians. From fish such as these arose the teleosts, the advanced modern fishes.

The teleosts lost the heavy armor that covered the bodies of most earlier fish. At first, all teleosts had soft-rayed fins. These fish gave rise to present-day catfish, eels, minnows, and other soft-finned fish. The first spiny-finned fish appeared during the Cretaceous Period, which began about 145 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.

Amphibians

According to paleontologists (scientists who study prehistoric life), the oldest fossils of amphibians date back to the end of the Devonian Period—about 360 million years ago. Amphibians are thought to have been the first group of vertebrates to emerge from the water and live on land. Most scientists believe the amphibians evolved (developed gradually) from the lobe-finned fish. Lobe-finned fish had lungs and enlarged fins supported by bones and muscles. They could use their fins as legs to come out of the water for brief periods. These fins probably developed into amphibian legs. Amphibians were the most important vertebrates on land during the Carboniferous Period—from 360 million to 286 million years ago. There were many more kinds of amphibians then than there are now.

The groups to which modern amphibians belong did not appear until the Mesozoic Era—from 248 million to 65 million years ago. By then, most of the other amphibians had died out. Scientists do not know all the reasons why amphibians became less numerous. The world's climate was becoming drier, and many of the ponds and lakes needed by amphibians were disappearing. Also, reptiles, which first appeared during the Pennsylvanian Period, were becoming more important. Reptiles are not so dependent on water because they have hard-shelled eggs that will not dry out on land. Reptiles probably ate the same foods as many of the early amphibians and probably preyed on amphibians themselves.