Prehistoric animal, animal that became extinct before human beings began to produce written records. Our knowledge of these animals is therefore derived almost completely from fossils. Although scientists believe life on earth began over 3 billion years ago, few fossils have been found that are more than 600 million years old. The earliest are all invertebrates, or animals without skeletal backbones. These include ammonites, snails, clams, worms, and animals resembling jellyfish. The most common prehistoric invertebrate seems to have been the trilobite—a kind of flat shellfish with jointed legs. The first fishes appeared about 480 million years ago. They had no jaws and were covered with heavy, bony armor. Fishes as we know them did not appear until about 130 million years later. Some of these had fleshy fins that probably evolved into legs. The first amphibians appeared about 400 million years ago.
An extensive fossil record indicates that the first land vertebrates—the reptiles—evolved about 290 million years ago. They were bigger and more powerful than the amphibians, and were able to hatch their eggs on land. Reptiles dominated the earth for about 100 million years. Dinosaurs are perhaps the best known of the prehistoric animals. Although some grew to enormous size and were very powerful, they all became extinct, although the reason why remains a subject of controversy. Among the dinosaurs was the carnivorous Tyrannosaurus rex, the 85-ton Brachiosaurus, the 87-foot-long Diplodocus, the horned Triceratops, and the armored vegetarian Stegosaurus. Flying reptiles began to appear during the Jurassic period. One of these—the Archaeopteryx—is believed to be the earliest ancestor of modern birds.
Placental mammals, or animals who carry their young within their bodies, have been on earth for about 65 million years. Later, when mammals came to dominate the land, larger variants of modern-day mammals existed. Megatherium was a 20-foot-long mammoth that resembled a large, hairy elephant. Some of these prehistoric mammoths have been found deep-frozen in the icy soil of Siberia.
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