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230 million to 65 million years ago, reptiles called dinosaurs lived on the
Earth. They varied tremendously in size and shape. Some were about the size of a
dog but others were huge, weighing many tons. The largest species ate plants,
but others were flesh eaters. Most were four-legged, but many walked and ran on
their hind legs only. Dinosaur fossils have been discovered nearly worldwide,
and major new finds are continually reported. These fossils include bones, eggs,
footprints, and even traces of feathers. Based on their study of fossils,
scientists have been able to classify dinosaurs into two main groups, the
Saurischia ("lizard hips") and the Ornithischia ("bird hips"). Also using
fossils as a guide, scientists have created life-size models that show how
different events in a dinosaur's life may have occurred. But many mysteries
remain. One of the biggest concerns the disappearance of the dinosaurs.
Scientists have proposed various theories to account for the extinction of these
fascinating animals, but none have been conclusively proven.
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