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About this unit

Dinosaurs first appeared on Earth about 240 million years ago. They varied greatly in shape and size. Some weighed more than 80 tons and were more than 120 feet long. Others were the size of a finch and weighed as little as 8 ounces. Most dinosaurs became extinct at the end of the Cretaceous Period 66 million years ago, but one group survived: modern birds, which are direct descendants of carnivorous dinosaurs.