Dinosaurs first appeared on Earth about 228 million years. 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 65 million years ago, but one group survived: modern birds, which share a common ancestor with non-avian dinosaurs.