Millions of years ago, dinosaurs(恐龙) controlled the earth. And there were different types of dinosaurs on the earth. Some of them were no bigger than chickens, but all of them had very small brains(大脑). Later on, there weren't any dinosaurs. What happened to them?
In the days of dinosaurs, the whole earth was warm and wet. There were many green forests. But the earth changed slowly all the time. When parts of the earth became cold and dry, the forests there died. Then dinosaurs could not find enough food to eat. This must be one reason why dinosaurs died out.
What's another reason? We can make a guess. New kinds of animals came to the earth. Some had big brains and were fast and strong. They could kill dinosaurs. Some ate dinosaur eggs, too.
There may be other reasons that we do not know about. Each year, people will find some dinosaur footprints(脚印) and bones. Scientists are trying to tell us more about dinosaurs.
