Millions of years ago, there were three hundred or more different species of dinosaur. then, about 65 Million years ago, something happened, and now there are no dinosaurs. At about the same time, 70 per cent of all species in the world became extinct. Most of the species in the sea disappeared before that, 250 Million years ago. About five times in the past, scientists think, there were big extinctions. A lot of species became extinct, and some different appeared for the first time.