About 75 million years ago there were more kinds of dinosaurs than ever, but 10 million years later all the birds had vanished as did land animals, Pterasaurs and sea creatures. There have been many theories about what exactly happened that would have caused this mass extinction. Many scientists believe it was caused by the collision of a massive comet. Clouds of rock and dust would have filled the atmosphere, hiding the sun. There would have been no heat and the dinosaurs would have died from the cold and lack of food. Nothing would have survived!