Dinosaurs went extinct about 66 million years ago, at the end of the Cretaceous Period. This extinction event was one of the most dramatic mass extinctions Earth has ever seen, and it claimed three-quarters of life on Earth. Many other animals also died out, including pterosaurs, large marine reptiles, and ammonites. The exact nature of this catastrophic event is still open to scientific debate, but evidence suggests that an asteroid impact was the main culprit, together with volcanic eruptions that caused large-scale climate change and more gradual changes to Earths climate that happened over millions of years. Birds were the only dinosaurs to survive the mass extinction event. Paleontologists have yet to discover rocks with a trace of a dinosaur younger than 66 million years, during the Cretaceous period.