Dinosaurs went extinct about 66 million years ago, at the end of the Cretaceous Period. This event was one of the most dramatic mass extinctions Earth has ever seen, and it claimed three-quarters of life on Earth. The fossil record shows that for the first 175 million years of their existence, dinosaurs took on a huge variety of forms as the environment changed and new species evolved that were suited to these new conditions. Dinosaurs that failed to adapt went extinct. The exact nature of the catastrophic event that caused the extinction is still open to scientific debate, but the leading theories are that it was caused by an asteroid impact or volcanic eruptions that caused large-scale climate change. 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.