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The end of the Cretaceous Period saw one of the most dramatic mass extinctions the Earth has ever seen. The fossil record shows that throughout their 160 million year existence, dinosaurs took on a huge variety of forms as the environment changed and new species evolved that were suited to these new conditions. Others that failed to adapt went extinct. But then 66 million years ago, over a relatively short time, dinosaurs disappeared completely (except for birds). Many other animals also died out, including pterosaurs, large marine reptiles, and other sea creatures such as ammonites. Although the number of dinosaur species was already declining, this suggests a sudden catastrophic event sealed their fate, causing unfavourable changes to the environment more quickly than dinosaurs and other creatures could adapt. The exact nature of this catastrophic event is still open to scientific debate. The catastrophe could have been an asteroid impact, volcanic eruptions or the effect of both, together with more gradual changes in the Earth’s climate over millions of years. Whatever the causes, the huge extinction that ended the age of the dinosaur left gaps in the ecosystem that were subsequently filled by mammals and birds, allowing them to evolve rapidly.