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The Mesozoic started with a mass extinction (Permian-Triassic). The Triassic Period then gave way to the Jurassic Period following another mass extinction. The Mesozoic Era ended with another extinction (Cretaceous - Paleogene). So my question: why are the Jurassic and Cretaceous periods separate?

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The Mesozoic started with a mass extinction (Permian-Triassic). The Triassic Period then gave way to the Jurassic Period following another mass extinction. The Mesozoic Era ended with another extinction (Cretaceous - Paleogene). So my question: why are the Jurassic and Cretaceous periods separate?	4