Phillips fixed as the dividing point between the Paleozoic and the Mesozoic what would now be called the end-Permian extinction, and between the Mesozoic and the Cenozoic, the end-Cretaceous event. (In geologic parlance, the Paleozoic, Mesozoic, and Cenozoic are “eras,” and each era comprises several “periods”; the Mesozoic, for example, spans the Triassic, the Jurassic, and the Cretaceous.)
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