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The Permian-Triassic extinction opened the way for the rise of dinosaurs — which themselves fell victim to another mass die-off 65 million years ago. Most experts agree that the catastrophic impact of a massive asteroid or crater played the primary role in that extinction, at the boundary between the Cretaceous and Tertiary geologic periods. The blast is thought to have formed the Chicxulub crater off Mexico’s Yucatan Peninsula.