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Native to the Yangtze River in China, the Chinese river dolphin is one of many species to have recently gone extinct in what some are calling "the sixth great mass extinction" in Earth's history. Far from being caused by humans, though, the five prior such events --- the Ordovician-Silurian (440million years ago, or mya), the Devonian (365 mya), the Permian-Triassic (250 mya), the Triassic-Jurassic (201 mya), and the Cretaceous-Tertiary ______ all predate us.