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Cases in which many species become extinct within a geologically short interval of time are called mass extinctions.There was one such event at the end of the Cretaceous period (around 70 million years ago).There was another, even larger, mass extinction at the end of the Permian period (around 250 million years ago).The Permian event has attracted much less attention than other mass extinctions because mostly unfamiliar species perished at that time.

托福真题（四科目）

https://docs.qq.com/doc/DSVFTUVNQVXNkalJS