Mass Extinctions Remain Mysterious

Sure, a giant asteroid wiped out the dinosaurs. But four other known mass extinctions are blamed mostly on other things or haven't been figured out.

And there might be a lesson in that. The end-Permian extinction 250 million years ago happened in essence because "the Earth got sick," USC earth scientist David Bottjer said this week at Joint Annual Meeting of the Geological Society of America.

Figuring out that planet's ancient ills could help researchers better understand the current decimation of coral reefs, he said.

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Greatest Mysteries: What Causes Mass Extinctions?

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