Mass extinction's cause: 'Sick Earth'

What really caused the largest mass extinction in Earth's history? USC earth scientists will reveal new clues at the annual meeting of the Geological Society of America in Philadelphia Oct. 22-25.

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<p><a href="http://www.physorg.com/news80653738.html">Mass extinction's cause: 'Sick Earth'</a></p> <p>What really caused the largest mass extinction in Earth's history? USC earth scientists will reveal new clues at the annual meeting of the Geological Society of America in Philadelphia Oct. 22-25.</p>