The Permian-Triassic extinction of 250 mya was the most severe. Up to 90% of all species lost (don't tell the EPA). The theory has always been a volcanic period, especially in Siberia. Now it appears to be verified:

http://blogs.scientificamerican.com/history-of-geology/2011/09/16/large-igneous-provinc...

New pinointing of the extinction:

http://www.sciencedaily.com/releases/2011/11/111117143955.htm