Today the Washington Post is carrying this article providing more details about the discovery of an impact crater that may have caused the great Permian extenction event approximately 2.5 billion years ago:

The researchers said that geological evidence suggests that an object about six miles in diameter crashed at the shoreline of what is now Australia's northwestern coast, creating climate changes and other natural catastrophes that wiped out 90 percent of marine species and 70 percent of land species.

As with the Cretaceous event, the other leading cause is a sudden, massive, increase in volcanism at about the same time. I guess geologists just haven't come up with a mechanism for how impacts can actually cause massive eruptions, but the ocurrance of two similar events seems more than just a coincidence to me.