More info? According to Wikipedia, the Phanerozoic Eon has been lasting for 541 million years (I rechecked because wasn't sure what My meant at first). How would we even detect extinction events from a few million years back? Also, hundreds of millions years divided by thousands of geomagnetic field reversals sounds like very rare (considering the briefness of human existance). Also, ecosystem collapses are probably not all the same: there's gigantic explosions (a la dinosaur killer), there could be ocean acidifaction, etc.