1. The snowball effect
Time: 700 MYA
Description: The first megacontinent (I forget it's name) blocked out the sun and destroyed almost all of earth's population with a mile-thick worldwide glacier. Surface temperatures became about -40° F.
2. Massive Meteor
Time: 65 MYA
Description: A meteor 6 miles long attacked Earth. 70% of the population was wiped out. An explosion from a distant volcano didn't help the dinos to survive.