Scientists recently discovered a 300 mile wide crater in Antarctica.  Over 250 million years ago the asteroid impact  triggered a mass extinction that made it possible for the dinosaurs to ascend to power.  They reigned for a brief 185 million years until another smaller asteroid took them out.

http://www.space.com/scienceastronomy/060601_big_crater.html

"The Permian-Triassic extinction, as it is known, wiped out most life on land and in the oceans. Researchers have long suspected a space rock might have been involved. Some scientists have blamed volcanic activity or other culprits.

The die-off set up conditions that eventually allowed dinosaurs to rule the planet.

The newfound crater is more than twice the size of the Chicxulub crater in the Yucatan peninsula, which marks the impact that may have ultimately killed the dinosaurs 65 million years ago. The Chicxulub space rock is thought to have been 6 miles wide, while the Wilkes Land meteor could have been up to 30 miles wide, the researchers said."