The K–T Extinction event at the end of the Cretaceous Period was the catastrophic mass extinction of extant animal species in a comparatively short period of time, approximately 65.5 million years ago.
K-T Extinction in "Before the Beginning"Edit
The time-viewer proved that an asteroid caused the K-T Extinction. That hit made the atomic bombing of Hiroshima look like a lit match next to a forest fire. However, the K-T was nothing compared to the Permian-Triassic event.