End Cretaceous - 65 million years ago
This event is also known as the K/T mass extinction, since it occurred between the Cretaceous and Tertiary periods. Although it was not the largest of Earth’s mass extinctions, it is probably the best-known because it ended the reign of the dinosaurs.
Cause:		Scientists largely agree that an impact event had something to do with the End Cretaceous extinction. The large crater at Chicxulub, Mexico, is thought by many to have been the site of the impact, but there is much controversy about this in the scientific world at present. Other theories posit that volcanic activity and falling sea levels either caused or contributed to the extinction.
Result:		On land, all non-avian (flying) dinosaurs and pterosaurs, and some mammals and birds went extinct. Global vegetation was also severely affected, with North America along thought to have lost up to 60% of its land plants. In the oceans, several groups of large reptiles (the mosasaurs and plesiosaurs), all ammonoids, and some clam groups went extinct.