The most famous mass extinction event in the history of the Earth is probably this one. About 65 million years ago, the Mesozoic Era, the Age of Reptiles, ended, ushering in the Cenozoic Era, the age of mammals. This is the Cretaceous/Tertiary extinction. It is famous because it is the one that killed the dinosaurs. Nearly all scientists agree that an extraterrestrial impact is probably the reason for this extinction, but not all scientists agree and we will discuss that in this lesson.