Mass extinction

Mass extinction is the wipe out of many species at one time. It is usually caused by a major catastrophic event like a meteor or a flood. It allows new organisms to come and rule the earth. 

The greatest example of this is the dinosaurs. Dinosaurs ruled the earth, they lasted for thousands and thousands of years as being the biggest creatures on earth, but that all changed when a mass extinction occurred and al the dinosaurs were wiped out. This allowed animals like mammals to rule.