The mass extinction that took place 250 million years ago and 65 million years ago was due to the catastrophic activities that took place at that time.
The extinction almost 95% of the population took place due to major climatic change that took place long time ago (250 million years ago).
The extinction of dinosaurs took place 65 million years ago due to the activity of the meteorites that suddenly hit the earth and killed almost all the dinosaurs.
Both of the event took place due to gradual process or catastrophic activities.
