Answer: Catastrophic events.
Explanation: The mass extinction of the organism that occurred 250 million of years ago and 65 million years ago took place due to the natural gradual processes or due to the Catastrophic events.
Catastrophic events can be defined as the events that has a massive impact on the population of the species.
These events can be hurricanes, tornadoes, et cetera. that can lead to a destruction of mass number of species.