Mass extinction is known to reduce the diversity greatly but it plays a major role in the evolution process.
Mass extinctions are responsible for creating new opportunities for different forms of life to evolve and thrive.
This causes reduced competition for resources, which surviving species could utilise and diversify.
Triassic-Jurassic Extinction: Occurred around 18 million years ago at the end of the Triassic age causing all such terrestrial and marine reptiles and plant species including the dinosaurs to be wiped out.
Cretaceous–Paleogene mass extinction: occurred 66 million years ago and killed all the non-avian dinosaurs causing mammals and birds to evolve.