How Many Mass Extinctions Have There Been

– Mass Extinction Event: a significant, global decrease in the diversity of life
– "Big 5": The five biggest mass extinction events since the Cambrian explosion of Life 550 million years ago
– Biodiversity Crisis: Like a mass extinction, a biodiversity crisis is a marked depletion in diversity in the fossil record. Some scientists prefer to call the late-Devonian extinction a "biodiversity crisis"