Extinctions are a relatively common occurrence in the geologic past, and are the result of biologic and environmental pressures and processes. Mass extinction (along with mass radiation) forms the boundary of geologic time units. Indeed, abrupt extinctions often allow for surviving organisms to enter, exploit and adapt to new environments. Major mass extinctions have included:
Cambrian-Ordovician boundary (510 mya)
Ordovician-Silurian boundary (440 mya)
Devonian-Carboniferous] boundary (360 mya)
Permian-Triassic boundary (245 mya) - over 90% of life was lost in this, the largest extinction ever.
Triassic-Jurassic boundary (208 mya)
Cretaceous-Tertiary boundary (65 mya) - and there go the dinosaurs!
Mass extinction is often caused by competition, reduction of available food, atmospheric changes, climate changes, sea level changes and plate tectonics - all of which can be wondrously intertwined at times.