Mass extinctions have occurred many times in Earth's history.

Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time.

Mass extinction is defined by some scientists as a rate of extinction 100 to 1,000 times the natural rate.

Mass extinctions are periods in Earth's history when abnormally large numbers of species die out simultaneously or within a limited time frame.

Mass extinctions seem to be a Phanerozoic phenomenon, with extinction rates low before large complex organisms arose.

Mass extinctions have sometimes accelerated the evolution of life on Earth.

Mass extinctions are thought to result when a long-term stress is compounded by a short term shock.

Mass extinctions have been recorded since the dawn of paleontology.

Mass extinction should only accelerate the process of predictable advance.

Mass extinctions in large part reset the evolutionary clock.