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SPOTLIGHT ON: MASS EXTINCTION

Mass extinctions are episodes in the history of life on Earth during which unusually large numbers of species die off. They stand in contrast to the background rate of extinction, which occurs even when the diversity of life is increasing.

Scientists recognize five major mass extinctions in the Earth’s history. The extinctions are measured in terms of large groups of related species, called families.

The five mass extinction episodes occurred because of major changes in the prevailing ecological conditions brought about by climate change, cataclysmic volcanic eruptions, or collisions with giant meteors. The sixth mass extinction appears to be in progress now, and the main cause is environmental change brought about by human activities.

Millions of Years Ago Name of Extinction Episode Percent of Families That Died Out

          Note

440 Late Ordovician 20 - 50 % Tropical areas are hit hardest. No terrestrial vertebrates have evolved yet.

360

 Late Devonian 20 - 30 % Many fish and marine invertebrates die out.

 

250

 End of Permian 50 % Greatest mass extinction ever.
213 Late Triassic 20 - 35 % Dinosaurs and mammals had just recently evolved; both live through this extinction.
66 Late Cretaceous 15 % Dinosaurs become extinct, possibly caused by a metorite colliding with the Earth. Mammals and fishes begin to proliferate. 65 million of years after this mass extinction, early humans evolve.
NOW Holocene ? Anthropogenic causes

 

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Words in bold italics can be found in the glossary.
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