A relatively sudden, global decrease in the diversity of life-forms on Earth. Some mass extinctions appear to have resulted, at least in part, from extraterrestrial catastrophes involving asteroid or comet impacts and, possibly, nearby supernovae or other stellar outbursts. See also K-T boundary.
The Five Largest Known Mass Extinctions
| Geological Period | Years Ago | Notes |
| Late Ordovician | 438 million | 100 families extinct, including more than half of all bryozoan and brachiopod species. |
| Late Devonian | 360 million | 30% of animal families extinct. |
| End of Permian | 245 million | Trilobites extinct. 50% of all animal families, 95% of all marine species, and many trees die out. |
| Late Triassic | 208 million | 35% of all animal families die out, including most early dinosaur families and most synapsids, except for the mammals. |
| Cretaceous-Tertiary Boundary | 65 million | About half of all life-forms died out, including the dinosaurs, pterosaurs, plesiosaurs, ammonites, and many families of fish, snails, sponges, sea urchins. |
Copyright © 2004 by Wiley-Blackwell. Wiley and the Wiley logo are registered trademarks of John Wiley & Sons, Inc. and/or its affiliates in the United States and other countries. Used here by license.
