03/20/02 - 12:36:38
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Of all the species that have ever lived (do we really know?), some 99 percent are now extinct. A small percentage of species provides the future diversity, but extinction is so common that it is the rule rather than the exception.|
Lest anyone use this knowledge to excuse the modern destruction of habitats and species, the number and rate of extinctions occurring today occupy that rarefied zone that paleobiologists call mass extinctions. Although most (probably over 90 percent of) extinctions occur as normal or background extinctions, in the past 540 million years during the Phanerozoic Era five mass extinctions have occurred when the numbers and rates of extinction have become so high that they stand out as peaks above the background extinctions. A sixth mass extinction is now underway, but this time WE are the culprits. Although the earlier five mass extinctions comprise a relatively small percentage of total extinctions during Earth's herstory (for Kevin!), they each represent a major reorganization of Earth's biota. The severest of the five mass extinctions occurred at the end of the Permian, some 250 million years ago (mya), possibly with near 95 percent species extinction. The most famous mass extinction, however, was the one that included the dinosaurs, the terminal Cretaceous mass extinction 65 mya.
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