mass extinction

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The extinction of a large number of species within a relatively short period of geological time, thought to be due to factors such as a catastrophic global event or widespread environmental change that occurs too rapidly for most species to adapt. At least five mass extinctions have been identified in the fossil record, coming at or toward the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous Periods. The Permian extinction, which took place 245 million years ago, is the largest known mass extinction in the Earth's history, resulting in the extinction of an estimated 90 percent of marine species. In the Cretaceous extinction, 65 million years ago, an estimated 75 percent of species, including the dinosaurs, became extinct, possibly as the result of an asteroid colliding with the Earth. Compare background extinction.
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“Was” is used for the indicative past tense of “to be,” and “were” is only used for the subjunctive past tense.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

How to use mass extinction in a sentence

Cultural definitions for mass extinction

mass extinction

Any of several events in the Earth's past in which large numbers of species (in some cases, up to eighty percent) became extinct.

notes for mass extinction

The most famous mass extinction included the destruction of the dinosaurs sixty-five million years ago. (See Alvarez hypothesis.)
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.