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The extinction of a large number of species within a relatively short period of time, as between the Cretaceous and Tertiary Periods when three-quarters of all species on earth, including most dinosaurs, became extinct.
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
mass extinctionA massive (>15%) and relatively abrupt reduction in the diversity and number of microbes, algae, fungi, protists, plants and animals on the planet.
Seven mass extinctions have have been identified according to the fossil record; the largest occurred in the Early Cambrian period (±500 million years ago) when there was ± 68% reduction in taxa.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.
mass extinctionany of the several episodes which have occurred throughout the evolution of biological organisms where there has been a period when organisms have disappeared from the fossil record relatively quickly and at a much greater rate than the background extinction. Such mass extinctions may well have resulted from several causes. The demise of the DINOSAURS at the end of the Cretacious (K/T boundary) was probably brought about by the earth being struck by an asteroid, the subsequent effects leading to the extinction of several groups of organisms. Climatic changes (some possibly caused by asteroid/comet impacts) also probably initiated some mass extinctions.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005