(Blacksburg, Va., May 14, 2002) -- Marine life had to re-evolve after two major extinctions in order for shrimp and whales and other sea life as we know it to come into being. But what is remarkable, according to an article published in the May 14, 2002 issue of the Proceedings of the National Academy of Sciences (PNAS), is not that marine life recovered from two mass extinctions, but that marine ecosystems have maintained very stable structure over the last 450 million years and only changed noticeably in the recovery from these two great extinctions.

Previous studies claim five mass extinctions decreased the diversity of ocean life. But the PNAS article, "Anatomical and Ecological Constraints on Phanerozoic Animal Diversity in the Marine Realm," by Richard Bambach, professor emeritus of geological sciences from Virginia Tech; Andrew H. Knoll of Harvard, and the late J. John Sepkoski Jr. of the University of Chicago, reports and analyzes evidence that only two of these mass extinctions led to major change in global ecosystem structure. These were the extinctions 251 million years ago, at the end of the Permian period, and 65 million years ago, at the end of the Cretaceous period.

The Permian extinction was massive. It is believed to have wiped out 95 percent of all species, possibly as a result of increased levels of dissolved CO2 in the oceans triggered by a comet or meteorite impact. (Inclusions in Permian rocks containing gases known only to exist in outer space were discovered only last year.) The Cretaceous extinction, also triggered by a large impact, was not as thorough, but is the one we are all familiar with because it killed the dinosaurs. Although it also devastated marine life, the pattern of extinction at the end of the Cretaceous was quite different than in the Permian, suggesting a different immediate cause of death than in the Permian event.

However, the change in ecosystems was not produced by the killing events, the articl
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Contact: Richard Bambach
rbambach@oeb.harvard.edu
617-495-7602
Virginia Tech
13-May-2002

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