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 article emphasizes, but by the way life evolved afterwards. “What we think happened was that only these two big extinction events were severe enough to so empty the world that the basic structure of the ecosystem itself had to be re-established,” says Bambach. “That took time, and as things evolved, the pattern of ecologic relationship between different adaptive types of organisms developed differently.”
The analysis treats several very general modes of life and how they relate to total global diversity. A similar pattern of change is seen in each analysis. In one analysis, the authors simply compare the proportion of the marine fauna that is motile, or capable of moving around, versus the proportion that is sedentary or attached and doesn’t move. “The difference is important for several reasons,” says Bambach. “For instance, non-mobile organisms all need to get food brought to them in water currents whereas motile organisms can feed in many ways.
“Before the end-Permian extinction, about two-thirds of the different kinds of marine animals were immobile,” Bambach explains. “After the recovery from that extinction, the ratio between motile and non-motile was nearly one to one until the end Cretaceous extinction. Then, as marine life recovered from that event, motile organisms became about two-thirds of the different kinds of life, the reverse of the ratio before the Permian extinction. And that ratio has persisted to the present.”
While the standard ideas of the severity of extinctions has been called into doubt by differences in the quality of the geologic record, the results of the work by Knoll, Sepkoski, and Bambach, published in PNAS, demonstrate that the end Permian and end Cretaceous events did affect life in general in very important ways.
Previous related publications include:
1996 Knoll, A. H., R. K. Bambach, J. P. Grotzinger, and D. Canfield, “Comparative Earth History and Late Permian Mass Extinction”, Science, v. 273, p. 452-457.
1998 R. K. Bambach, “A Theory of Marine Mass Extinction,” p. 218-219 in Parker, S. P. (Ed.), McGraw-Hill 1999 Yearbook of Science and Technology, McGraw-Hill, Inc.
