Ocean ecosystems only altered following two great mass extinctions

e 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.


Contact: Richard Bambach
Virginia Tech

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