Impact of Meteorites, Drop in Sea Level Caused Mass Extinctions 65 Million...( r 50 to 75 percent of the ammonite ext...)

r 50 to 75 percent of the ammonite extinctions, which is a much higher figure than some scientists had expected, but lower than others anticipated, he says. The substantial drop in sea level, which had peaked at least 10,000 years before the comet struck, accounts for between zero and 25 percent of the extinctions. In addition, about 25 percent of the extinctions were due to factors that would have occurred regardless of the meteorite or the change in sea level -- what scientists call "background extinction."

"Some scientists thought the extinctions were due solely to the impact of the asteroid or comet, others thought they were due to the sea level change, and still others thought that background extinction accounted for most of the extinctions; we're seeing evidence that all of them were factors," says Marshall.

Marshall and Ward found evidence for a decline in the abundance of species, and perhaps the extinction of more species than would have occurred normally, during the time sea level dropped substantially. In addition, the scientists found that there was a mass extinction of species most likely caused by the impact of the asteroid or comet.

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Contact: Cheryl Dybas
cdybas@nsf.gov
703-306-1070
National Science Foundation
21-Nov-1996

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