Impact of Meteorites, Drop in Sea Level Caused Mass Extinctions 65 Million Years Ago

November 21, 1996

The scientific dispute over what caused the extinction of 70 percent of all species worldwide 65 million years ago is closer to a resolution, with new research by scientists from UCLA and the University of Washington. The research was funded by the National Science Foundation (NSF).

A cover story in the November 22 issue of the journal Science reports the researchers' evidence supporting the controversial theory that the extinction of dinosaurs and many other species was caused by the impact of a huge meteorite that crashed to Earth some 65 million years ago.

"The end-Cretaceous extinction is one of the largest mass extinctions in Earth's history, and its cause has been among the most contentious, hotly debated issues in paleontology," says Chris Maples, program director in NSF's division of earth sciences. "This work by Marshall and Ward is important because it may result in more agreement among proponents of different end-Cretaceous extinction scenarios."

However, the scientists -- paleontologist Charles Marshall of UCLA and geologist Peter Ward of the University of Washington in Seattle -- also present evidence that other factors, including a drop in sea level prior to the assault by the massive asteroid or comet, also may have caused some of the extinctions at the end of what is known as the Cretaceous period. Marshall and Ward present evidence demonstrating that a combination of factors caused the mass extinctions.

Marshall applied statistical analysis to a well-preserved and well-documented fossil record which Ward has collected -- some 40 species of sea creatures from the Cretaceous period that include clams and squid-like creatures with shells called ammonites.

"For the first time, we can estimate the relative importance of all the major factors that led to the extinction of the ammonites," says Marshall. The impact of the asteroid or comet accounts for 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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National Science Foundation

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