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Thursday, December 5, 1996

RESEARCH:

Study results in unique, statistical extinction explanationBy Teresa Jun

Daily Bruin Contributor

For many people, news concerning the extinction of dinosaurs may seem as old as dinosaur fossils themselves. But those who study the field know that the question of what exactly caused the extinction 65 million years ago is still hotly debated among scientists today.

For Earth and Space Sciences Associate Professor Charles R. Marshall, this debate has finally progressed toward a resolution. In a new study co-authored with Geology Professor Peter Ward of the University of Washington, Marshall provides an innovative statistical explanation for the extinction of dinosaurs and other species.

Recently, the Science journal published the results of their research in a front cover story.

The classic extinction debate centers around what exactly caused the mass extinction of most species world-wide 65 million years ago. Ward and Marshall's study presents evidence that while more than one factor caused the extinctions, a meteorite impact was responsible for the majority of the mass extinction.

"Some scientists claim that a huge meteorite struck the earth during this time and caused a sudden extinction," said Professor Marshall. "Others say that the impact was nothing. They say that the fossil record shows a pattern of natural extinctions with species gradually tapering off long before the impact."

According to Marshall, the problem with the on-going dispute is that the fossil record is too incomplete to draw conclusions from it by observing the record literally.

"Imagine that a woman dies," he explains. "And the only record of her existence that she leaves behind is her checkbook. Now, how would you go about determining what day she died?" Simply looking at the date of the last check she wrote would not be helpful because that may not necessarily be the day that she died, according to Marshall, who said a more accurate approach would be to observe the pattern of her previous check-writing.

"We wouldn't be able to precisely pinpoint the exact date," said Marshall. "But by observing the frequency of the checks, a span of time in which she died can be somewhat accurately determined. This margin of error is the same approach we must take with all of our available fossil data."

Marshall uses this analogy to clarify the unconventional approach he took in this research, which has been commended by his colleagues in the field.

"This study statistically proves that the last time you see a fossil of a particular species is not necessarily the last time the species existed," supported Frank Kyte, researcher and geochemist in the Institute of Geophysics and Planetary Physics.

By applying a statistical analysis to the well-documented fossil record collected by Ward, Marshall's analysis avoids the shortcomings of previous scientific explanations. Two common shortcomings are reliance on either a literal interpretation of the fossil data or a broad "guesstimate" approach to the fossil record.

The conclusions of the study indicate that a combination of factors contributed to the mass extinctions. According to the study, about 50 percent of the extinctions were caused by the impact of the meteorite ­ a surprising statistic for scientists at both extremes of the debate. Another 25 percent became extinct due to a substantial drop in sea level, and the remaining 25 percent became extinct through "background extinction," extinctions that would have occurred naturally with time.

"Ward's and Marshall's research is the first really proper analysis of what happened at the end of the Cretaceous," commented Earth and Space Sciences Department Chairman Bruce Runnegar. "They apply a thorough, mathematical analysis to the imperfect, incomplete fossil record."

Their thorough analysis was the result of years of hard work in the field. Ward had been accumulating a rich fossil data set for several years before Professor Marshall approached him with his hypothesis.

"I kept myself updated with his records. His data sets were a classic in the field," Marshall recalled. "I wanted to test my hypothesis by applying it to his exceptional fossil record."

As a result of this cooperative union, Marshall and Ward spent a week in the south of France, analyzing fossil deposits during the summer of 1994.

Their days were spent doing field work in the rich fossil deposits of the northern coast of Spain, near the French border.

One of the photographs taken of the fossil site during this expedition was chosen by the Science committee to appear on the front cover of the magazine.

This past summer, when Marshall found out that the research article he and Ward had submitted to Science was accepted for print, he said he was very excited, since it was the first time that he had ever submitted anything to Science for publication. Several weeks later, when the journal committee announced its decision to print his photo on the front cover and promote his article to a cover story, "Cool!" was his initial, giddy reaction.

The next step for Marshall in this research is to create statistical models and conduct further critical analyses of the extinction. But for the most part, the toughest part is over for the UCLA professor of six years.

"Scientists will never all agree about what really caused the extinction," added Kyte. "But Professor Marshall's study provides an important statistical tool to go about analyzing the imperfect fossil record."

JUSTIN WARREN/Daily Bruin

Charles Marshall, Associate Professor-Earth & Space Sciences, conducts research which focuses on a more statistical explanation for the cause of dinosaur and other species extinction.