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The paper, written by Dr. Stanley and his former student, Xiangning Yang, a professor at China's Nanjing University, will be published on Nov. 25 in the journal Science.

The researchers studied fossil records to learn more about the characteristics of a mass extinction that marked the end of the Permian period, about 245 million years ago. Scientists have traditionally estimated that as many as 95 percent of the species on Earth died at that time. The mass extinctions were possibly the result of gigantic lava flows in Siberia that might have altered the planet's atmosphere.

Dr. Stanley said his figures indicate the number of species killed during the second of the two extinction pulses was about 80 percent, still the largest mass extinction in Earth's history, but considerably less dramatic than previous estimates indicated.

"People just keep perpetuating this idea that the extinction might have removed as many as 95 percent of species, but clearly it did not," said Dr. Stanley, a professor in the Department of Earth and Planetary Sciences.

Dr. Stanley and Dr. Yang studied fossils left by several groups of animals, ranging from ancient clams and snails, to nautilus- like predators called ammonoids and a group of shelled, single- celled creatures called fusulinaceans, relatives of amoebas that looked like grains of wheat or rice.

The spindle-shaped fusulinaceans provided the researchers with some of the most concrete data they needed to determine that the Permian extinctions were not the result of one event. Instead, the species were first killed off during a time referred to by geologists as the Guadalupian stage of the Permian period. A hiatus of five million years followed before the second, and largest extinction pulse, which came during a time referred to as the Tatarian stage, which marks the end of the Permian period.

The fusulinaceans were abundant on seafloors during the Permian, and their tiny skeletons are found in limestone deposits around the world. Their extinction pattern shows that all of the largest genera (groups of species) of fusulinaceans, those longer than about six millimeters, were killed during the Guadalupian interval. Included among the genera that died during that time were all of those possessing a distinctive wall structure resembling a honeycomb.

The Guadalupian interval would have been recognized by geologists as a major period of mass extinction if not for the overshadowing period five million years later, Dr. Stanley said.

By lumping extinctions from both intervals together, scientists have overestimated the Tatarian extinction rate, he said.

Of the 59 genera of fusulinaceans that existed at the Guadalupian stage, 45 were killed, leaving only 14 surviving. Five new genera emerged in the Tatarian, but the mass extinction at the end of that interval wiped out all of the remaining 19 genera, Dr. Stanley said.

"If we take the record at face value, clearly this earlier event was a major event," he noted. Perhaps slightly more than 70 percent of species were killed during the Guadalupian.

Fusulinaceans provide an excellent fossil record, since they preserved well and were distributed globally, making them ideal for illustrating the double-pulse extinction conclusion.

One paleontologist who has reviewed the work said Dr. Stanley's research may reflect a trend in which scientists find better and better evidence for such extinction pulses, challenging the more accepted idea that mass extinctions happened in single catastrophic die-offs.

"I think this is a very good development," said David Raup, professor emeritus at the University of Chicago's Department of Geophysical Sciences. "His data and his interpretation seem very credible to me. As we do more rigorous analysis, and as we get more data, I think we are going to see a lot more pulses of extinction."

John Sepkoski, another University of Chicago paleontologist who reviewed Dr. Stanley's findings, said he had been skeptical about the double-event hypothesis, until he analyzed the data, which he called "quite convincing."

Researchers are finding that a similar pattern marked mass extinctions during the late Devonian period, about 367 million years ago. In that period, many organisms first became extinct in a similar pulse, followed by a more devastating event at the boundary between the Devonian and Carboniferous periods, Dr. Stanley said.

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