Hopkins Scientist Challenges Theories of Extinction By Emil Venere Hopkins paleobiologist Steven M. Stanley has added a new chapter to the constantly evolving understanding of mass extinctions, periods when huge percentages of life on Earth died off. He and his former student, Xiangning Yang, a paleontologist at China's Nanjing University, have discovered strong evidence for a previously unknown surge of extinctions that rivaled the infamous die-off that felled the dinosaurs. Their research also challenges basic assumptions about Earth's most devastating mass extinction, 230 million years ago. Evidence in the fossil record shows that the spectacular extinction period was actually not as big as scientists have assumed. Moreover, it was not a single event, but a combination of two extinction "pulses" 5 million years apart, according to the Hopkins scientist. For more than a decade scientists have believed that up to 95 percent of all species on Earth ceased to exist at a point in geologic history marking the end of the Permian period. Scientists also have accepted that, although those extinctions might have taken place gradually, they probably resulted from a single deadly event or chain of events. But Dr. Stanley's work casts doubt on both assumptions. The researchers studied fossil records and developed tests to analyze the pattern of the extinctions, which possibly were the result of gigantic lava flows in Siberia that might have altered the planet's atmosphere. Their figures indicate that 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. "People just keep perpetuating this idea that the extinctions might have been as high as 95 percent of species, but it clearly was not," said Dr. Stanley, a professor in the Department of Earth and Planetary Sciences. Perhaps more startling is strong evidence that the first pulse, a previously undiscovered mass extinction, was a major die-off in its own right, possibly as large as the cataclysm that ended the age of dinosaurs 65 million years ago, marking the boundary between the Cretaceous and Tertiary periods. The scientists 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, many species were first killed off during a time referred to by geologists as the Guadalupian stage of the Permian period. A hiatus of 5 million years followed before the second, and largest, extinction pulse, which came during a time referred to as the Tatarian stage, marking 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 the largest genera, or groups of species, of fusulinaceans, those longer than about 6 millimeters, were killed during the Guadalupian interval. Included among the genera that died during that time were all of those possessing a distinctive, complex wall structure resembling a honeycomb. By lumping extinctions from both the Guadalupian and the Tatarian intervals together, scientists have overestimated the Tatarian extinction rate, Dr. Stanley said. Only a handful of scientists are aware of the findings, since the research has not yet been published. One paleontologist who 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 has reviewed Dr. Stanley's findings, said he was skeptical about the double-event hypothesis, until he analyzed the data, which he called "quite convincing." One of the problems in gathering data in the past has been that some of the most vital fossil evidence is located in obscure and politically touchy places, such as China, Tibet and Iran, noted Dr. Raup, who was a member of the Hopkins geology faculty from 1957 to 1965. Scientists, however, are now gaining access to places that had been off-limits, and new information is constantly coming in. In fact, the Permian extinctions are not the only die-offs being characterized as having happened in a pulse of catastrophy. 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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