THE greatest crisis in the history of animal life on the planet - wiping out the vast majority
of species 200 million years ago - was a combination of two mass extinctions, according
to scientists.
The research, published today in the journal Science, may reflect a trend in which scientists
are increasingly challenging the accepted idea that mass extinctions throughout the history
of life on the planet were catastrophic die-offs. Instead, they believe they occurred in a series
of extinction "pulses".
The study of tiny single-celled creatures, clams, snails and other creatures, also suggests that
when a mass extinction occurred, it wiped out only 80 per cent of all species, rather than 95
per cent, the usual estimate. "People just keep perpetuating this idea that the extinction might have removed as many as 95 per cent of species, but clearly it did not," said Dr Steven Stanley of John Hopkins University.
"However, the two extinction "pulses", separated by five million years, still mark the largest
extinction in Earth's history," said Dr Stanley, who conducted the work with Dr Xiangning
Yang, of Nanjing University, on fossil records that marked the end of the Permian period,
about 245 million years ago.
Scientists have traditionally estimated that as many as 95 per cent of the species on Earth died at that time, perhaps as a result of gigantic lava flows in Siberia. Now, a second cause of mass extinction may have to be sought. Drs Stanley and Yang based their estimate on the fossils of several groups of animals.
Among them were spindle-shaped fusulinaceans. These shelled and single-celled creatures look like grains of wheat or rice, and provide an excellent fossil record. They were distributed globally, demonstrating that the Permian extinctions were not the result of only one event.
The species were first killed off during a time known as the Guadalupian stage of the Permian
period. A hiatus of five million years followed before the second, and larger extinction pulse.
