The extinction, which marks the end of the Permian period and the beginning of the Triassic, has intrigued paleontologists because of its massive scale; more than twice as many kinds of vertebrates died out than in the extinction that killed the dinosaurs 186 million years later. And unlike the end of the dinosaurs, exactly what happened is a mystery. Some scientists see evidence for a gradual series of natural disasters, such as glacier-induced changes in the oceans, while others believe only the rapid punch of a massive asteroid could have wreaked so much havoc so quickly.
To find out whether everything went wrong all at once, paleontologist Doug Erwin of the National Museum of Natural History in Washington, D.C., and his colleagues at the Nanjing Institute of Geology and Palaeontology in China analyzed 333 extinct species, collected from a well-studied section of the Permian-Triassic boundary near the town of Meishan in South China. The strata at the Meishan site have previously been dated, so the researchers knew when each species last appeared in the fossil record. Erwin and his colleagues also took into account the number of fossils of each species. This is an important factor in determining the probability that they didn't find the last fossil, which would mean the extinction took place later than it appears.
After crunching the numbers, the researchers conclude that it's most likely that 94% of the species discovered at the end of the Permian are, in fact, the last survivors. This means that they died off more or less simultaneously, about 251.4 million years ago. As further evidence for a one-time mass extinction, Erwin's group found volcanic dust in the same stratum as the extinct species, as well as a change in the carbon isotope profile, an indication that fewer plants were alive and photosynthesizing. Erwin thinks the event most likely was triggered by massive lava flows but doesn't rule out an impact from outer space.
The researchers are the first team to study such a large number of species at this extinction event, says Charles Marshall, a paleontologist at Harvard University. The data "do seem to suggest there was one major event, that's for sure," he says. But Marshall is skeptical of extraterrestrial influence, saying his guess would be that a massive change in ocean circulation or ocean chemistry, possibly after a continental collision or rift, caused the extinction.