Hunkered down. Burrowing animals, such as these Thrinaxodon mammal-like reptiles, seemed to have survived the extinction better.

The Last Gasp of the Permian

The biggest mass extinction of all time, at the end of the Permian era 251 million years ago, wiped out the vast majority of species. Now, two papers lay out scenarios for how deadly gas might have killed most land animals--and why some survived. One sketches an apocalyptic vision of methane erupting from the sea floor, forming vast clouds and exploding. In the other, paleontologists propose that at least one creature escaped the suffocating gases because it was already adapted to thin air.

The idea that methane contributed to the Permian mass extinction is not new. The anoxic oceans at that time could have stored massive amounts of methane, either dissolved in bottom waters or frozen solid. Most geologists imagined the methane gradually seeping into the atmosphere over 10,000 years or so. Gas can escape much more quickly, though, as it did at Lake Nyos, Cameroon, in 1986, when clouds of lethal carbon dioxide suddenly bubbled out of the waters.

Chemical engineer Gregory Ryskin of Northwestern University in Evanston, Illinois, wondered if that type of eruption might have happened on a larger scale during the Permian extinction. He calculated that the oceans could easily hold a whopping 10,000 gigatons of methane. After a disturbance, such as an earthquake, bubbles would form. This would lower the pressure of the water, which would then release even more dissolved gas. "It's a real kind of snowball effect," Ryskin says. Over days, massive amounts of methane would enter the atmosphere and form an explosive mixture with air, Ryskin argues in the September issue of Geology. Blown inland, the deadly clouds would be detonated by lightning. The smoke and soot from widespread fires might lead to a nuclear winter, or the gases might cause global warming; it's hard to predict which, Ryskin says.

Greg Retallack of the University of Oregon, Eugene, also believes that massive amounts of methane left the ocean, but not as catastrophically. Still, the gas would also have immediate impact. With the relative amount of oxygen in the air lowered to something akin to that at 4000 meters, animals would suffer from pulmonary and cerebral edema. "The reptiles died of mountain sickness," he says. Retallack and colleagues noticed that surviving animals had traits that would help deal with low oxygen, such as thick ribs and a barrel chest for efficient breathing.

The paper, which appears in next month's issue of the Geological Society of America Bulletin, is winning points for originality. "I think it's a really neat story," says Christian Sidor, a paleontologist at the New York College of Osteopathic Medicine in Old Westbury, who considers it a plausible explanation. Some scientists who study the Permian oceans have trouble imagining Ryskin's conflagration, however. "I just don't find compelling evidence that the methane levels needed were present in the Permian ocean," says geochemist Lee Kump of Pennsylvania State University, University Park.

Related sites
Overview of the extinction
More about Lake Nyos