|This 18-inch skull belonged to a gorgonopsid the largest land predator alive at the time of the Permian extinction.|
Photograph courtesy of Peter Ward/University of Washington
When it comes to extinctions, dinosaurs get all the press. Their history reads like the life story of an oil tycoon or a rock star: the steady rise from obscurity, the heady days of world dominance, and then, 65 million years ago, a literally meteoric demise brought about by an impact near the Yucat‡n Peninsula. What the tabloids won't tell you is that the fall of the dinosaurs, while spectacular, is second-rate. In the annals of earthly catastrophe, nothing beats the Permian extinction, a deadly convulsion 250 million years ago that wiped out 90 percent of all life on the planet.
The Permian extinction isn't likely to get the Hollywood treatment anytime soon for at least two reasons. One is the cast of characters: on land, an ensemble of stupid, slow-moving creatures likened by one scientist to "naked turtles" and "sausages with beaks." The Permian ocean also lacks charisma unless snails, clams, fish, and trilobites do it for you. "Getting people passionate about brachiopods isn't easy," concedes Doug Erwin, a paleontologist with the Smithsonian Institution who has tried.
But the real trouble with pitching the Permian extinction is that no one knows how the story ends. The plants and animals of the Permian Period were diverse and thriving. Hippo-sized reptiles roamed Pangaea, a single vast tract of land that eventually split up into today's continents. Trophy fish swam in the surrounding sea. Then, a quarter of a billion years ago, the party ends. Thousands of species simply vanished from the fossil record. It took life millions of years to recover.
Until recently, geologists and paleontologists thought the Permian extinction itself occurred over millions of years, the result of gradual changes in climate and sea level that are common in Earth's history. But new studies have unearthed increasing evidence of sudden death. In 1997, analyses of radioactive decay in Permian sediments showed that the extinction may have taken place over a period of less than half a million years. Subsequent studies of other sediment features reduced that figure to 10,000 years or less. And according to an exhaustive fossil census Erwin and his colleagues conducted last year, the Permian extinction may have gone down virtually overnight. "Something happened very suddenly, very catastrophically," says geoscientist Michael Rampino of New York University.
Experts are divided on what that something might be. The most sensational candidate is, of course, a comet or asteroid impact, which has a record of offense at the boundary of the Cretaceous and Tertiary periods. An extraterrestrial assault like the one that did in the dinosaurs would have launched clouds of corrosive gas and debris into the atmosphere, blocking out the sun and showering the planet with acid rain and snow. Temperatures would have dropped, plants would have starved for light, and the rest of the food chain would shortly have followed.
But evidence for any such calamity in the Permian Period has been lacking. The Cretaceous-Tertiary impact left its signature in fractured quartz crystals, glass fragments, and heightened levels of the heavy metal iridium, which is abundant in meteorites. Although some investigators have reported finds of shocked quartz and modestly elevated levels of iridium in end-Permian rock layers, the markers at the Cretaceous-Tertiary boundary are far more obvious and plentiful. That doesn't preclude the possibility of an impact, says Rampino. "If it was an ocean impact, it wouldn't make much shocked quartz, because there's very little quartz in ocean crust," he says. And if the killer was a comet, it wouldn't leave much iridium, he notes, because comets are mostly ice.
Earlier this year, a research team led by geochemist Luann Becker of the University of Seattle published analyses using a new kind of impact marker: noble gases trapped in closed carbon lattices, called fullerenes. Ratios of argon and helium isotopes in Earth rocks differ from those found in meteorites, and Becker's work shows that the fullerenes from end-Permian rocks contain isotope ratios typical of meteorites, not Earth. "Thus, it would appear that extraterrestrial fullerenes were delivered to Earth at the Permian-Triassic boundary, possibly related to a cometary or asteroidal impact event," the scientists concluded.
But Becker's methods are controversial, and alternative theories abound. The main terrestrial contender is an extended series of volcanic eruptions that coincide with the Permian extinction. The so-called Siberian Traps are the largest continental eruptions in the history of multicellular life. "We don't even have a concept of what it's like to be around during such eruptions," says Erwin. In less than a million years, they poured a blanket of lava two miles thick over five time zones from the Ural Mountains to Lake Baikal. The lava alone wouldn't have been enough to cause a global extinction, especially in the ocean. But the eruptions could have spewed debris and noxious vapors, creating the same kind of cold, dark, acidic envelope that an asteroid impact would.
Critics of the vulcanism theory point out that the eruptions were prolonged, while the Permian extinction seems to have been abrupt. And because of technical limitations, it has been difficult for geologists to prove that the eruptions actually started before the extinction did. Faced with such uncertainties, some scientists are taking an ˆ la carte approach. Perhaps an asteroid impact on the opposite side of the planet focused energy under the Siberian fault system, triggering the eruptions. Or maybe the eruptions mixed up ocean circulation, causing oxygen-poor currents from the deep to suffocate marine communities in the shallows. Maybe deadly carbon dioxide percolated up from the depths of the ocean and put the brachiopods to sleep with the fishes. Maybe trouble came in threes: impact, eruption, asphyxiation.
"I tend not to like these Rube Goldberg mechanisms," says Rampino, who favors the impact theory for its simplicity and track record. One team of geologists has already proposed a possible impact site in Western Australia, where they found shocked minerals in drill-core samples and craterlike deformations in buried rock.
Erwin is chronicling the resurgence of life among Permian leftovers. He'd like to understand why some species made it while so many others perished. "From an evolutionary perspective, the most interesting thing about mass extinctions is the aftermath, not the extinction itself," he says. "The tide pool that you look at today reflects who won and who lost 250 million years ago."
On land, the story is a little more complicated: Post-Permian vertebrates went through many more evolutionary turnovers. But at the dawn of the Triassic, the worst was over. The stupid, slow-moving survivors of the Permian carnage gave rise to the marginally brighter cynodonts. The cynodonts spawned the crafty mammals, and some of the mammals got brainy enough to contemplate the causes of mass extinctions.