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The greenhouse extinction
Discover, August, 1998 by Peter D. Ward
Life disappeared almost completely 250 million years ago. Now the fossils of the victims tell a tale of hideous global warming.
After a long day collecting cores of rock, it was finally time to relax. Along with my colleague Joseph Kirschvink of Caltech, I sat in the cool protection of the afternoon's lengthening shadows with field gear and sample bags strewn about my feet. Slabs of eroding brown anti red shale were piled around us; randomly scattered through this shale were bits--and occasionally whole skeletons--of white fossilized bone. We were about halfway up a wide canyon, and as I stretched out my tired muscles I looked back, and down, in the direction we had traveled since morning.
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At first light we had made our way to the banks of the broad Caledon River, which torpidly snakes across the region of parched South African countryside known as the Karroo. The Caledon River had created the canyon, near the town of Bethulie, by carving its way through rock dating back 245 to 255 million years ago. These ancient strata have escaped the tossing and turning of plate tectonics, and so they still lie in the same horizontal plane in which they were laid down. Thus as we climbed the canyon wall, we ascended into ever more recent times. From the river's edge to where we now rested, the strata were green and olive sandstone and shale formed on an ancient floodplain. Above us, however, the somber green suddenly gave way to red and brown. For as much as a thousand feet above us, I could make out rocks painted every imaginable hue between carmine and ocher gleaming in the sun.
Our resting place was balanced on the fulcrum of this great color shift--green rocks below, red above, and around us hovered the unnumbered flies of Africa. They seemed fitting company, for this place is actually a vast ancient graveyard. The division between green and red marks the greatest mass extinction in Earth's history: the end of the Permian Period and the beginning of the Triassic 250 million years ago, when 90 percent of species in the ocean and 70 percent of species on land disappeared.
Despite its scale, the Permian extinction remains a deep mystery. Kirschvink and I had traveled to this isolated piece of Africa to look for new clues about what had actually happened during this global catastrophe. Did the old species slowly die away over millions of years, one after another, to be gradually replaced by dinosaurs and other creatures? Or was the old fauna suddenly destroyed by a catastrophic event of much shorter duration? And most important, what caused this greatest of mass extinctions? Could it happen again?
Sitting in the Bethulie canyon, I could see the fossilized bodies of the victims scattered around me, eroding into green and red dust, and I wondered about their death. Perhaps some shift in the way our planet worked--in its climate, its sea level, or the chemistry of its oceans and atmosphere--killed them off slowly. Or perhaps their passing was a sudden result of an extraterrestrial event, such as the impact of a comet or the explosion of a nearby star. I felt a bit like a gumshoe looking for a murder weapon in a cemetery, hoping that the murderer wasn't still lurking nearby.
In the 530 million years' worth of fossils that animals and plants have left behind in any great numbers, we have found evidence of many mass extinctions but only five that have killed more than half the world's species. The best known is the Cretaceous-Tertiary (also called the K/T) event 65 million years ago, caused by the impact of a comet or an asteroid. Yet despite its fame for claiming the dinosaurs, the K/T event destroyed only about 50 percent of the species on Earth, making it far gender than the Permian-Triassic event.
It's been difficult to determine the cause of the Permian-Triassic extinction--largely because very few places on Earth preserve the sedimentary rocks formed at the time, most of which were deposited at sea rather than on land. The marine rocks demonstrate that a monstrous extinction swept away most of the animals then living in the sea; the few terrestrial deposits suggest that some extinctions happened at about the same time on land. But most of the land fossils have turned up in scattered layers below and above the Permian-Triassic boundary, making it hard to tell what the speed of extinction on land really was, or to determine whether it was happening at the same time as the extinction at sea.
South Africa may hold the solution to these mysteries. The great Karroo desert has long been revered by paleontologists as an exquisite boneyard, with the world's best fossil collection of land-dwelling Permian vertebrates. Most of the Karroo fossils belong to one of three groups of animals: amphibians, big plant-eating reptiles called pareiasaur's, and therapsids--the reptilian-looking ancestors of mammals. We know that therapsids were mammalian ancestors because despite superficial appearances, they share a number of distinctive features with us. Their teeth, for example, are differentiated for biting, chewing, and grinding, as ours are.
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