There was no hiding from the greatest mass extinction of all time. Two hundred and fifty million years ago, in a geologic blink of an eye, 85% of the species in the sea and 70% of the vertebrate genera living on land vanished. Now evidence shows that the extinction of land plants was equally brutal and swift, but the body count doesn't reveal the cause of the carnage.

Whatever pummeled Earth's species at the end of the Permian period and the opening of the Triassic did its dirty work in a geologic moment of less than half a million years (Science, 15 May 1998, p. 1007). Many suspects have been implicated in the so-called P-T extinction: a comet or meteorite hitting the planet, massive environmental change due to continental drift, or huge volcanic explosions. Earlier this year, paleontologists showed that the devastating extinctions in the sea took place even faster than thought, probably in less than 160,000 years (Science, 21 July, p. 432). Taking into account the blurring of the fossil record, all the extinctions could have occurred in a single bad day 251.4 million years ago, says Douglas Erwin of the National Museum of Natural History in Washington, D.C.

Now researchers have found signs of an abrupt ecological catastrophe at the time of the P-T extinction. Paleontologist-geologist Peter Ward of the University of Washington, Seattle, and colleagues report in the 8 September issue of Science that rocks in South Africa's Karoo basin tell of an abrupt switch in style of sedimentation, as if the land had been permanently stripped of the rooted plants that held it in place. At seven spots scattered across 400 kilometers of the basin, Ward and colleagues found the same pattern. Their benchmark was the P-T boundary, a layer of rock marked by evidence of extinctions and a globally recognized shift in carbon isotopes. In tens of meters of rock laid down before the boundary, the researchers found sandstones filling broad channels, as if deposited by meandering rivers. Above the extinction bed, the river deposits are entirely different. They are typical of quick-flowing river systems carrying large amounts of water and sediment in narrow, interconnecting channels.

The diagnosis is grim. "It looks a lot like we just lost the forests," says paleontologist Gregory Retallack of the University of Oregon, Eugene. His research has uncovered the extinction of large rooted plants, especially along the banks of streams and rivers, at the time of the P-T extinction in Australia and Antarctica, and other researchers have seen braided rivers making an appearance at the start of the Triassic in the United Kingdom. Although they still don't know what caused the ecological catastrophe, Ward says, the evidence shows that "it was fast."

--RICHARD A. KERR