WASHINGTON — The greatest mass extinction on Earth, which wiped out up to 85 percent of all animal life 250 million years ago, also killed off most of the plants, scientists say.
Evidence from South Africa's Karoo region suggests strongly that many species of plants disappeared at the end of the Permian period, along with oceans teeming with creatures.
So many plants were lost that once-meandering rivers turned into straight-flowing steams laden with sediment stripped from land no longer held in place by roots, the team at the University of Washington and the South African Museum found.
Dave Montgomery, a geologist at the University of Washington who worked on the study, said the finding did not explain what caused the huge extinction.
"Maybe the same thing killed both (animals and plants) at the same time," he said in a telephone interview. "With what we have we can't say. But it does suggest that it was global and it was big."
Before the Permian extinction, the Earth's seas were filled with odd and fantastic life, from the first fish to nautilus-like molluscs called ammonites.
Trilobites, once found almost everywhere, disappeared. There were 10,000 different species of the arthropods, which could roll up like pill bugs.
Fossil evidence suggests at least 85 percent of all marine species and 70 percent of the land animal species, mostly reptiles and amphibians, perished in less than 1 million years — very quickly by geological standards.
So many creatures died that fungi dominated the land for a short time, according to some studies.
Theories as to what could cause such devastation include an asteroid impact — now accepted as the probable cause of the extinction of the dinosaurs 65 million years ago.
Scientists also believe radiation from a supernova, excessive volcanic activity, a huge "burp" of trapped gases from beneath the ocean or perhaps global warming may have caused the extinctions.
Montgomery and colleagues were looking for physical evidence of what may have happened in the Karoo Basin, a relatively untouched wasteland that is prized as a kind of living laboratory for paleontologists.
They found evidence of ancient riverbeds dating to the time of the extinction 250 million years ago. These riverbeds took a form known as braided streams, which run straight and fast and branch out for short distances before merging back to the primary stream.
These are rarely found today. Most rivers meander, making long, swooping curves. And meandering rivers had developed before the Permian extinction, as well.
Writing in Friday's issue of the journal Science, the researchers said the conclusion is clear. Whatever caused the Permian extinction was major.
"That change triggered changes in vegetation that were catastrophic and global and those may have triggered basic and catastrophic changes in the basic morphology of rivers," Montgomery said.
"It is very hard to get a meandering river to go into a braided river. It really suggests that it was biggie."
Such changes are seen today in places where volcanos have killed all the vegetation on land. "It is the kind of change that we are seeing at Mount Pinatubo in the Philippines, in response to the eruption there," Montgomery said.
"A massive amount of sediment is being dumped into the rivers there. Narrow, steeper mountain rivers are being turned into braided channels."
The finding could also help explain what other scientists have found—that huge amounts of carbon showed up at the bottom of the seas at the time of the extinction. Carbon is found in all life forms.
"All that dead stuff had to go somewhere," Montgomery said.