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The mass extinction, known as the "Great Dying," extinguished 90 percent of sea life and nearly three-quarters of land-based plants and animals.
There has been recent evidence that a big asteroid or meteor hit the Earth and triggered the catastrophe, but researchers say they now have evidence that something much more long-term -- global warming -- was the culprit.
Kliti Grice of Curtin University of Technology in Perth, Australia, and colleagues studied sediment cores drilled off the coasts of Australia and China and found evidence the ocean was lacking oxygen and full of sulfur-loving bacteria at that time.
This finding would be consistent with an atmosphere low in oxygen and poisoned by hot, sulfurous, volcanic emissions, they wrote in a report published in the journal Science.
A second team led by Peter Ward at the University of Washington looked at fossil evidence in South Africa and found little evidence of a catastrophe and instead signs of a gradual die-off.
They examined 126 reptile and amphibian skulls from the Karoo Basin in South Africa, where there is an exposed piece of dried sediment from the end of the Permian Era and the beginning of the Triassic, 250 million years ago.
They found two patterns, one showing gradual extinction over about 10 million years leading up to the time of the extinction, and then a spike in extinction rates that lasted another 5 million years, Ward's team reported.
"Animals and plants both on land and in the sea were dying at the same time, and apparently from the same causes -- too much heat and too little oxygen," Ward said in a statement.
Ward also believes mass volcanic eruptions may have pumped greenhouse gases into the air, which would have trapped heat in the atmosphere and raised temperatures.
"I think temperatures rose to a critical point. It got hotter and hotter until it reached a critical point and everything died," Ward said. "It was a double-whammy of warmer temperatures and low oxygen, and most life couldn't deal with it."
Last May researchers said they found evidence of a giant asteroid striking Earth off the coast of what is now Australia 251 million years ago. But others have disputed their conclusions.
Most experts agree there is a great deal of evidence to show an asteroid wiped out the dinosaurs 65 million years ago, forming what is now the Chicxulub crater off Mexico's Yucatan Peninsula
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