Paleontologists concluded that the disappearance of 90 percent of all marine species and 75 percent of land plants and animals at the boundary between the Permian and Triasic periods was caused by atmospheric warming because of greenhouse gases triggered by erupting volcanoes.

The most commonly held theory on the biggest extinction of species in Earth's history is that a huge meteorite or comet impacted the globe causing a drastic, worldwide climate change.

"The marine extinction and the land extinction appear to be simultaneous, based on the geochemical evidence we found," said University of Washington paleontologist Peter Ward, lead author of the paper.

"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," he said, adding that there was little evidence of a sudden catastrophe caused by an asteroid.

Ward and his colleagues from the University of Washington, the California Institute of Technology, the Washington-based Smithsonian Institution and the South African Museum, examined 126 reptile or amphibian skulls from a nearly 300-meter (1,000-foot) thick section of exposed Karoo sediment deposits from the time of the extinction in the Karoo basin of South Africa.

The researchers were able to use chemical, biological and magnetic evidence to establish that the mass extinction took place gradually over a period of 10 million years, followed by a sharp increase in the extinction rate that lasted another five million years.

A second team of paleontologists led by Kliti Grice, of Curtin University of Technology in Perth, Australia, examined sediments from the same period along the coastline of Australia and China, finding chemical evidence of a low oxygen content in the oceans.

The results confirmed the South African data, showing the atmosphere had less oxygen and higher levels of sulfur gas emanating from volcanic eruptions.

"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."

Most experts concur that the disappearance of the dinosaurs some 65 million years ago was caused by a huge asteroid that fell near Mexico's Yucatan peninsula.

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