Palaeozoic World

Scientists studying mollusc fossils reveal that their rise to prominence some 250 million years ago suggests the most devastating mass extinction in Earth's history took a long time as opposed to being the result of a catastrophic extraterrestrial cause such as an asteroid strike.

The largest die-off in Earth's history was not the cataclysm that ended the Age of the Dinosaurs some 65 million years ago. Instead, it was the so-called end-Permian mass extinction, which eliminated as much as 95 percent of the planet's species before even the earliest dinosaurs strode the planet.

One alleged consequence of this mass extinction was the dominance of oysters, snails and other molluscs all over the world some 8 million years before the end-Permian.

"Our results aren't really consistent with a more catastrophic extraterrestrial cause, such as an asteroid impact — although they don't directly contradict the impact theory either," said researcher Matthew Clapham at Queen's University in Kingston, Canada.

Instead, these findings support theories suggesting the end-Permian was triggered by ocean changes long in the making, "the climax of a prolonged environmental crisis," Clapham said.

The whole Permian period, from about 300 million to 250 million years ago, saw a gradual warming. This would have slowed down circulation in the ocean, eventually leading to very low levels of oxygen in the water. Massive volcanism near the end of the Permian might have caused more damage to the environment.

"Molluscs are better adapted to such stressful and changing environments, and so could have thrived," Clapham told LiveScience. "The abundance of molluscs we see are symptoms of the conditions that ultimately caused the extinction."

The research involved gleaning more than 33,000 Permian fossils from blocks of limestone that researchers gathered from China, Greece, Thailand, Nevada and Texas over the course of four years.

These blocks were then dunked in vats of hydrochloric acid. Although the acid dissolved the limestone, over millions of years the building blocks of the fossil shells were replaced one by one with silica. This silica resisted the acid and helped the fossils survive.

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