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Palaeozoic World

Molluscs Ruled the World before Dinosaurs

Edited from Mathaba News, 1 August 2007

Scientists studying mollusc fossils revealed that the rise to prominence of these molluscs some 250 million years ago are evidence that 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 which ended the age of the dinosaurs some 65 million years ago (see the feature, Wilkes Land Permian Impact Crater, via the sidebar link, right). Instead, it was the so-called end-Permian mass extinction, which eliminated as much as ninety-five 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 eight million years before the end-Permian.

The results of the research are not really consistent with a more catastrophic extraterrestrial cause, such as an asteroid impact - although they don't directly contradict the impact theory either, according to researcher Matthew Clapham at Queen's University in Kingston, Canada.

Instead, these findings support theories which suggest that the end-Permian was triggered by ocean changes long in the making, the climax of a prolonged environmental crisis. 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 were better adapted to such stressful and changing environments, and so could have thrived. The abundance of molluscs was a symptom of the conditions which ultimately caused the extinction. The research involved gleaning more than 33,000 Permian fossils from blocks of limestone which 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 to survive.



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