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 that 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 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