Searching for the secrets of extinction
Five times in the last half a billion years, tremendous, global-scale extinctions have wiped out a significant fraction of life on Earth.
The most recent and most familiar is the extinction that wiped out the dinosaurs - between the Cretaceous and Tertiary periods, about 65 million years ago.
But before that, 205 million years ago, was the "End-Triassic Event" - it set the stage for the Jurassic Period, which saw the rise to prominence of the dinosaurs.
Just what happened that killed off half the species on the planet, though, remains a mystery.
One researcher from the US has visited the UK to hunt for evidence of this event - and what he finds may undermine what he had built up for years.
Science reporter Jason Palmer went to Somerset to meet Professor Paul Olsen of Columbia University.
13 Oct 2011