Mass extinctions have occurred many times throughout Earth's history, the most famous being that of the dinosaurs, 65 million years ago.
While meteorite impacts are commonly believed to have caused these global extinctions, new evidence is keeping the debate alive.
Professor Saunders, a geologist from Leicester University in the UK, is one of many scientists promoting an alternate theory that the dinosaurs were wiped out by climate change resulting from massive volcanic eruptions.
"Impacts are suitably apocalyptic. They are the stuff of Hollywood. It seems that every kid's dinosaur book ends with a bang.
"But are they the real killers and are they solely responsible for every mass extinction on earth?" he said in a press release.
"There is scant evidence of impacts at the time of other major extinctions, for example, at the end of the Permian, 250 million years ago, and at the end of the Triassic, 200 million years ago," he said.
According to Saunders, massive volcanic eruptions evidenced today by layers of "flood basalt" would have released enough greenhouse gases such as sulphur dioxide and carbon dioxide to seriously affect the climate.
The largest flood basalt eruptions at Deccan Traps in India and the Siberian Traps appear to coincide with the largest mass extinctions, 65 and 250 million years ago.
Professor Saunders's team hopes to prove the dates match and further investigate what could have led to the mass extinctions.
"Dinosaur Killer" Denied
Dinosaurs disappear from the geological record at a layer called Cretaceous-Tertiary (K-T) boundary, 65 million years ago.
In the 1980s scientists discovered traces of a meteorite impact – a layer of iridium – in sediment at the K-T boundary around the world. A large crater named Chicxulub on the Yucatan peninsula, Mexico, was linked to this meteor impact and quickly became popularised as the "dinosaur killer".
However, Dr Markus Harting of the University of Ultrecht, Netherlands, questions this theory with evidence that the Chicxulub crater was formed by an impact many thousands of years before the extinctions at the K-T layer.
Dr Harting found that glass droplets, said to be formed when molten rocks were ejected on impact at the Chicxulub crater, were concentrated in a layer 300,000 years "below" the K-T layer. He says this shows the Chicxulub impact and the mass extinction at the K-T boundary are probably unrelated.
Another burning question is whether the massive Chicxulub impact – which undoubtedly occurred and was certainly catastrophic – is responsible for any extinction at all. Maybe, answers Dr Harting. Ammonites, the once widespread nautilus-like sea creatures, died out about the same time as the Chicxulub impact and before the K-T boundary, he said.
But whether the impact even killed the ammonites is not at all clear, according to Harting. Early models of the Chicxulub impact envisaged a "nuclear winter" scenario, in which a dust-shrouded world went cold and plant life died away for years, causing the mass extinctions.
Yet sun-loving animals such as crocodiles and turtles appear to have glided right through without any ill effects. And that is perhaps the silver lining to Chicxulub's fall from the status of most-massive-of-all-killers: Even giant impacts aren't necessarily global catastrophes.
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