The five generally acknowledged mass extinction events that have hit the Earth over the last 450 millions years should be joined by a sixth, an international research team is proposing.

There is sufficient evidence to promote an event known as the Capitanian extinction, first identified in the fossil record some 20 years ago, to the classification of a mass extinction, they say.

The event, which occurred during the Middle Permian period around 260 million years ago, should be ranked equally with the "Big Five" events already recognized by most scientists, suggests study leady David Bond, a paleontologist at the University of Hull in England.

Since the Capitanian extinction first came to the attention of scientists there has been dispute as to whether it was global and thus truly catastrophic, since the only evidence for it has been gathered from tropical regions.

In addition, the deadliest mass extinction in the planet's history, known as the End Permian extinction or the "Great Dying" - which killed off 96 percent of all species on Earth - occurred just 12 million years after the Capitanian event, raising speculation they were, in fact, one extended event.

Bond and his research colleagues decided they needed evidence from some other region of Earth besides the tropics, so they headed for Norway in three separate expeditions from 2011 to 2013.

On the island of Spitsbergen, which had once been at the bottom of the sea in the Middle Permian, they found the evidence they were looking for - a complete absence of fossils for the period in question.

"They all drop out," says study co-author Paul Wignall of Britain's University of Leeds, who is also a paleontologist. "It's like a blackout zone and there's nothing around."

A little later in the rock record, some species are seen to recover, then there's a mass takeover of mollusks - until the complete devastation of the following Permian extinction some 8 million years later, the researchers report in the Geological Society of America Bulletin.

The findings from Norway and the earlier discoveries in the tropics, both of which left distinguishable marks in the fossil record, are strong evidence the Capitanian extinction was global, not just regional, and therefore qualifies for mass extinction status, Bond says.

"It's the first time we can say this is a true global extinction," he says.

Not everyone agrees; Matthew Clapham, a paleontologist at the University of California, Santa Cruz, says he believes Bond and his team have in fact found something dated a bit younger - perhaps 255 million years old, which would put it much closer to the Permian event.

"They've definitely identified a real event, which is really interesting," he acknowledges. However, he says, "Their age model is less convincing."

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