The Permian-Triassic mass extinction event 252 million years ago was not quite as bad as had previously been estimated. After reassessing the number of species going extinct before the onset of the event – also known as the Great Dying – researchers showed losses were less severe than previously predicted.

The Late Permian mass extinction lasted for approximately 60,000 years. Past research has indicated that up to 96% of marine animals and 70% of terrestrial life was wiped out, making it the most severe mass extinction event to ever occur.

There were a number of contributing factors to the event, with several phases of extinction taking place. Causes are thought to include massive volcanic eruptions and asteroid impacts that would have resulted in ocean acidification and extreme global warming.

In a study published in the journal PNAS, Steven Stanleya, from the University of Hawaii, looked at how many species were disappearing in the intervals between mass extinctions – known as background extinctions.

He found there was a correlation between the length of intervals between mass extinction events and the fraction of marine animals that were wiped out. Findings showed background extinctions precede mass extinctions. This means a proportion of species that died out during mass extinctions would have gone extinct anyway. As a result, Stanleya argues, they should be eliminated from models showing the percentage of animals wiped out during mass extinctions.

Calculations show that the Great Dying mass extinction was responsible killing off 81% of marine animals – not 90-96% as was previously thought. Indeed, he found many extinctions attributed to the Late Permian took place far earlier, while 90 orders and 220 families of marine life survived the mass event.

"Numbers belie the frequent Armageddon-like claim that life in the ocean nearly disappeared at the end of the Permian," he wrote. "Because in this event the losses were unequally distributed among taxa (there was clustering), some orders and classes absorbed a disproportionate percentage of species losses, and others did not even come close to disappearing."

He said the often quoted 96% extinction rate is "unquestionably wrong" and should not be used. "Life did not nearly disappear at the end of the Permian," he concluded.