Mass Extinction that Killed Most Life Came in Two Stages on Land and Sea

New evidence may rock long-held assumptions about a catastrophic mass extinction event that occurred 250 million years ago. Scientists have found that there were two phases of the extinction: one on land and one in the oceans.

In this latest study, the researchers looked at a key volcanic ash deposit that was discovered in rock layers that chronicled the mass extinction. By dating the volcanic ash-bearing deposit, the researchers concluded that two phases of this extinction occurred about 1 million years apart, as opposed to roughly at the same time.

Based on previous dating of fossils, researchers concluded that the die-off among marine species occurred about 251.9 million years ago. The extinction on land, though, was more difficult to date-until now. The volcanic deposits revealed a more definitive date.

"There has been some concern in the scientific community about whether the extinction among vertebrates on land was actually coincident with that in the marine realm in terms of their timing," said John Geissman, one of the researchers, in a news release. "Nonetheless, many researchers have just tacitly assumed that the land event occurred roughly concurrently with the marine extinction."

It appears, though, this isn't the case. The latest findings place terrestrial vertebrate turnover about 1.5 million years earlier than the accepted estimated age of the marine end Permian-extinction.

The findings reveal a bit more about this extinction event and show that marine and terrestrial extinctions did not, in fact, occur at the same time.

The findings are published in the journal Geology.

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