You may have heard of the sixth mass extinction, which (unlike all others that preceded it) is not resigned to the history books but is taking place in the present – or is due to take place in the near future, depending on who you ask. The timeline for this particular doomsday scenario is muddy.

But according to a paper published in Historical Biology, it should really be the seventh mass extinction. This is because researchers at New York University, US, and Nanjing University, China, have identified a previously underestimated mass-extinction event that occurred some 260 million years ago: the end-Guadalupian extinction event. The event seems to have been sparked by the Emeishan flood-basalt eruption, which produced an extensive rock formation in southern China we still see today called the Emeishan Traps.

"It is crucial that we know the number of severe mass extinctions and their timing in order to investigate their causes," co-author Michael Rampino, a professor in New York University’s Department of Biology, said in a statement.

Up until now, five mass extinctions have been described. Each of which decimated (and restructured) life on the planet, while marking the end of a geological period. First, there was the end of the Ordovician (444 million years ago), then the Late Devonian (372 ma ago), the Permian (252 ma ago), the Triassic (201 ma ago), and the Cretaceous (66 ma ago). The new research, which includes data and analysis collected over the last 40 years or so, focuses on the Guadalupian (aka Middle Permian) period, 272 to 260 million years ago.

"Notably, all six major mass extinctions are correlated with devastating environmental upheavals – specifically, massive flood-basalt eruptions, each covering more than a million square kilometers with thick lava flows," said Rampino.

The end-Guadalupian extinction event hit species and ecosystems on land and sea and was triggered by the eruption that produced the Emeishan Traps – comparable, Rampino says, to those that brought about other known mass extinctions. 

"Massive eruptions such as this one release large amounts of greenhouse gases, specifically carbon dioxide and methane, that cause severe global warming, with warm, oxygen-poor oceans that are not conducive to marine life," said Rampino.

In three out of four extinction severity ranking tables calculated since 1996, the newly identified extinction comes third in terms of taxonomic severity – only outranked by the end-Permian and end-Ordovician extinctions. In the fourth, it comes ninth out of nine.

Although the number of species wiped out as a result varies depending on the study, estimates suggest approximately 36 to 47 percent of marine genera were lost. And the study's authors conclude that "In terms of both losses in the number of species and overall ecological damage, the end-Guadalupian event now ranks as a major mass extinction, similar to the other five."