Today’s biodiversity crisis is often referred to as the “sixth mass extinction” by those who believe we’re headed for a collapse in global species counts. But perhaps, some scientists argue, we should be calling it the seventh.

In 1982, quantitative paleontologists Jack Sepkoski and David Raup at the University of Chicago took stock of the Earth’s worst mass extinctions, naming them the Big Five. That set includes the end-Permian, the greatest extinction event of all time, which occurred around 252 million years ago and eliminated 95 percent of marine species.

At the time, the carnage of the end-Permian overshadowed another extinction event just eight million years earlier at the end of the Guadalupian epoch. Over the last three decades, though, geologists have been digging deeper into the end-Guadalupian, and it’s more widely recognized as a distinct crisis. Now, some scientists are arguing that this ancient die-off was big enough to rank among the pantheon of past apocalypses, and they propose renaming the group of major extinction events the Big Six.

In the history of life, there have been many flameouts and setbacks. But by singling out and studying the biggest ones, geologists can begin to unearth patterns and search for common causes. Increasing evidence suggests that many global extinction events were associated with oxygen-depletion in the oceans, a symptom of greenhouse warming, and that has worrying implications for the present-day effects of climate change. The end-Guadalupian fits this trend.

“I think there’s a problem with hanging on to the number five,” says Richard Bambach, a marine paleoecologist and professor emeritus of paleontology at Virginia Tech who was a reviewer on the landmark Sepkoski-Raup paper. The end-Permian came much closer to wiping out all life when you look at the percentages. But, he says, the end-Guadalupian was astonishingly bad for biodiversity.

“If you actually look at the raw numbers,” he says, “the loss of taxa in the Guadalupian is actually greater than it was in the Permian.”

Floods of lava

The end of the Guadalupian epoch is marked by the Emeishan Traps in southwestern China, which are a monument to a flood of lava that erupted in the ocean 260 million years ago and oozed over a million square kilometers. The event unleashed plumes of methane and carbon dioxide that wreaked havoc on the climate, killing off as much as 60 percent of marine species, mostly in the shallow tropical waters around the supercontinent Pangea.

So-called flood basalts like the Emeishan Traps exist all over the world, and they have been shown to line up with the Big Five major mass extinctions. “It’s a one-to-one correlation,” says Michael Rampino, a geologist at New York University.

But geologists studying mass extinctions weren’t always looking for flood basalts. Starting in the 1980s, after Luis and Walter Alvarez hypothesized that a meteor impact wiped out the nonavian dinosaurs, teams of geologists searched fruitlessly for evidence of meteor strikes that could explain the other mass extinctions.

Coming up empty-handed, Rampino soon turned his gaze to flood basalts, noting that the Deccan Traps in India had formed around the same time as the Chicxulub impact and the end-Cretaceous extinction. The end-Permian, too, was marked by the even bigger Siberian Traps. (See how scientists recently captured the last day of the dinosaurs’ reign in stunning detail.)

“I went from being an impact man to a volcanism man,” he says. His research over the last decade has focused on correlating flood basalts with the other major mass extinctions, and also with periods of oxygen-depletion and acidification in the oceans.

Investigating such relationships was fraught in the 1980s, when technologies for dating fossils and rocks were less reliable. But in the past five years, advanced radiometric dating methods have provided increasingly accurate timestamps for geologic events. Uranium-lead zircon dating has superseded the much less accurate argon-argon dating, and margins of error that used to span millions of years now span thousands, greatly increasing the resolution of the data.

With this new specificity, geologists can confidently say that the end-Guadalupian lava flood happened within a hundred thousand years—instantaneously in geologic terms—and that it coincided with the extinction crisis documented in the fossil record.

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In a study recently published in Historical Biology, Rampino and coauthor Shu-Zhong Shen of Nanjing University brought together the newest data on the Emeishan Traps and an analysis of the ecological severity of the end-Guadapulian extinction to argue for its inclusion among the Big Six.

Ecosystem changes in the end-Guadalupian were dramatic, the team says. Huge coral and sponge reefs that filled the seas suffered widespread collapse, along with other organisms that built shells out of calcium carbonate, which would have dissolved in the acidifying water. Giant mollusks with flanged shells that looked like alien cruisers disappeared forever, and many species of nautilus-like cephalopods called ammonites also went extinct.

Paleontologists know less about the casualties on land, but among them were a group of large, thick-skulled proto-mammals called dinocephalians. After the crisis, the dominant seedless ferns were supplanted by seed-bearing gymnosperms such as conifers and ginkgoes.

Revising the record

New calculations have also clarified the timing of the first and last appearances of species in the overall fossil record. The researchers cite a study from 2016 which argues that, due to poor dating, many species that had gone extinct in the Guadalupian were mistakenly attributed to the end-Permian, giving the latter an inflated extinction record of 95 percent of marine species when it was likely closer to 80 percent.

Bambach notes that he has some doubts about the study’s assessment of the ecological severity of the end-Guadalupian. Global sea levels reached their lowest point during the Guadalupian and rose again after the extinction event, meaning that relatively few Guadalupian reefs are preserved in rock where paleontologists can access them.

“Some of the disappearance of ecosystems may be simply the deterioration in the quality—or the amount—of the preserved record,” he says. One exception is in China, where Shen has been dating marine fossils across the Permian period and refining the biological portrait of the Guadalupian.

Still, Bambach ultimately agrees with Rampino and Shen that it’s time to add the end-Guadalupian to the Big Five: “It does rank right up there with the big ones.”