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,