End-Permian mass extinction

The greatest mass extinction the world has ever seen was not as great as we thought, a new paper suggests.

Calculations by Steven Stanley from the University of Hawaii in the US downgraded the end-Permian “great dying” from wiping out between 90 to 96% of all species to around 81%. It’s still a mass extinction, but, he writes, “life did not nearly disappear at the end of the Permian, as has often been claimed”.

He published his work in the Proceedings of the National Academy of Sciences.

Marking mass extinctions, where most of the planet’s species died out in a few million years or so (there’s no hard or fast rule for a “mass extinction”), may seem simple enough – dig into the fossil record and find a time point when the number and diversity of species dropped off a cliff.

But there are complications.

Animals go extinct all the time, so palaeontologists must tease out the rate of “background extinction” from extinction events.

Then only a fraction of a fraction of animals become fossilised. It takes very specific conditions to preserve bits of plants and animals, such as sediments in the bottom of a lake.

There’s also what’s called the Signor-Lipps effect. Put forward by Philip Signor and Jere Lipps in 1982, the principle states that species go extinct after the most recent evidence for it is found in the fossil record. How likely is it that we uncover the last member of a species?

The most famous example of the Signor-Lipps effect in action today is the ancient fish, the coelacanth. It was thought to have gone extinct around 66 million years ago – but a live specimen was caught off the coast of South Africa in 1938.

So instead of looking for fossils to drop off a cliff, a mass extinction might look like rarer species disappearing before the more common, for instance.

Taking Signor-Lipps into account, Stanley calculated the extinction at the end of the Permian, around 250 million years ago, killed off 81% of marine species – fewer than the oft-quoted 96% (including on Cosmos).

The remaining species comprised 90 orders and 220 families, which “embodied an enormous amount of morphological, physiological and ecological diversity”, Stanley writes.