Earth's worst mass extinction might not have been as bad as we thought

The Permo-Triassic extinction event happened 251 million years ago, killing off 96% of all marine species, and 70% of those on land. As bad as all that was, it now appears that land-based species recovered much faster than previously believed.

Extinction events are devastating not just because of the massive amount of individual deaths and species-wide extinctions, but also because of the cumulative loss of biodiversity. Without sufficient genetic diversity, species are less equipped to deal with new threats and diseases, and ecosystems as a whole suffer without a variety of species to fill different ecological niches.

The loss of biodiversity in the Permo-Triassic extinction event was so gargantuan that it took millions of years to recover, with previous studies estimating as much as 15 to 30 million years for terrestrial species to fully recover. But now two researchers from the University of Rhode Island are challenging that estimate.

URI professor of geosciences David Fastovsky explains:

"Our results suggest that the cause of the extinction didn't spill over as severely into the terrestrial realm as others have claimed. There was still a terrestrial extinction, but its repercussions weren't more long term than those in the marine realm, and possibly less."

Fastovsky and research partner David Tarailo looked at two sets of fossil lists from northeastern Arizona, one of which dated to the Middle Triassic and the other to the Late Triassic. According to previous studies, that first list should have still shown signs of the loss of biodiversity caused by the earlier extinction events. But the levels of biodiversity in the two lists was pretty much exactly the same, strongly suggesting biodiversity had already long since recovered by the Middle Triassic.

That finding means it only took about five million years for biodiversity to fully recover, nowhere close to the 15 to 30 million years previously suggested. The result isn't definite - after all, they only looked at one area, and it's at least conceivable that particular region recovered much faster than the rest of the world. Fastovsky and Tarailo say they plan to continue their analyses with more geographically diverse fossil deposits to provide a more robust picture of the fossil recovery.

This finding doesn't affect our understanding of how long it took marine species to recover, which is still confidently thought to be between 4 and 10 million years, which is roughly twice as long as the average marine recovery time in typical mass extinctions.

That's a crucial point, as it suggests the Permo-Triassic extinction event was more devastating in the oceans, which means the crux of the event was centered there and not shared equally. That would lend support for ocean-based explanations of the event like sudden emissions of methane clathrate from the ocean floor, a big fluctuation in sea levels, or a massive drop in ocean oxygen levels. On the other hand, this finding would generally argue against explanations like massive volcano eruptions or an impact event like the one thought to have wiped out the dinosaurs.

Via the Geological Society of America. Image of modern animal skull by YXO on Flickr.