After The Largest Extinction Event on Earth, These Animals Were The First to Recover

The Permian-Triassic extinction event, which occurred around 252 million years ago, is widely known as the Great Dying because of the manner in which it decimated life on Earth, almost entirely destroying it. It is the most devastating extinction catastrophe in human history.

However, life did rebound, and recent study shows that deposit feeders like worms and shrimps - organisms that feed on organic materials collected at the ocean's bottom - were the first to recover in terms of population numbers and diversification.

Suspension feeders, which eat organic debris suspended in water, appeared significantly later, according to a comprehensive date of tracks and burrows on the South China sea bed. This research uncovered a plethora of ichnofossils, or trace fossils - not genuine animal remains, but remnants of animal activity.

How the oceans may have looked before (A) and after (B-F) the extinction. (X.Feng/Z.-Q.Chen/M.J. Benton/Y. Jiang)

"We were able to look at trace fossils from 26 sections through the entire series of events, representing 7 million crucial years of time,"  explains paleontologist Michael Benton of the University of Bristol in the United Kingdom.

"Showing details at 400 sampling points, we finally reconstructed the recovery stages of all animals including benthos, nekton, as well as these soft-bodied burrowing animals in the ocean."

Because soft-bodied animals have no bones to leave behind, trace fossils are critical in determining how these organisms lived. The research team was also able to integrate body fossils into their analysis to examine how other species began to recover as the deposit feeders established themselves.

"The end-Permian crisis – which was so devastating to life on Earth – was caused by global warming and ocean acidification, but trace-making animals may be selected against by the environment in a way that skeletal organisms were not," says paleoecologist Xueqian Feng of the China University of Geosciences.

"Our trace fossil data reveal soft-bodied animals' resilience to high CO2 and warming. These ecosystem engineers may have played a role in benthic ecosystem recovery after severe mass extinctions, potentially, for example, triggering the evolutionary innovations and radiations in the Early Triassic." 

When gauging recovery, the scientists looked at four separate metrics: variety (the different varieties of an animal), disparity (how varied those different types were), how space was used (ecospace utilization), and how the animal adapted surroundings (ecosystem engineering).

Life began to return first in the deepest waters. Suspension feeders such as brachiopods, bryozoans, and bivalves - mostly sedentary and sometimes rooted to the ocean floor - followed after deposit feeders, but considerably later.

Even later, corals began to reappear. Soft-bodied sediment dwellers required around 3 million years to recover to pre-extinction levels.

"Maybe the deposit feeders were making such a mess of the seafloor that the water was polluted with mud, the churned mud meant suspension feeders could not properly settle on the seafloor, or the muddy water produced by those deposit feeders just clogged the filtering structures of suspension feeders and prohibited them from feeding efficiently," says Alison Cribb, a geobiology graduate student at the University of Southern California.

It's no surprise that the Permian-Triassic extinction catastrophe wiped off 80-90 percent of Earth's marine life, so recovery took a long time. Scientists can acquire a better picture of what happened next by combining trace fossil records with body fossils.

Climate change, global warming, a decline in oxygen, and rising ocean acidity are considered to be the key causes of the mass extinction - which means the discoveries here can tell us more about what's going on now.

Understanding how particular animals survived and recovered in the aftermath of the Great Dying allows us to better predict how these creatures will survive the present time of warming, and which species will be the most robust.

The research has been published in Science Advances.