After the biggest extinction event on Earth, these animals were the first to recover

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The Permian-Triassic extinction event, which occurred around 252 million years ago, is colloquially known as the Great Death because of how it wiped out life on Earth – the almost completely ending. It is the most severe extinction event in history.

Life has recovered, however, and new research has identified that deposit feeders like worms and shrimp – animals that feed on organic matter deposited on the ocean floor – were the first to rebound in terms of population and of biodiversity.

Suspension feeders, which nibble on organic matter suspended in the water, followed much later, according to detailed dating of trails and burrows on the seabed in southern China. This analysis revealed a wealth of ichnofossils or traces of fossils – not real animal remains, but remains of animal activity.

What the oceans looked like before (A) and after (BF) the extinction. (X.Feng/Z.-Q.Chen/MJ Benton/Y.Jiang)

“We were able to observe fossil traces from 26 sections across the entire series of events, representing a crucial 7 million years,” says paleontologist Michael Bentonfrom the University of Bristol in the UK.

“By showing details of 400 sample points, we finally reconstructed the recovery stages of all animals, including benthos, nekton, as well as those soft-bodied burrowing animals in the ocean.”

Since soft-bodied animals don’t have skeletons to leave behind, trace fossils are key to understanding how these creatures lived. The research team was also able to incorporate body fossils into their study to see how other species began to recover once the depositivores became established.

“The Late Permian Crisis – which was so devastating to life on Earth – was caused by global warming and ocean acidification, but tracer animals may be selected by the environment in ways that skeletal organisms were not”, says paleoecologist Xueqian Feng from China University of Geosciences.

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

The team looked at four different measures when measuring recovery: diversity (the different types of animals), disparity (how these different types were varied), how space was used (use of ecospace) and how habitats were modified by the animal (ecosystem engineering).

Life began to return to the deepest waters first. Once deposit feeders had largely recovered, suspension feeders such as brachiopods, bryozoans and bivalves – largely sedentary and often rooted to the ocean floor – followed, but much later.

Even later still, the corals started to come back. It took approximately 3 million years for the inhabitants of the soft-bodied sediments to return to pre-extinction levels.

“Perhaps the deposit feeders made such a mess of the seabed that the water was polluted with mud, the churned mud meant the suspension feeders couldn’t settle properly on the seabed, or the muddy water produced by these deposit feeders simply obstructed the filtering structures of suspension feeders and prevented them from feeding effectively,” says geobiology graduate student Alison Cribbfrom the University of Southern California.

The Permian-Triassic extinction event killed an estimated 80-90% of marine life on Earth, so it’s no surprise that recovery took a long time. By adding trace fossils to the data alongside the body fossils, scientists can get a more complete picture of what happened next.

Climate change, global warming, a decline in oxygen and increased ocean acidification are considered the main drivers behind the mass extinction – and of course that means the discoveries here can tell us more about what’s happening in the modern age.

By understanding how certain animals survived and recovered in the aftermath of the Great Death, we are better able to understand how these creatures might survive the current warming times we are experiencing and which species might be the most resilient.

The research has been published in Scientists progress.

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