The Permian–Triassic extinction event, which happened roughly 252 million years ago, is colloquially known as the Great Dying because of the way it obliterated life on Earth – almost ending it completely. It's the most severe extinction event in history.
Life did recover however, and new research identifies that deposit feeders like worms and shrimps – animals that feed off organic matter settled at the bottom of the ocean – were the first to bounce back in terms of population numbers and biodiversity.
Suspension feeders, which snack on organic matter suspended in water, followed much later, according to a detailed dating of trails and burrows on the South China sea bed. This analysis revealed a wealth of ichnofossils or trace fossils – not actual animal remains, but remains 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," says paleontologist Michael Benton, from the University of Bristol in the UK.
"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."
As soft-bodied animals don't have any skeletons to leave behind, trace fossils are vital in working out how these creatures lived. The research team were also able to incorporate body fossils into their study to look at how other species began to recover once the deposit feeders became established.
"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."
The team looked at four different metrics when measuring recovery: diversity (the different types of an animal), disparity (how varied those different types were), how space was used (ecospace utilization), and how habitats were modified by the animal (ecosystem engineering).
Life began to return in 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 on, but much later.
Even later still, corals started to come back. It took around 3 million years for soft-bodied sediment dwellers to get back 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 geobiology graduate student Alison Cribb, from the University of Southern California.
The Permian–Triassic extinction event killed off around 80–90 percent of the marine life on Earth, so it's no surprise that recovery took a long time. By adding trace fossil records to the data alongside body fossils, scientists can get a fuller picture of what happened next.
Climate change, global warming, a drop in oxygen and increased ocean acidification are thought to be the primary drivers behind the mass extinction – and of course that means the findings here can teach us more about what's happening in the modern era.
By understanding how certain animals survived and recovered in the wake of the Great Dying, we're better able to figure out how these creatures might survive the current period of warming we're going through, and which species could be the most resilient.
The research has been published in Science Advances.