The world's worst mass extinction that occurred about 252 million years ago - known as the 'Great Dying' - was followed by two other events, according to a new study which explains why it took ecosystems around the globe millions of years to recover.
The extinction events are linked to climate change caused by massive volcanic activity, researchers said.
The study is a step toward understanding how lifeforms survived during the extinctions, which could help scientists understand how modern ocean life evolved and how it might respond to climate change in the future.
"The early evolution of modern marine ecosystems happened during the recovery period of these extinction events," said William Foster, postdoctoral researcher at The University of Texas at Austin in the US.
Earth has experienced five mass extinctions that killed the majority of species living on the planet at the time.
The end-Permian extinction or "Great Dying" about 252 million years ago was the worst, with an estimated 95 per cent of marine life and 70 per cent of terrestrial life perishing.
The extinction is linked to climate change caused by prolonged volcanic eruptions in Russia's Siberian Traps.
The eruptions covered an area larger than Alaska with lava and released massive amounts of greenhouse gasses into the atmosphere, which had dire consequences for life.
"This release of carbon dioxide and sulfur started this whole climate warming scenario that caused the extinction," Foster said.
The end-Permian extinction had the longest recovery time, lasting five million to eight million years.
Researchers found the first fossil and geochemical evidence for two distinct extinction events following the end-Permian that probably played a role in the slow recovery.
The evidence comes from rock samples with spikes of carbon 12 relative to carbon 13, a chemical ratio associated with large disruptions in the carbon cycle that were probably caused by the volcanic eruptions.
A carbon 12 spike occurred in samples from the Dienerian, a period about half a million years after the end-Permian extinction that was previously recognized from fossil evidence as an extinction event.
A second carbon 12 spike was found at the boundary of the Smithian/Spathian periods, which occur about 1.5 million years after the end-Permian extinction.
At both sites researchers noted a decreased diversity of marine fossils compared with surrounding periods, with the dominant survivors of the extinction events being mollusks, such as snails and clams, only a few centimetres in size at most.
After the second extinction event, the fossil record shows an increased ecological diversity.
This is a sign, researchers said, that the environmental stresses that limited recovery from the first extinction event and instigated the second were beginning to lessen.
The study was published in the journal PLOS ONE.
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