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Earth’s worst mass extinction event was not the asteroid that is thought to have killed the dinosaurs, but the ‘Great Dying’ 252 million years ago.
Massive volcanic eruptions brought about rapid climate change, and when the dust settled, the planet entered the age of dinosaurs.
But new research has shown that extinction did not come at the same speed, and that the process took far longer on land after many species in the seas had already died.
It could have important implications for our present-day battle against climate change, the researchers have warned.
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Pia Viglietti, a postdoctoral researcher at Chicago's Field Museum, said: "People assumed that because the marine extinction happened over a short period of time, life on land should have followed the same pattern, but we found that the marine extinction may actually be a punctuation to a longer, more drawn-out event on land."
Viglietti, senior author Ken Angielczyk and their colleagues examined fossils from 588 four-legged animals that lived in what's now South Africa's Karoo Basin at the time of the Permian mass extinction.
The researchers created a database and separated the fossils by age, grouping together specimens by 300,000-year time intervals, allowing the researchers to quantify the appearance and disappearance of different species and look at the bigger picture of life over time.
Viglietti said: "Our approach unifies the data and says, OK, within this time bin we have these species, but as we go up, we have these other species. By applying sampling methods to these bins, we can help correct for issues like having more or fewer specimens collected in different time intervals or places.
"Ultimately, it lets us quantify how much extinction is happening and how quickly new species are appearing.
“Instead of putting too much focus on any one fossil, you compile hundreds of observations roughly in the same time interval."
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The fossils showed the researchers that the Permian extinction looked very different on land than it did in the oceans, where it lasted a mere 100,000 years.
The extinctions on land took 10 times as long, although it’s not clear why it was so different.
"The changes to the Earth's climate were cumulative and added up over time. Ecosystems were slowly disrupted, and then it just got to a point where everything collapsed, like the straw that breaks the camel's back," Viglietti added.
“Everything's fine, until it's not."
One reason for the discrepancy could be that the oceans can absorb chemical changes and stabilise themselves, up to a point.
Viglietti said: "In today's climate crisis, the oceans can absorb a lot of carbon dioxide or rise in temperature without people realising, and then all of a sudden you get sudden ecosystem breakdowns like ocean acidification and coral bleaching."
Fossils are preserved more easily in sediment in the oceans, so paleontologists have known for a while that 252 million years ago a mass extinction hit at the end of the Permian period
Within 100,000 years, more than 85% of the species living in the ocean went extinct.
But the process was very different on land, said Angielczyk, curator of vertebrate paleontology at the Field Museum.
"The focus for studying terrestrial extinction has basically been, 'Can we match up the pattern in the terrestrial realm with what's observed in oceans?' And the answer is, 'Not really.'
"This paper is the first really focusing on vertebrates and saying, 'No, something was going on that was unique to the terrestrial realm.'"
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