New research published in the journal PNAS has found that, while life in the ocean rapidly disappeared during the Great Dying at the end-Permian mass extinction, the loss of life on land was a much more drawn-out affair.
The Earth has suffered five mass extinction events throughout its history, each permanently altering life’s evolutionary trajectory. But the most devastating was the end-Permian event, 252 million years ago.
Nicknamed the “Great Dying”, it is thought to have been triggered by catastrophic volcanic eruptions, resulting in dramatic environmental changes – including a runaway greenhouse effect and ocean acidification – that wiped out 95% of both land and ocean species.
A great deal of what scientists know about this event comes from the ancient oceans, because the fossil record is much more abundant in marine environments: when an animal dies, its body is rapidly covered with sediment and thus is more likely to be preserved.
The marine fossil record tells us that when the end-Permian event hit, it took a mere 100,000 years for more than 85% of ocean species to disappear.
But the Great Dying on land followed a different pattern, says lead author Pia Viglietti, from Chicago’s Field Museum in the US: “We found that the marine extinction may actually be a punctuation to a longer, more drawn-out event on land.”
To reach this conclusion, Viglietti and colleagues from the US, UK and South Africa created a database from the fossils of nearly 600 four-legged vertebrate animals found in the region of South Africa’s Karoo Basin. The fossils spanned four million years, but the study separated them by age and grouped them into 300,000-year intervals. Applying statistics-based algorithms revealed the bigger picture of when certain species appeared or disappeared.
The results showed that a high rate of extinction persisted for around a million years on land – ten times longer than in the oceans.
One key species the team traced through time was the Lystrosaurus, a plant-eating mammal relative. It is what paelaeontologists call a “disaster taxon” – it proliferated while most other life disappeared.
Researchers had assumed the Lystrosaurus thrived only in the aftermath of the extinction, but this new research suggests otherwise.
“We see Lystrosaurus appearing before the extinction even got started – it was already abundant,” Viglietti explains.
The team hypothesise that Lystrosaurus was able to better adapt to the environmental changes that caused the extinction of other species.
Comparing the abundance of Lystrosaurus with the disappearance of other species helped the team quantify extinction rates and show that the entire event dragged out over a million years on land.
The reasons for this drawn out extinction are still unclear, but may be related to how changes to the Earth’s climate are cumulative over time, building up slowly until they hit a sudden point of collapse – analogous to the tipping points we face today.
“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,” Viglietti explains.
The researchers argue that the same may have been true for the oceans, 252 million years ago.
Studying such events provides a powerful analogue to the mass extinction humans are driving today.
“The environmental changes that we are causing and the impacts we are having on animal and plant species are getting to the point where the scale is such that there isn’t really anything in human history that is comparable,” warns co-author Ken Angielczyk, curator of vertebrate paeleontology at the Field Museum.
“The fossil record can give us some idea of what massive biodiversity crises are like and how they proceed.”