Scientists detect Earth's deadliest extinction signs in today's oceans: Great Dying started with warming seas and oxygen loss; 70% of land animals vanished

Scientists have presented what they describe as the strongest evidence yet explaining what caused Earth's largest mass extinction around 252 million years ago, concluding that rapidly warming oceans and falling oxygen levels drove the catastrophic event that wiped out most life on the planet.

The new Stanford University-led study, published in the Proceedings of the National Academy of Sciences (PNAS), also explains why some marine animals survived while many others almost completely disappeared, reshaping ocean life for the next 252 million years.

What was the 'Great Dying'?

Known as the Permian-Triassic mass extinction or the Great Dying, the event remains the most devastating extinction in Earth's history.


According to the researchers, around 96% of marine species and 70% of land animals disappeared during the catastrophe.

While many groups were nearly wiped out, others managed to survive and eventually came to dominate today's oceans. The study set out to understand why that happened.

Lead author Jose Andres Marquez said the research aimed to answer a question that can still be observed on modern beaches.

"With this study, we essentially wanted to solve the mystery of why, when you go to the beach, you collect the shells of clams and snails rather than those of brachiopods."

Why did some species survive while others disappeared?

The researchers found that survival largely depended on how different animals responded to warmer, oxygen-poor oceans.

Animals such as brachiopods, sea lilies (crinoids) and several bottom-dwelling filter feeders had slow metabolisms that struggled to cope as temperatures rose and oxygen levels fell.

By contrast, groups including clams, snails, fish, sea urchins and other mollusks were better equipped to survive these harsh conditions.

According to the study, the species that experienced the highest extinction rates were those least able to tolerate rising water temperatures and declining oxygen availability.

How did scientists reach this conclusion?

The new research expands on an earlier 2018 study that had already linked ocean warming and oxygen loss to the Great Dying.

This time, the researchers collected living representatives of both ancient Paleozoic animals and modern marine groups to directly compare how they respond to changing ocean conditions.

Fieldwork was conducted in Washington state's San Juan Islands, where brachiopods still exist today.

Scientists then measured how much oxygen different species consumed under varying water temperatures.

The experiments showed that ancient Paleozoic animals could survive in low-oxygen water better than many modern species when temperatures remained stable.

However, once temperatures increased, their oxygen requirements rose sharply while their slow metabolisms struggled to keep pace, making them especially vulnerable.

Ancient oceans changed after massive volcanic activity

According to the researchers, the extinction was triggered by enormous volcanic eruptions that released vast amounts of carbon dioxide and methane into the atmosphere.

Those greenhouse gases warmed the planet, reduced oxygen levels in the oceans and created conditions that many marine animals could not survive.

Senior author Erik Sperling said the findings effectively settle one of palaeontology's longest-running debates.

"This study is really the final nail in the coffin for what caused the Permian-Triassic mass extinction."

He noted that the extinction began from a world with relatively cool, oxygen-rich oceans before a massive release of greenhouse gases dramatically altered Earth's climate.

Why today's climate makes the findings significant

Beyond solving an ancient mystery, the researchers say the study carries an important message for the present.

Modern oceans are once again experiencing rising temperatures, declining oxygen levels and increasing acidification because of human-driven greenhouse gas emissions.

The team believes understanding how marine life responded during Earth's greatest extinction could help scientists predict which species may face the greatest risks in the future.

Sperling said the planet is moving towards levels of warming that resemble those associated with the Great Dying under the worst climate projections.

"The bad news is, we are on track for Permian-Triassic levels of warming in worst-case scenario projections."

He added that there is still time to alter that trajectory.

"But the good news is, we're still at the point where we can change things and do something about it."

What happens next?

The researchers now plan to study additional marine animal groups to better understand how warming, oxygen loss and ocean acidification interact to affect marine ecosystems.

While the study found that ocean acidification may also have contributed to the ancient extinction, it concluded that warming and oxygen depletion were the dominant drivers behind Earth's largest known mass extinction.