Snails narrow down the impacts of the largest ever extinction

The fossilised shell of a marine snail seen under an electron microscope
The fossilised shells from Norway suggest that ocean acidification did not persist after the Permian-Triassic mass extinction event. Image © Foster et al., licensed under CC BY 4.0 via the Journal of Systematic Palaeontology

A group of fossilised snails and clams are challenging suggestions that ocean acidification contributed to the largest extinction in history. 

An international team of researchers found limited evidence of acid damage to shells following the Permian-Triassic mass extinction event, suggesting acidic conditions may have been more limited than expected.

Fossils from the Museum collection have helped rule out one possibility for why the Earth’s largest ever extinction was so devastating. 

The Permian-Triassic mass extinction event took place over 250 million years ago and is estimated to have wiped out up to 80% of marine species as well as two thirds of all terrestrial vertebrates. Despite its catastrophic impact, questions surrounding its initial cause and following impacts remain. 

A recent study, published in Scientific Reports, has found no fossil evidence that the extinction caused long-lasting ocean acidification as had previously been suggested. Fossils of ancient molluscs dated to the years immediately following the extinction showed no signs of repair, suggesting that their shells were not being damaged by acidic seawater while they were alive and growing. 

Professor Richard Twitchett, a Research Leader in Earth Sciences at the Museum, says, ‘One of the exciting aspects of this research is that we now know we can use fossil shells of aragonitic marine animals from millions of years ago as bioindicators of past ocean acidification.  

‘Where the fossil record is good enough, we can test the severity and extent of ocean acidification during major episodes of past climate change and mass extinction, from seafloor to surface waters. What better source of information is there on past climate change than the animals that lived through it themselves?’ 

What was the Permian-Triassic mass extinction? 

The Permian-Triassic mass extinction is sometimes known as the ‘Great Dying’ due to its severe impact on life. Even some long-lived groups such as the trilobites, which had survived for almost 300 million years, were wiped out. 

While a variety of causes have been proposed, including asteroid strikes and the release of large amounts of methane by microorganisms, one of the leading suggestions is that volcanism in what is now Siberia resulted in the release of large amounts of carbon dioxide into the atmosphere and caused the planet to warm. 

As with modern anthropogenic climate change, the increasing levels of carbon dioxide in the atmosphere caused the oceans to become more acidic.  

While the oceans act as one of the largest carbon sinks, taking it from the atmosphere and locking it away in carbonate minerals or in decaying animal matter, the carbonates can then break apart into carbonate ions and hydrogen ions. These hydrogen ions make seawater more acidic, which can impact marine life by dissolving shells and other natural structures, such as the skeleton of coral

While direct measurements of modern ocean acidification are relatively straightforward, studying ocean acidification from over 250 million years ago is much more challenging. Studies using different sources of evidence can come back with opposing answers on when and to what degree the Earth’s oceans were acidified during the Permian. 

These conflicting studies used geological evidence, such as the levels of different mineral isotopes, but for this new paper the researchers turned to fossilised snails and clams from Norway

Lead author Dr William Foster says, ‘The fossil shells we analysed are exquisitely well preserved. If the animals experienced severe ocean acidification when they were alive millions of years ago we would see that evidence as dissolution scars and repair marks on the fossil shells.’ 

What do the shells reveal? 

James Ashworth, Natural History Museum, 31 January 2022. Press release.

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