Snails narrow down the impacts of the largest ever extinction

Fossils from the Museum collection have helped rule out one possible impact of the Earth's largest ever extinction. 

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 subsequent effects 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 that were living soon after 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, 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 and seafloor sediments, carbon dioxide dissolving in seawater increases the concentration of hydrogen ions in the ocean. 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 and geochemical evidence, such as the levels of different isotopes, but for this new paper the researchers turned to fossilised snails and bivalves from a Norwegian archipelago.

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? 

Dating to immediately after the Permian-Triassic extinction event, the individual mollusc fossils were separated out and examined under an electron microscope.

Jaime Hirtz, one of the study's co-authors, says, 'The shells are beautiful. They are so well preserved you can see perfect growth lines and ornamentation, even on the tiny and delicate larval snail shells.'

While two-thirds of the mollusc fossils showed some minor evidence of being dissolved, these weren't associated with the signs of repair, suggesting that the animal was already dead when the damage occurred. 

Fossilised shells preserving the juvenile stage of these animals, some of which spend time as plankton-feeding larvae in the surface ocean, also showed very limited amounts of damage and no repair marks. Overall the results show that ocean acidification was not present on the seafloor where the adult animals lived or in the surface waters at this time.

However, while ocean acidification may not have affected these animals in this location, they can't rule out that other parts of the world may have experienced more acidic seas at this time, although as other fossilised molluscs from found across the world seem to also show limited damage it is probable that ocean acidification was very restricted.

If ocean acidification did play a role in the extinction of Permian life, the study's results suggest that it was limited to occurring during the extinction event itself, and did not persist into the start of the Triassic. 

Once the conditions associated with the Permian-Triassic mass extinction began to diminish, it paved the way for life to rapidly diversify into many new forms, including ichthyosaurs, dinosaurs and early mammals.