Does the fossil record tell the true story about mass extinctions?

The fossil record could be misleading us about when mass extinctions took place, suggest new findings based on the study of deep sedimentary cores drilled from where the Po river delta in northern Italy opens out into the Adriatic Sea.

The Adriatic Sea off the coast of Italy. The Po river delta can be seen entering the Adriatic in the most northern part of the sea. Image credit: Jacques Descloitres/MODIS Land Rapid Response Team/NASA/GSFC.

Identifying when mass extinctions occurred, and what environmental factors were coincident with them that could have led to those extinctions, is a vital part of piecing together the history of life on Earth. Paleontologists are able to determine when a species went extinct based on its last appearance in the fossil record. Any variations in the fossil record had been assumed to be down to either random chance, or the incompleteness of that record.

However, previous computer models have suggested that this might not be the case, and that a species’ ecological preference – what kind of habitat it prospers in – could affect their placement in the fossil record, not because they died off, but because climatic change had altered their environment.

To put this to the test, researchers from the Florida Museum of Natural History, writing in Proceedings of the Royal Society B, analyzed 130-foot (39.6-meter) deep cores drilled in Po Plain, which is a large river valley that runs into the Adriatic Sea off the east coast of Italy. The scientists, led by Michal Kowalewski and Rafal Nawrot, searched for mollusks – i.e hard-shelled sea life, like oysters – in the cores. Because the various species of mollusk have not gone extinct yet, their fossils should be found in the uppermost layers of the cores, where they would have been recently deposited. However, they were instead found to be more widely spread throughout the cores, giving the impression that there had been multiple mass extinctions over time.

Following their habitats

The cores gave the impression that most offshore species had gone extinct in one large burst a long time ago, with their last fossils found in the lower parts of the cores, followed by smaller pulses of extinction of shallow water species. This was incorrect; those species of sea life still exist in the Adriatic today, but what has changed is their local environment. The Po river delta that opens into the Adriatic has expanded and the sea level changed, skewing the way material is deposited in the sea bed as the environment changed and species followed the changing habitats.

“It’s important to admit that fossil species – just like modern ones – have specific ecological requirements, which sounds obvious but is not always acknowledged,” says Nawrot.

He points out that by ignoring this, and by mistakenly assuming that fossils are deposited randomly, statistical analysis can provide results that are incorrect by millions of years.

“We’re not saying that you cannot study mass extinctions – you can,” says Kowalewski. “What we’re saying is that the nature of the geological record is complicated, so it is not trivial to decipher it correctly.”

The solution, says Kowalewski, is the application of statistical methods that do not assume randomness, but which instead are able to reflect ecological preferences and changes in environment over time. By doing so, paleontologists will be able to better analyze the fossil record and discover when mass extinctions really took place.