Rare Fossil Points to Toxic Oceans in Devonian Period

A well-preserved crab-like fossil that was found by scientists from Curtin University, Australia, has provided evidence of a toxic ocean environment in the Devonian Period, potentially responsible for the mass extinction 380 million years ago.

A study, published in the journal Geology, shows that hydrogen sulphide dependant organisms – known as Chlorobi – and sulphate-reducing bacteria had preserved the shell and the muscles of the crab-like creature.

“The research presents organic geochemistry as a new tool for paleontologists, enabling them to identify invertebrate fossils and reconstruct their environments from a molecular point of view,” explained lead author Ines Melendez, a PhD student at the Curtin University.

“It’s like walking in on a crime scene, when all the evidence is still intact. Not only do we know the organism was a crustacean from the abundance of cholestane it contained, but we also know it was in a toxic ocean environment, from the biomarkers associated with the sulfate–reducing bacteria and Chlorobi. By looking at the biomarkers and stable isotopes of fossils, we are able to reconstruct past environments, and can apply this technique to other ages of geological time,” Melendez said.

Curtin University scientists collected the unique fossil from the Gogo Formation in the Kimberley Region of Western Australia.

“This research suggests the Devonian Period had similar paleoenvironmental conditions to the largest extinction event in the past 600 million years, where it was proved toxic concentrations of hydrogen-sulfide in ancient oceans, rather than a meteorite, were largely responsible for wiping out mass populations,” said study co-author Prof Kliti Grice of the Curtin University.

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Bibliographic information: Ines Melendez et al. 2012. Biomarkers reveal the role of photic zone euxinia in exceptional fossil preservation: An organic geochemical perspective. Geology, first published online November 6, 2012; doi: 10.1130/G33492.1