A recent Yale study, which suggests that the high silica content of ancient oceans is responsible for the preservation of the Ediacara biota fossil group, discovered that these fossils provide an accurate window into seafloor life as it was half a billion years ago. The study was published in the journal Geology on Oct. 3.
The Ediacara biota is Earth’s earliest fossilized ecosystem of complex, multicellular, macroscopic organisms, according to the study. Many of the group’s fossil assemblages come in “cast and mold” or “Ediacara-style” as a result of the sandstone impressions left by these soft-tissue organisms.
Lidya Tarhan, a postdoctoral fellow at Yale and first author of the study, said she was interested in studying the mechanism of this fossilization. She added that most soft-tissue organisms don’t leave behind fossils as detailed as the Ediacara biota.
“To have all of these details captured means that this fossilization process happened very early,” Tarhan said. “And when you look at the sandstones, the sandstones themselves are very rich in silica, so I started to wonder whether that might potentially have played a role in the fossilization of these organisms.”
Modern-day oceans contain almost no silica because organisms including plankton and sponges use it to build their skeletons, according to Tarhan. However, these organisms had not evolved when Ediacara biota organisms lived, approximately 580 to 540 million years ago, so the oceans were much richer in silica. Tarhan added that understanding mechanisms of fossilization is important to understanding ancient communities of organisms.
According to the study, high concentrations of dissolved silica can influence the silicification — a process involved in petrifaction — of organic matter, resulting in cellular-level preservation. Tarhan and four others tested the hypothesis that silica was responsible for the preservation by using paleontological, petrographic and geochemical techniques to determine the composition of cements associated with Ediacara macrofossils and the timing of their formation.
“You need to understand how fossilization occurs to accurately reconstruct what these organisms were and what these communities looked like,” Tarhan said. “It’s almost like a Pompeii effect — we have entire communities preserved together. To reconstruct their community structure and their ecology, you need to first understand how these organisms were fossilized.”
Derek Briggs, a professor of geology and geophysics at Yale who co-authored the paper, said that the team studied the Ediacara Member, which is a particular assemblage of Ediacara biota fossils found in South Australia.
Briggs reiterated that in proving early silicification, the study helps to paint a more accurate picture of the seafloor from over half a billion years ago.
“The argument is we can now be pretty confident that at least the Ediacara Member in South Australia is giving us a very complete picture of the diversity of creatures on the seafloor at the time,” said Briggs, who is also the curator of invertebrate paleontology at the Yale Peabody Museum of Natural History.
The paper was also co-authored by Mary Droser of the University of California, Riverside, James Gehling of the South Australian Museum and the University of Adelaide and Ashleigh Hood, another Yale postdoctoral associate.