A reexamination of microfossils found in the Brazilian state of Mato Grosso do Sul shows that the marks previously interpreted as traces of worms or other small oceanic animals are actually communities of fossilized microscopic bacteria and algae.
These findings imply that during the Ediacaran period, approximately 540 million years ago, there may not have been enough oxygen for the evolution of invertebrates that left traces in the substrate, as previously thought. The study was published in the journal Gondwana Research.
"Using microtomography and spectroscopy techniques, we observed that the microfossils have cellular structures – sometimes with preserved organic material – consistent with bacteria or algae that existed during that period. These aren't traces of animals that may have passed through the area," says Bruno Becker-Kerber , the first author of the study. He conducted the study as part of his postdoctoral research at the Institute of Geosciences at the University of São Paulo (USP) and at the Brazilian Center for Research in Energy and Materials (CNPEM), with a fellowship from FAPESP .
Currently conducting postdoctoral research at Harvard University in the United States, Becker-Kerber explains that if the records were marks left by animals, it would imply the existence of meiofauna – invertebrates less than one millimeter in length – that were already quite active during the Ediacaran period. These would be the oldest animals of this type ever recorded.
This period precedes the Cambrian explosion, when increased oxygen levels in the environment enabled the evolution of complex organisms and the diversification of animal species. The presence of meiofauna is confirmed in the Cambrian fossil record.
The study is part of the "Rio de la Plata Craton and Western Gondwana" project, which is supported by FAPESP and coordinated by Miguel Angelo Stipp Basei . Basei is a professor at IGc-USP and an author of the study.
Lucas Warren , a professor at the Institute of Geosciences and Exact Sciences at São Paulo State University (IGCE-UNESP) in Rio Claro, is also among the authors. He is supported by FAPESP .
The reanalyzed fossils were found in the municipality of Corumbá, and the newly analyzed fossils were found in a rock outcrop in Bonito, in the Serra da Bodoquena region. Both cities are in the current state of Mato Grosso do Sul, where the Tamengo geological formation is located.
The rocks formed in a marine environment on the continental shelf during the final stages of the formation of the supercontinent Gondwana, which later gave rise to Africa and South America.
In another study, the group described the first known lichen in the fossil record, which was also found in Mato Grosso do Sul and is younger than the recovered bacteria and algae (read more at agencia.fapesp.br/57029 ).
Tomography
The MOGNO beamline at Sirius, the CNPEM particle accelerator in Campinas, made it possible to precisely examine such tiny fossils, which range in size from a few micrometers to a few millimeters.
There, the samples underwent micro- and nanotomography, which produces images on the scale of micrometers (one-thousandth of a millimeter) and nanometers (one-billionth of a meter), respectively.
"When you have a large sample and want to image a structure inside it, the resolution obtained is often insufficient. The MOGNO beamline is one of the few in the world that performs so-called zoom tomography, in which we focus on something inside the sample and analyze it at the nanoscale without destroying the sample," says Becker-Kerber. He points out that the study suggesting the fossils were animal traces did not have access to this technology.
Other, more accessible techniques, such as Raman spectroscopy, revealed the organic composition of the cell walls of the fossils, for example. This information was essential to corroborate the hypothesis that the marks were body fossils.
Some samples contain pyrite, a mineral composed of iron and sulfur. Therefore, based on the shape of some of the fossils, one hypothesis is that they contain remnants of sulfur-oxidizing bacteria, a group that uses sulfur as part of its metabolism.
"This group of bacteria is surprising. Some of the largest ever recorded belong precisely to this category. Unlike the common image we have of microscopic bacteria, certain species can reach diameters larger than a strand of hair and are visible to the naked eye," says Becker-Kerber.
However, there are no preserved parts that would allow for a more precise species distinction, such as reproductive structures. Nevertheless, the fossils exhibit preserved cells, cell wall divisions, and organic remnants at the different sites where they were collected. This would not be possible with traces of animals that merely passed through the sediment.
Furthermore, the distribution of the fossils in the material indicates three distinct size categories, which suggests that they represent different species that may have lived in a microbial consortium. The larger population shares similarities with green or red algae, while the smaller populations may be algae, cyanobacteria, or sulfur-oxidizing bacteria.
"There are concave and convex partitions, coiled filaments, cells without sediment but containing organic matter. This evidence is much closer to bacteria or algae than to mere marks of disturbance caused by animals," the researcher concludes.
These findings help paint a more accurate picture of the period leading up to the Cambrian explosion and provide further evidence that may help us better understand the conditions surrounding the event that transformed life on Earth.
About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at www.fapesp.br/en and visit FAPESP news agency at www.agencia.fapesp.br/en to keep updated with the latest scientific breakthroughs FAPESP helps achieve through its many programs, awards and research centers. You may also subscribe to FAPESP news agency at http://agencia.fapesp.br/subscribe.
