Scientists studying ancient microscopic fossils from Brazil have discovered that structures once thought to be traces left by small animals were actually formed by communities of microscopic bacteria and algae. The findings challenge previous ideas about when small animals first appeared on Earth and suggest that oxygen levels in ancient oceans may have still been too low to support some forms of animal life about 540 million years ago.
The research focused on fossils found in the Brazilian state of Mato Grosso do Sul and was published in the journal gondwana research. Earlier studies had interpreted the marks as evidence of worm-like creatures or other small marine animals moving through sea floor sediments during the Ediacaran period, which came just before the Cambrian explosion.
Bruno Becker-Kerber, first author of the study, says, “Using microtomography and spectroscopy techniques, we observed that the microfossils contain cellular structures – sometimes with preserved organic material – that are consistent with bacteria or algae present during that period. These are not traces of animals that would have passed through the area.” He conducted research with the support of FAPESP during postdoctoral work at the Institute of Geology at the University of São Paulo (USP) and the Brazilian Center for Research in Energy and Materials (CNPEM).
Baker-Kerber, who is now conducting postdoctoral research at Harvard University, explains that if the marks were indeed left by animals, they would represent evidence of meiofauna during the Ediacaran period. Meiofauna are small invertebrates less than a millimeter in length. Finding them in such old rocks would have pushed the fossil record of these creatures back a long way.
Ancient ocean before the Cambrian explosion
The Ediacaran period preceded the Cambrian explosion, a major evolutionary turning point when increasing levels of oxygen helped complex organisms rapidly diversify in Earth’s oceans. Fossil evidence clearly shows that meiofauna existed during the Cambrian, but new findings suggest that they did not exist earlier in the way some scientists had proposed.
The project is part of the “Rio de la Plata Craton and Western Gondwana” study, supported by FAPESP and coordinated by Miguel Angelo Stipe Bessi, professor at IGC-USP and co-author of the paper.
Another co-author, Lucas Warren of São Paulo State University in Rio Claro (IGCE-UNESP), also received support from FAPESP.
The researchers re-examined fossils collected in Corumbá and also analyzed newly studied material from Bonito in the Serra da Bodoquena region. Both sites are located in Mato Grosso do Sul within the Tamengo geological formation.
These rocks formed in a shallow marine environment along a continental shelf during the final stages of the formation of Gondwana, before the supercontinent eventually split apart to form the regions that became South America and Africa.
The same research group previously identified what may be the oldest known lichen fossil, which was also discovered in Mato Grosso do Sul and is even younger than the bacteria and algae described in the current study.
High resolution fossil imaging reveals hidden structures
To examine the fossils in more detail, the team used the MOGNO beamline at Sirius, the particle accelerator facility of CNPEM in Campinas. The technology allowed researchers to study fossils ranging from just a few micrometers to a few millimeters in size.
Scientists used both microtomography and nanotomography, techniques capable of generating images at extremely small scales, including micrometers (one thousandth of a millimeter) and nanometers (one billionth of a meter).
“When you have a large sample and you want to image a structure inside it, the resolution achieved is often insufficient. The MOGNO beamline is one of the few in the world that does 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.
They noted that earlier studies interpreting the structures as animal tracks did not have access to this level of imaging technology.
The researchers also used Raman spectroscopy to investigate the chemical composition of the fossils. The technique identified organic material within the fossilized cell walls, strengthening the interpretation that the structures were preserved microbial bodies rather than traces left by passing animals.
Giant Ancient Bacteria and Algae
Some fossil specimens included pyrite, a mineral composed of iron and sulfur. Based on the size and chemistry of the samples, researchers believe some may represent sulfur-oxidizing bacteria, organisms that use sulfur in their metabolism.
“This group of bacteria is astonishing. Some of the largest bacteria ever recorded belong to this category. Contrary to the common image we have of microbes, some species can reach diameters larger than a strand of hair and are visible to the naked eye,” says Becker-Kerber.
Although the fossils do not preserve enough detail to identify the exact species, researchers have found preserved cells, partitions within cell walls, and traces of organic material at several collection sites. According to the team, these features would not exist if the structures were simply disturbances generated by roaming animals.
The fossils also appear to be in three different size ranges, suggesting that multiple species may have lived together in microbial communities. The largest forms resemble green or red algae, while smaller fossils may represent algae, cyanobacteria, or sulfur-oxidizing bacteria.
The researcher concluded, “There are cells with concave and convex partitions, coiled filaments, without sediment but containing organic matter. This evidence is much closer to that of bacteria or algae than to traces of disturbance caused by animals.”
The findings provide scientists with a clearer picture of the world before the Cambrian explosion and may help researchers better understand the environmental conditions that paved the way for the rise of complex animal life.