Scientists studying ancient microfossils in Brazil have discovered that structures once believed to be traces left by small animals were actually formed by microscopic communities of bacteria and algae. The discovery challenges previous ideas about when small animals first appeared on Earth and suggests that around 540 million years ago, oxygen levels in ancient oceans may still have been too low to support certain forms of animal life.
The study focused on fossils discovered in Brazil’s Mato Grosso do Sul state and was published in the journal Science. Gondwana research. Previous studies interpreted the traces as evidence that worm-like creatures and other small marine animals were moving through seafloor sediments during the Ediacaran Period, just before the Cambrian Explosion.
“Using microtomography and spectroscopy techniques, we observed that the microfossils have cellular structures consistent with bacteria and algae present at that time, and sometimes contain preserved organic material. These are not traces of animals that may have passed through the area,” says Bruno Beckerkerber, lead author of the study. He conducted this research during his postdoctoral studies at the Institute of Earth Sciences of the University of São Paulo (USP) and the Brazilian Center for Energy and Materials Research (CNPEM), with support from FAPESP.
Dr. Becker Kerber, currently a postdoctoral researcher at Harvard University, explains that if the traces were indeed left by animals, they would be evidence of Hadesan animals from the Ediacaran period. Meiofauna are small invertebrates less than 1 millimeter long. Their discovery in rocks this old would have significantly set back the fossil record for these creatures.
Ancient ocean before the Cambrian explosion
The Ediacaran period preceded the Cambrian Explosion, a major evolutionary tipping point where rising oxygen levels caused complex organisms to rapidly diversify across Earth’s oceans. Fossil evidence clearly indicates that meiofauna existed during the Cambrian period, but new discoveries suggest that meiofauna may not have existed as early as some scientists claim.
This project forms part of the “Rio de la Plata Craton and Western Gondwana” study supported by FAPESP and coordinated by Miguel Angelo Stip Basei, IGc-USP professor and co-author of the paper.
Another co-author, Lucas Warren from the State University of São Paulo in Rio Claro (IGCE-UNESP), also received support from FAPESP.
The researchers reexamined fossils collected at Columba and also analyzed newly studied material from skipjack tuna in the Bodokena Mountains region. Both sites are located within the Tamengo Formation in the state of Mato Grosso do Sul.
These rocks formed in a shallow marine environment along the continental shelf during the final stage of Gondwana formation, before the supercontinent ultimately broke apart to form the regions that would become South America and Africa.
The same research group previously identified what may be the oldest known lichen fossil, also found in Mato Grosso do Sul state, and younger than the bacteria and algae reported in this study.
High-resolution fossil imaging reveals hidden structures
To examine the fossils in more detail, the researchers used the MOGNO beamline at Sirius, CNPEM’s particle accelerator facility in Campinas. This technology allows researchers to study fossils that are just a few micrometers to a few millimeters in size.
The scientists used both microtomography and nanotomography. This is a technology that can produce images at very small scales, such as micrometers (one-thousandth of a millimeter) or nanometers (one-billionth of a meter).
“When you have a large sample and want to image its internal structure, the resolution you get is often not enough. The MOGNO beamline is one of the few beamlines in the world that performs so-called zoom tomography, which focuses on something inside the sample and analyzes it at the nanoscale without destroying it,” Becker-Kerber said.
He points out that earlier studies that interpreted structures as animal tracks did not have access to this level of imaging technology.
The researchers also used Raman spectroscopy to examine the fossil’s chemical composition. This technique identified organic material within the fossil cell walls, strengthening the interpretation that the structures are preserved microbial bodies rather than traces left by passing animals.
giant ancient bacteria and algae
Some fossil samples contained pyrite, a mineral made of iron and sulfur. Based on the shape and chemistry of the specimens, researchers believe some may be sulfur-oxidizing bacteria, or organisms that use sulfur for their metabolism.
“This group of bacteria is amazing. Some of the largest ever recorded are precisely in this category. Unlike the typical image we have of microscopic bacteria, certain species can reach diameters larger than a hair and are visible to the naked eye,” Beckerkerber says.
Although the fossils did not preserve enough detail to identify the exact species, researchers found preserved cells, divisions within cell walls, and traces of organic matter at multiple collection sites. According to the researchers, these features would not be present if the structures were simply disturbances caused by animal migration.
The fossils were also found in three different size ranges, suggesting that multiple species may have lived together in a microbial community. The largest forms resemble green or red algae, but smaller fossils may represent algae, cyanobacteria, or sulfur-oxidizing bacteria.
“Rough septa, coiled filaments, and cells without sediment but containing organic matter are present. This evidence is much closer to bacteria or algae than to mere traces of disturbance caused by animals,” the researchers conclude.
The discovery provides 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 emergence of complex animal life.
