For years, a set of tiny structures locked inside Brazilian limestone served as one of paleontology’s most tantalizing clues: evidence that animals were already tunneling through the seafloor more than 540 million years ago, right at the dawn of complex life. A study published in May 2026 in Gondwana Research dismantles that interpretation. The structures are not animal burrows at all, the researchers argue. They are the pyritized remains of filamentous bacteria and algae, preserved in iron sulfide minerals so faithfully that they fooled scientists into seeing worm tunnels where only microbes had grown.
“These filaments branch, taper, and cross-cut in ways that are entirely consistent with microbial growth and inconsistent with directed animal movement,” the study’s authors write in the paper, noting that the uniform diameters of the structures further distinguish them from excavated tunnels.
The finding strips away a key piece of evidence from the record of early animal behavior and reopens a fundamental question: when did animals first begin reshaping the ground beneath them?
The fossils that fooled paleontologists
The structures come from the Tamengo Formation, part of the Corumba Group in southwestern Brazil. This Ediacaran-age carbonate platform preserves some of the final chapters of life before the Cambrian explosion, the evolutionary burst that produced most major animal groups starting around 538.8 million years ago. Previous researchers had classified the tiny features as meiofaunal burrows, tunnels dug by worm-like creatures small enough to live between grains of sediment. That reading placed animal burrowing behavior at roughly 541 million years ago and suggested that sediment-mixing organisms were already at work just as complex life was diversifying.
The new study re-examined the structures using morphological analysis, spatial mapping, preservation style, and multiple microanalytical techniques. In thin sections and three-dimensional reconstructions, the filaments branch, taper, and cross-cut one another in patterns far more consistent with microbial colony growth than with the directed, purposeful movement of a burrowing animal. Their diameters are strikingly uniform, another signature of biological filaments rather than excavated tunnels, which typically vary in width as an animal pushes through different sediment layers.
Pyritization, the process by which iron sulfide minerals replace organic material, can preserve delicate biological structures in extraordinary detail. But it can also create shapes that mimic animal-made tunnels when the original organisms were thread-like microbes rather than worms. The researchers argue that is exactly what happened here: sulfate-reducing bacteria in the sediment converted organic matter and dissolved sulfate into sulfide minerals during early diagenesis, locking the filaments in place and giving them a deceptively burrow-like appearance.
A seafloor ruled by microbes, not worms
The Tamengo Formation also hosts fossils of Cloudina, one of the earliest known organisms to build a mineralized skeleton. A 2017 study by Warren et al. published in Nature Scientific Reports documented how these tube-building creatures lived in close association with microbial mats, with the formation’s age constrained to approximately 543 million years ago (plus or minus 3 million years). That earlier work, now well-established context for the Tamengo Formation’s paleobiology, helps distinguish between microbial mat communities and early biomineralizing animals, two categories of life that shared the same seafloor but left very different kinds of traces.
The reinterpretation of the supposed burrows fits squarely into the microbial mat category. Carbon isotope data from the Tamengo Formation tie these fossil-bearing layers to the Shuram-Wonoka anomaly, a dramatic negative swing in carbon isotope values recorded in Ediacaran rocks worldwide that reflects major shifts in ocean chemistry driven largely by microbial processes. In the Tamengo carbonates, those strongly negative values coincide with abundant microbial textures, laminated sediments, and pyrite-rich fabrics, all hallmarks of seafloors stabilized by bacterial mats rather than churned by burrowing animals.
Sedimentological studies of the formation describe a low-energy, shallow-marine setting with localized microbial buildups and early cementation. In such an environment, filamentous organisms could grow in place and be preserved before physical reworking destroyed them. Tiny animals might still have existed nearby, but their traces would have competed with rapid mineral precipitation and microbial overgrowth, making preservation far less likely than for the robust microbial fabrics that dominate the rock record.
What this changes about the animal timeline
Removing the Tamengo burrows from the evidence base does not settle every question about when animals first disturbed sediment. A peer-reviewed synthesis by Mangano and Buatois, published in the journal Philosophical Transactions of the Royal Society B and available through PubMed Central, has laid out conservative benchmarks for what counts as unambiguous bilaterian activity: consistent directionality, specific branching patterns, cross-cutting relationships, and clear evidence of sediment displacement. Some Ediacaran trace fossils from other sites may still meet those criteria. But the Tamengo case removes one of the most frequently cited data points from the animal column, and it fits into a broader trend of re-evaluating supposed Ediacaran animal traces with stricter standards.
The age of the formation itself carries some uncertainty. The approximately 543-million-year constraint from Cloudina-focused work and the approximately 541-million-year estimate in the Gondwana Research paper overlap within error margins, but precise radiometric dating of carbonate-dominated sequences is inherently difficult. If the formation proves slightly younger than current estimates suggest, the loss of these supposed burrows becomes even more consequential, because it would further compress the gap between the first clear animal traces and the onset of widespread Cambrian bioturbation.
Independent replication would strengthen the case. Sulfur isotope measurements on the pyrite filaments themselves could provide a decisive test: if the isotopic signatures match those expected from microbial sulfate reduction, a bacterial origin would be strongly supported. If they diverge, the door might reopen to more complex biological contributors.
Why the Tamengo microbes still matter for Ediacaran science
The Tamengo Formation preserves a specific combination of carbonate sedimentation, microbial growth, and early diagenetic chemistry that may not have existed everywhere in the late Ediacaran ocean. It is possible that small animals burrowed into seafloors in other settings but left traces that were either destroyed or have not yet been correctly identified. The reinterpretation narrows the window for early animal sediment disturbance without closing it entirely.
What it does establish, with multiple independent lines of evidence, is that at least one prominent piece of the early burrowing record belongs to bacteria, not animals. The picture that emerges from the May 2026 study is of a late Ediacaran world where microbial communities still dominated the seafloor and where the earliest animals, even if present, had not yet begun the large-scale sediment mixing that would transform marine ecosystems during the Cambrian. The revolution was coming, but in the Tamengo Formation, the microbes were still in charge.
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*This article was researched with the help of AI, with human editors creating the final content.
