Here’s what you’ll learn when you read this story:

• Meiofauna are microscopic invertebrates that live between grains of sand, and it was thought that filaments on a fossil half a billion years old was the earliest evidence of their existence.
• To see if the filaments really were traces left behind by meiofauna, a research team studied them with advanced tech.
• Morphological features such as cell walls and certain shapes showed that these were actually the body fossils of single-celled microbes.

Strange forms of life emerged during the Ediacaran period. Creatures such as the flat, soft-bodied Dickinsonia and wormlike Spriggina (which crept along ancient seafloors) became some of the first animals to appear on Earth, and something unknown left behind mysterious filaments that fossilized into rock.

For a long time, these filament fossils were thought to be from even older animals. But paleontologist Bruno Becker-Kerber was suspicious that the fossils were not what they seemed. When they surfaced in the late Ediacaran Tamengo formation of the Corumbá Group in Brazil back in 2017, researchers thought they had found evidence of the oldest meiofauna—a diverse group of invertebrates (including nematodes and copepods) that live between the grains of underwater sands and sediments—on Earth. The earliest metazoan body fossils on record date from the Ediacaran, but meiofauna that old have never been found. Becker-Kerber and his research team decided to revisit the specimen because of some inconsistencies with what the filaments should have been if they really did come from meiofauna.

“Fossils from multiple localities exhibit features such as preserved cells, cell wall divisions, organic remains at cell walls, shared alignment, extreme diameter variations, and a lack of cross-cutting relationships,” Becker-Kerber said in a study recently published in the journal Gondwana Research. “These characteristics are inconsistent with burrow-like trace fossils but are fully consistent with the interpretation as remains of pyritized filamentous organisms.”

It is possible that metazoans from the Ediacaran period could have been preserved, but none have been identified, which is why the 2017 discovery was a rare find. Becker-Kerber and his team closely observed the fossils (which ranged in size from mere micrometers to several millimeters) through micro-and nanotomography with the MONGO beam line at the Sirius CNPEM particle accelerator in Campinas. Also known as zoom tomography, this method makes it possible to focus on areas inside a sample without inflicting any damage. The team who originally dug up these fossils had not had access to such advanced technology at the time of the find.

Becker-Kerber’s team further analyzed the fossils with Raman spectroscopy and optical microscopy, and found that pyrite and other minerals had crystallized in the filaments before they were preserved in iron oxides. Some had a linear morphology while others were more sinuous, which suggested the presence of several species of microorganisms. From the evidence of cell walls and organic residue that nothing else could explain, the filament fossils were determined to instead be microbial body fossils—imprints left behind by microbes whose soft bodies had long since decomposed. They were also closer in size to remnants of microbes like cyanobacteria than they were to traces or burrows made in sediment by meiofauna.

What the researchers did not find in these rocks was also indicative of something other than meiofauna. Many species burrow and line their burrows with grains of clay and sediment particles, which were nowhere to be seen in the filaments, and it was unlikely that the pyrite covering them had filled in traces. Instead, the researchers believe that the pyrite may have come about as a result of sulfate-reducing bacteria processing organic material (pyritization has been known to happen this way with existing bacteria and algae).

Unfortunately, there were other details that the researchers were unable to make out. “Although the available evidence supports their interpretation as body fossils, the lack of preservation of fine morphological details and overall simple morphology prevent an unambiguous identification of their biological affinities,” they said. “The size and preserved morphology suggest [multiple species].”

Frozen in time, these fossils are probably the remains of red or green algae, larger sulfur-oxidizing bacteria, and cyanobacteria. Whether there any microscopic metazoans had died with them is unknown. Either way, meiofauna had apparently not entered the primordial chat just yet.