Earliest Animal Remains Found—But Scientists Question Discovery

Here's what you'll discover as you read this tale:

• Microscopic invertebrates known as meiofauna reside among sand particles, and it was previously believed that filaments found on a fossil dating back half a billion years represented the earliest signs of their presence.
• To determine if the filaments were indeed remnants created by meiofauna, a group of researchers examined them using cutting-edge technology.
• Morphological characteristics like cell walls and specific shapes indicated that these were indeed the body fossils of single-celled microorganisms.

Unusual types of organisms appeared during theEdiacaranperiod. Beings such as the flat, soft-bodiedDickinsonia and wormlike Spriggina(which moved across ancient ocean floors) were among the earliest creatures to emerge on Earth, and an unknown entity left behind strange threads that turned into fossilsrock.

For an extended period, these filamentous fossils werethought to befrom even older creatures. However, paleontologist Bruno Becker-Kerber was doubtful that the fossils were genuine. When they were discovered in the late Ediacaran Tamengo formation of the Corumbá Group in Brazil in 2017, scientists believed they had uncovered proof of the earliest meiofauna—a wide range ofinvertebrates(including nematodes and copepods) that reside between the particles of underwater sands and sediments—on Earth. The earliestmetazoanFossilized remains from the Ediacaran period are documented, but meiofauna of that age have never been discovered. Becker-Kerber and his team chose to re-examine the specimen due to some discrepancies regarding what the filaments should have looked like if they truly originated from meiofauna.

Fossils found in various locations show characteristics like retained cells, divisions of cell walls, and organic remnants atcell walls, common alignment, significant differences in diameter, and an absence of cross-cutting relationships," Becker-Kerber stated in a recent study published in the journalGondwana Research. These traits do not align with burrow-like fossil structures but are entirely in line with the understanding as remnants of pyritized thread-like organisms.

It's possible that metazoans from the Ediacaran era might have been preserved, yet none have been found, which is why the 2017 discovery was considered exceptional. Becker-Kerber and his team meticulously examined the fossils (ranging in size from just micrometers to several millimeters) using micro- and nanotomography at the MONGO beam line at theSirius CNPEM particle acceleratorin Campinas. Also referred to as zoom tomography, this technique allows for examining internal sections of a sample without causing any harm. The group that first uncovered these fossils did not have access to such sophisticatedtechnologyat the moment of the discovery.

Becker-Kerber's team further examined the fossils using Raman spectroscopy and optical microscopy, discovering that pyrite and other minerals had formed crystals within the filaments prior to being preserved in iron oxides. Some exhibited a straight shape while others were more winding, indicating the existence of multiple types of microorganisms. Based on the evidence of cell walls and organic remnants that could not be explained by anything else, the filament fossils were identified as microbial body fossils—imprints created by microbes whose soft bodies had since decomposed. They were also similar in size to remains of microbes such ascyanobacteriathan they were to signs or tunnels created in sediment by small bottom-dwelling organisms.

What the researchers did notthe presence in these rocks also suggested something beyond meiofauna. numerous species dig and line their tunnels with clay and sediment particles, which were absent in the filaments, making it improbable that thepyritecovering them had concealed the evidence. Instead, the scientists think the pyrite might have formed due to sulfate-reducing bacteria breaking down organic matter (pyritization is known to occur in this manner with current bacteria and algae).

Regrettably, there were additional aspects that the researchers could not decipher. "While the existing evidence suggests they are body fossils, the absence of detailed morphological features and their overall simplistic structure makes it difficult to definitively determine their biological relationships," they stated.said. "The dimensions and well-maintained"morphology suggest [multiple species].”

Preserved in time, these fossils may be the remnants of red or green organisms.algae, larger sulfur-oxidizing bacteria, and cyanobacteria. It remains unclear if any microscopic metazoans perished alongside them. Regardless, meiofauna had seemingly not yet entered the early environment.