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More than met the eye

Fossilized burrows suggest complex organisms arose before the Cambrian explosion of diversity

LÉO RAMOS CHAVES A trace fossil of the genus Multina minima reveals burrowing by worms 542 million years agoLÉO RAMOS CHAVES

Very discrete, string-like fossils embedded in rock from quarries in Corumbá, Mato Grosso do Sul, Brazil, escaped researchers’ notice for years. “We were oblivious to what they were,” recalls paleontologist Juliana Leme of the Institute of Geosciences at the University of São Paulo (IGC-USP). She began investigating these materials in 2010 when studying fossils of Earth’s earliest organisms with skeletons (see Pesquisa FAPESP, issue No. 199). However, it has since become clear that in these rocks lies an even greater treasure to be explored: the string-like structures have been found to be traces left by organisms more complex than would be expected to exist between 541 million and 555 million years ago, when extant organisms were thought to be much simpler.

The fossils are—according to a paper published in September in Nature Ecology & Evolution—burrows left by tiny worms known as nematodes. At this stage of the evolution of life, when soft-bodied organisms inhabited the surface, it was surprising to find traces of worms that could burrow into the sediment through muscular contraction, even if reaching only a few millimeters (mm) in depth. This is the oldest fossil record of animals known as meiofauna—organisms no larger than 1 mm in size that reside in marine and freshwater bottom sediments.

The Corumbá region, now part of the Pantanal wetlands, was during that period a sea. The rocks from the region have long attracted interest from paleontologists for their traces of life forms known as Ediacaran biota, such as the genera Corumbella and Cloudina, which first appeared between 580 million and 560 million years ago. So when British paleobiologist Martin Brasier of Oxford University came to USP as a visiting professor in 2012, he went on a field trip to the area with his then master’s student Luke Parry. Just as other researchers before them, they too could not discern the subtle furrows in the rock samples they collected for further investigation.

It wasn’t until Parry examined the samples using 3D X-ray CT scanning back in the UK that he was able to see the burrows, some crossing up to 7 mm of stratigraphic thickness. The traces left by the former occupants of the burrows—punctuated by slightly wider, pouch-like spaces—also reveal the type of movement they made. According to the researchers, these are the marks of the muscular contractions of organisms classified as bilaterians, which already possessed a certain degree of morphological complexity. The burrows are filled with pyrite, a material dissimilar from the outer layer of sediment, denoting the presence of organic mucus.

This was surprising to find in rocks from the Guaicurus Formation, of early Cambrian age. That period, 541 million years ago, witnessed the great diversification of life forms known as the “Cambrian Explosion,” when many of the phyla that gave rise to our current biodiversity emerged. Parry lost his mentor, Brasier, to a road accident in 2014, but continued his research and this year completed his PhD. He is the lead author of the newly published paper.

In 2016, working alongside paleobiologist Alex Liu of the University of Cambridge, Parry and the Brazilian researchers found traces of the same organisms in samples taken from another, yet older layer in the Tamengo Formation. The find was all the more exciting as the fossils found there not only were older but also could be reliably dated thanks to the presence of volcanic ash collected by geologist Paulo Boggiani of IGC-USP. Repeated ash-bed dating in the UK confirmed that these organisms predated the Cambrian. Volcanic ash beds have not yet been found in the Guaicurus Formation, making the fossils it contains more difficult to date.

Back to the past
Nothing can ever be said to be proven in science, especially when it comes to events so far in the past, so the researchers are cautious about their findings. “If our interpretation is correct, there were already complex organisms prior to the Cambrian Explosion,” says Leme of IGC-USP. These animals would already have been modifying their environment by excavating into the sediment and bringing oxygen into its inner layers, possibly making the environment more hospitable to colonization by other life forms. That is what biologist Cleber Diniz intends to investigate in greater depth in his doctoral research under Leme. “I was initially studying Corumbella, but discovered there was something far more unknown to explore,” he says. Diniz has already thoroughly sampled the region’s quarries, layer by layer, and knows where the nematode trace fossils are to be found.

In the coming years, a group of around 15 Brazilian and foreign professors and their students plans to study these findings as part of a research project led by geologist Ricardo Trindade of the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP). Meanwhile, Leme can celebrate Brazil’s new-found place on the global Precambrian research scene—when the USP researchers and their British colleagues first initiated their collaboration, the state of Mato Grosso do Sul was not considered a significant region for research on this geological period.

The Neoproterozoic Earth System and the rise of biological complexity (No. 16/06114-6); Grant Mechanism Thematic Project; Principal Investigator Ricardo Ivan Ferreira da Trindade (USP); Investment R$4,305,689.93.

Scientific article
PARRY, L. A. et al. Ichnological evidence for meiofaunal bilaterians from the terminal Ediacaran and earliest Cambrian of Brazil. Nature Ecology & Evolution. No. 1, p. 1455–64. Sept. 11, 2017.