Fossils from the Ediacaran Period are discovered in Argentina

Brazilian and Argentinian researchers describe a disc-shaped marine organism from 565 million years ago.

By Peter Moon  |  Agência FAPESP – The oldest animal fossils found in South America date from 565 million years ago (mya), in the period known as the upper Ediacaran. They are tens of thousands of small circular impressions belonging to a marine organism, Aspidella. Discovered in sandstone quarries in Argentina, these fossils are approximately 15 million years older than those of the most ancient South American animals previously discovered.

The Aspidella fossils are described by four Brazilian and three Argentinian researchers in an article published in Scientific Reports, the open-access version of the journal Nature. The second author is geologist Lucas Warren from São Paulo State University (UNESP) in Rio Claro (São Paulo State, Brazil). He was supported by FAPESP in this research.

The Ediacaran (635-541 mya) was the last stage of the Precambrian Era, the period preceding the explosion of multicellular life that occurred in the Cambrian (541-485 mya), when all the original biological lineages from which all animals, extinct and living, descend emerged in the oceans.

The Ediacaran fauna is divided into three groups or assemblages. The oldest is the Avalon assemblage (575-560 mya), exposed in Canada and China. Next comes the White Sea assemblage (560-550 mya) in Australia and Russia. The most recent is the Nama assemblage (550-541 mya), initially described in Namibia.

Ediacaran fossils are evidently very rare due to their antiquity and are not found in large numbers. The first report of vestiges of Ediacaran organisms in South America was in the 1980s in Corumbá, Mato Grosso do Sul, Brazil. Fresh evidence was identified in Argentina in the same decade, followed by findings in Paraguay and Minas Gerais State, Brazil, since 2010. “All South American forms found to date belong to the Nama assemblage, with the sole exception of the Aspidella fossils from 565 mya described in this study,” Warren said. “They’re dated to the Ediacaran and are similar to Australian fauna.”

Because Ediacaran fossils are so rare and often very poorly preserved, it is a challenge for paleontologists to prove without a shadow of a doubt that these strangely shaped, faint rock impressions are of biological origin. “Not so in the case of the Aspidella fossils found in Argentina,” Warren said. “There are tens of thousands of well-preserved individuals.”

They were found in 2015 in piles of mining waste from two limestone quarries in Olavarría, approximately 300 km southwest of Buenos Aires. The sandstone occurring on top of the carbonates in these quarries consists of very fine reddish grains deposited more than half a billion years ago in ancient tidal mudflats. 

“We went to investigate the quarries because we knew there were ichnofossils there,” Warren said. Unlike body fossils, the remains of organisms in which minerals have replaced organic matter, ichnofossils or trace fossils preserve impressions such as tracks, trails, burrows, borings and other evidence of activity. “If there were ichnofossils, we’d probably have a chance of finding body fossils. We decided to look in the carbonate for Cloudina, considered an Ediacaran guide fossil. It’s found in Brazil and ten other locations around the world. However, we found none. Instead, we found Aspidella.” (For more on this topic, see “Life protected by armor” and “The last Minas coastline” published in Pesquisa FAPESP magazine).

The Aspidella impressions found in Olavarría are small disc-shaped trace fossils with diameters of 6-140 mm, although most are 10-26 mm. “They’re like tiny biscuits trapped in the rock,” Warren said. Specimens found in other parts of the world show that petal- or frond-like structures emerged from the center of the disc and probably swayed to and fro with the tide or current. 

“Very few petals were found in Olavarría. Most of the Aspidella fossils were discoid. The best hypothesis to explain the absence of petals is that they were torn off by waves and current during storms. Alternatively, fossilization conditions in Olavarría may have prevented the preservation of external parts such as fronds, so only the discoid part that anchored the organism to the substrate has remained,” Warren said.

Having published the first description of Aspidella from Olavarría, the researchers are now engaged in other studies using 3D microtomography and scanning electron microscopy to explore the impressions in more detail. They are also performing taphonomic studies to identify the exact conditions of deposition, fossilization and conservation for such a long period.

Aspidella was the first Ediacaran animal identified by scientists, back in 1872 in Newfoundland, Canada. However, for 140 years, scientists did not realize how old these animals really are, believing that they were Cambrian. Until the 1940s, paleontologists thought bacteria were the only living organisms in the Precambrian. That changed in 1946, when rock impressions of the first Precambrian animals were found at a place called Ediacara in South Australia.

Ediacaran sites have since been found on all continents. Although many more kinds of Ediacaran organisms are known to science, so far all those discovered seem to have been evolutionary experiments that failed to prosper. Aspidella is a case in point. It is not clear whether this organism belonged to a lineage that appeared and that became extinct in the Ediacaran or whether this lineage survived into the Cambrian. In other words, paleontologists are still unable to find Ediacaran fossils evidencing lineages that flourished in the Cambrian, although they must exist. They remain to be identified.

The article “Ediacaran discs from South America: probable soft-bodied macrofossils unlock the paleogeography of the Clymene Ocean” by María Julia Arrouy, Lucas V. Warren, Fernanda Quaglio, Daniel G. Poiré, Marcello Guimarães Simões, Milena Boselli Rosa & Lucía E. Gómez Peral, published in Scientific Reports (doi:10.1038/srep30590), can be read at nature.com/articles/srep30590.