Last updated February 14, 2018 at 11:02 am
New study using hydrocarbon biomarkers leads to partial breakthrough in identifying famous South Australian fossils.
Ever since they were first discovered in South Australia in 1942, the world’s oldest large fossils have presented palaeontologists with a mystery: what manner of creature made them?
Now, however, a joint Russian and Australian research project has finally solved part of the mystery.
The fossils, dubbed the Ediacara biota because the first examples were discovered in the Ediacara Hills region of South Australia, are up to two metres wide and date from the Cambrian age, around 570 million years ago.
Earliest evidence of explosion of life
The period saw a sudden dramatic increase in the amount and diversity of life on Earth, a phenomenon dubbed the Cambrian explosion.
Ediacara biota represent some of the earliest evidence of that explosion, and the first evidence of biological life above the minuscule. But what exactly formed the fossils has been the subject of robust debate.
In form, many examples so far found take the form of low circular mounds, often with concentric rings around the edges.
They have variously been interpreted as the remains of jellyfish-like creatures, or ancient corals, or sponges. Some researchers have even suggested that they are not of biological origin at all.
Now, however, a team led by Ilya Bobrovskiy of the Australian National University has discovered the origin of one major species, using a Russian example of an Ediacaran fossil known as Beltanelliformis.
Beltanelliformis was for years thought to be one of two Ediacaran species found at a site at Lyamtsa in the White Sea region of Russia.
It was believed to be the result of algal or bacterial activity. The second species, Nemiana, was thought to have an unknown animal microorganism origin.
In 2014, however, a Russian-Ukrainian team using a scanning electron microscope determined that the two fossils were in fact transitional examples of a single lifeform.
The scientists determined that the variants – now formally incorporated under the Beltanelliformis name – had lived in extremely shallow water, and were clearly microbial in origin. They did not definitely establish what type of microorganism, however.
That task has now been accomplished by Bobrovskiy and his colleagues. To achieve it, they also used fossilised Beltanelliformis from Lyamtsa, and ran a molecular comparison between it and a deposit of macro-algae found right next to it on the fossil bed.
The team extracted hydrocarbon biomarkers from both fossils and subjected them to range of tests using gas chromatography-mass spectrometry.
The analysis criteria were simple and elegant: because the Beltanelliformis and algal fossils had grown right next to each other at the same time, they must therefore have also experienced identical geophysical or bacterial degradation after death.
Any differences between the two, therefore, must represent the original organisms.
Looking at the results, those differences were stark. The algal deposit was found to comprise mostly the remnants eukaryotic cells – that is, cells containing a nucleus and mitochondria.
Beltanelliformis, on the other hand, yielded quite different findings.
The analysis showed that the components of the fossil were largely bacterial. The presence of compounds known as hopanoids narrowed the range of possibilities still further – leading Bobrovskiy and his team to conclude that the large and mysterious disc-like fossils were the remains of cyanobacteria.
The researchers say the use of biomarkers in making the identification is a significant achievement.
Other Ediacara biota fossils continue to baffle the experts, with interpretations ranging from animals to lichens. Bobrovskiy and colleagues expect their new approach to soon start providing answers.
The research is published in the journal Nature Ecology and Evolution.