Fossil Dickinsonia

Looking like not much more than a pancake, this is one of the first animals to have evolved © The Australian National University

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Fat molecules preserved in a 558-million-year-old fossil help settle decades-old debate

For decades researchers have been debating where the early organism Dickinsonia fits in the evolutionary tree. Now the discovery of cholesterol in this Ediacaran fossil helps argue that it was one of the first animals to have evolved.

Looking like little more than quilted pancakes, the fossils of Dickinsonia have sparked numerous debates in the palaeontological world. Dating to around 558 million years and growing up to 1.4 metres in length, they represent some of the most significant organisms from this early period.

But the discovery of cholesterol preserved in Dickinsonia fossils from Russia provides new evidence that they are indeed animals, not to mention some of the earliest identified animals to have evolved.  The work has been published this week in the journal Science

Associate Prof Jochen Brocks, lead senior researcher of the paper, says, 'Scientists have been fighting for more than 75 years over what Dickinsonia and other bizarre fossils of the Ediacaran biota were: giant single-celled amoeba, lichen, failed experiments of evolution or the earliest animals on Earth.'

While the discovery of cholesterol suggests that these creatures were indeed animals, it is crucial to note that this does not confirm the theory.

Dr Greg Edgecombe, a Merit Researcher at the Museum who investigates the evolution of early animals, explains, 'The cholesterols they find associated with this organic film on Dickinsonia are not necessarily diagnostic of animals, because they are also shared with two of animals' closest relatives.

'There are two other linages of unicellular organisms that are also contained within the holozoan group with animals, and these microscopic organisms also produce cholesterol.'

A piece of fossilised biofilm

A piece of the 558-million-year-old biofilm preserved on the Dickinsonia fossil © The Australian National University

The evidence of large amounts of fat found in the biofilm that covers the Dickinsonia fossils is therefore not proof within itself that these creatures were animals - rather this feeds into further evidence that has been mounting on where they fit on the evolutionary tree.

Previous work published earlier this year looking into how the organisms grow indicates that the bands of tissue in the fossils seem to form in a subterminal position, therefore indicative of Dickinsonia being an animal. This latest work in detecting the cholesterol helps to corroborate this notion.

Finding the right fossils

One of the most significant aspects of this study is simply the fact that they have managed to find a site with organic material preserved in 558-million-year-old fossils.

Ilya Bobrovskiy, the PhD researcher who discovered the fossils, says, 'The problem that we had to overcome was finding Dickinsonia fossils that retained some organic matter.

'Most rocks containing these fossils have endured a lot of heat, a lot of pressure, and then they were weathered after that - these are the rocks that palaeontologists studied for many decades.'

The new location, near the White Sea in northwest of Russia, seems to have the perfect conditions. This is likely due to how unexposed the fossil site has been.

While similarly old Ediacaran fossils have been found in Australia, those sites have been more exposed and therefore have endured far more weathering than the site in Russia, which has been far better protected. This has enabled the organic molecules to survive for over half a billion years.

If Dickinsonia truly nests inside the animal tree or branched off early from the lineage that gave rise to animals, as the researchers suggest, then it may have some further implications.  

'A lot of other Ediacaran biota that lived alongside Dickinsonia will also be transferred to that group,' says Greg.

'There are other quilted organisms that have these offset units and develop in the same way that will follow it into the animal group, and some of them are even older still.'