Inside out? Ediacaran fossils might represent internal structures, researchers say
At 560 million-years-old, Ediacarans are truly ancient organisms, but do palaeontologists really know what they looked like? Elizabeth Finkel reports.
The Ediacarans are weird-looking things. Estimated to be over 560 million years old, they offer the best answer to Darwin’s dilemma: what preceded the Cambrian explosion which spawned the prototypes of most modern animals 541 million year ago?
But Ediacaran organisms, the ancient fossils of which have been found worldwide, throw up their own dilemmas. For one, what exactly were they? Some resemble plants but aren’t because they lived in seas too deep to access light; others have a trifold symmetry that doesn’t exist anymore. But some were bilaterally symmetrical and these are the best candidates for prototypes of today’s animals.
But beyond Darwin’s dilemma lies another one – more mundane, but no less pressing to scientists.
Ediciacarans were soft bodied, evidenced by the fact that fossils depict them folded over or with bits torn off. How then did they fossilise? They aren’t like trilobites or dinosaur bones. Rather than petrified 3D replicas of the original, they are impressions stamped into sandstone.
Ediacaran-style fossils are almost never seen again in the later record. Probably because the sediments became infested with burrowing creatures that churned the sand and destroyed any traces.
Still, even without burrowers, researchers have been hard-pressed to explain why soft things persisted long enough to make impressions before being crushed.
The prevailing theory is that storm events deposited a light rain of sediment on the medium-shallow depth sea floors where the Ediacarans grazed on lawns made of microbes. Once buried, the microbial goo mixed with minerals, such as pyrite, in the sands to cement a death mask of the creatures.
Now palaeontologist Ilya Bobrovskiy from the Australian National University (ANU) and colleagues have come up with a new explanation. No death masks are required, they say. Rather, they propose in an article in the journal Nature Ecology and Evolution, the flow properties, or rheology, of different sediments could explain the imprints.
“It’s a welcome addition,” says Guy Narbonne, a palaeontologist from Queens University in Canada, who works on Ediacaran sites in Newfoundland, “but it does not necessarily do away with other models.”
The death mask theory to explain Ediacaran impression fossils holds up well for places such as Australia’s Flinders Ranges – where the fossils were first discovered in 1947 at a site called Ediacara Hills – and Mistaken Point in Newfoundland.
The Australian fossils are rich in the types of cementing minerals required, while in Newfoundland the death masks came from volcanic ash which formed “a quick concrete” on the creatures below, explains Narbonne.
But while that’s a good explanation for these places, it doesn’t hold up well for fossils found at another prolific Ediacaran site, Russia’s White Sea. There, Bobrovskiy found little evidence that cementing minerals were needed.
The Russian fossils haven’t been exposed to huge temperature and pressure changes like the ones in the Flinders Ranges. Extremes like that alter the rocks and complicate the ability to analyse how fossils first formed, says Bobrovskiy.
Nevertheless, in the White Sea, Ediacarans still resemble those in the Flinders Ranges, in that they appear to be stamped on the underside of sediments.
That got Bobrovskiy wondering how rocks with such radically different histories nevertheless produced similar fossils imprints. He resurrected an old theory, first proposed for the Flinders Ranges by palaeontologist Mary Wade in 1968.
Rather than a cementing death mask over the top, the idea is that the preserving agent was sand from below, oozing into the space vacated as the creature began to decompose.
Eventually the sand filled the entire space, preventing overlying material from collapsing, and thus preserving the imprint. The White Sea fossils indeed lie on top of clay, which is more fluid that the overlying sandstone, so that fit with the theory.
In the Flinders Ranges, Bobrovskiy says, the underlying sands tend to be smaller grained and more rounded than the overlying ones – all things that would make the lower material better at flowing.
Bobrovskiy decided to test his theory in the lab. To simulate the disintegrating fossil, he used a death star: a dome- shaped ice block with a brick-like pattern on its surface, and a concertina strip of cardboard embedded inside.
In a White Sea simulation, he sat the Death Star on a clay base and buried it with fine sand under pressure equivalent to a 40-centimetre-thick sand layer.
Then in a Finders Ranges simulation, he sat it on a base of rounded sand, overlain by angular sands and applied pressure equal to 90 centimetres of overlying material.
In both cases, as the ice melted, the lower sediments flowed directly above into the space vacated by the melting death star and prevented the sands above from collapsing. The final imprint registered on the surface. However, it was not that of the external brick pattern but the internal cardboard corrugation.
It appeared to have created an overprint, says Bobrovskiy. The upshot, he says, is that if this method of preservation is indeed at work, then the fossil imprints we see may not represent the true surface of the organism, but an internal, more resistant structure.
Bobrovskiy’s theory is seen as a welcome addition, but with caveats.
When it comes to the mechanism of preservation at the White Sea, “it’s been a blank”, says Narbonne, “I think this paper is important, arguing that the prevailing preservation mechanisms are not part of the equation.”
He adds, though: “It doesn’t eliminate other possibilities. Like all good science, it raises nearly as many questions as it answers.”
For Jim Gehling, a palaeontologist at the South Australian Museum and expert on the Flinders Ranges Ediacarans, the theory “fails to explain the vast array of photographed and cast specimens of Dickinsonia fossils that include evidence of torn specimens, folded over specimens and serial ‘resting traces’ — regarded as evidence of staged movement.”
Palaeontologist Alex Liu from Cambridge University, UK, sees a middle path. “I think that the most likely scenario is that both early cementation and the effects of rheology acted in combination to preserve Ediacaran fossils in the Flinders and elsewhere,” he says.