How do you become a fossil if you are an animal that lived 560 million years ago?
The organisms of the Ediacara biota that lived on the ocean floor 571-539 million years ago are the oldest large complex organisms we know. These creatures had no shells or skeletons, so they really should not be preserved as fossils – and yet they are found in sedimentary rocks around the world. Mixing palaeontology, sedimentology, a bit of physics, and a Death Star ice cast, we show how they managed to do that.
When you walk along the beach, you will leave a track of footprints behind. Yet, these will be gone within hours when the tides emerge. Members of the Ediacara biota left prints on sand… but they managed to do so from below, forming concave impressions at the bottom of sandstone layers, and these imprints remained for hundreds of million years. How did they do it?
That was the question I did not ask when I started working on Ediacaran fossils some 5 years ago. An elegant answer already existed. The idea behind it was that organisms living on the seafloor were covered by sand alive, and that the sand was instantly cemented by carbonates, silica or a pyrite crust, creating a momentary detailed imprint of how the organisms looked like - a ‘death mask’.
When I started my work, I was looking at sedimentology of Ediacaran deposits and searching for organic molecules in Ediacaran fossils in the White Sea area in Russia. The sediments there are extremely well preserved – the clay exposed at the cliffs of the White Sea is so fresh, it would make a good material for pottery. Looking at these rocks, I could not help noticing that minerals, which could have created the ‘death mask’, appear to be absent there. Together with Anna Krasnova, who was a Masters student at the Moscow State University, we decided to search systematically for potential ‘death masks’. And indeed, found that they did not exist in the White Sea area.
But how then did the soft Ediacara biota get preserved? Several stupid ideas later (so stupid that my PhD supervisor Dr. Jochen Brocks still loves bringing them up) I realised that their preservation follows simple physics. Because of a contrast in mechanical properties, the sand overlying the organisms is always more firm than the underlying sediment, even if it is not cemented. Under pressure, the overlying sand will not move as the organisms slowly decay, and the softer sediment beneath the carcass will flow upwards into the newly formed space, as it would flow into a tread pattern on shoes when you step on mud.
With this in mind, we took a fresh look at fossils that have imperfections: distorted, contracted and half-hidden in sand. In these imperfections we could now see how the sediments flowed. We only needed to make other people see the same – to simulate the preservation of Ediacaran organisms in the laboratory.
This took a team effort. We found a Death Star mould for ice – the melting ice represented a rotting organism, which we put between two types of sediment in a container. To make sure that the ice does not melt too early, our experiments were done in a beer fridge – they have an excellent temperature control. And eventually, we recreated Ediacaran-like fossils simply by applying pressure on the sediment. The new model implies that we may be oblivious of the external shape of Ediacaran organisms: for many fossils, the impressions could be left by most resistant tissues of the creatures, and not their whole bodies. On the bright side, looking at how sediments flowed, we can actually detect different types of tissues in some Ediacaran organisms, which brings us closer to understanding what these mysterious creatures were.