The Ediacaran era, around 571-541 million years ago, saw the appearance of the first animals in the fossil record, including Dickinsonia, whose prints, while beautifully preserved, have left paleontologists puzzled about how they fed. New research suggests this is partially because we have misunderstood the process by which Dickinsonia fossilized and that they may have had body parts we can no longer see.
Dickinsonia fossils look more like modern plants than animals, but last year Ilya Bobrovskiy, a PhD student at the Australian National University, found molecules in Dickinsonia fossils that settled their status as animals.
As the oldest definite animals, people are naturally fascinated whether these are our ancestors. As Bobrovskiy put it to IFLScience: “Are these our great, great grandparents, or our great, great uncles?” We currently have no way to answer that question, since we have no Dickinsonia genetic material. As long as it remains a live possibility that we are their direct descendents, however, there will be plenty of interest in learning more about them.
None of the Dickinsonia fossils we have found show signs of a mouth, gut, or anus, leading many paleontologists to conclude they lacked these organs. In Nature Ecology and Evolution, Bobrovskiy provides a new explanation for the fossilization process, revealing Dickinsonia could have been quite advanced, with organs that would not have been preserved.
Ediacaran fossils have three different preservation styles. One is the result of a layer of volcanic ash burying the organism, but the other two have proven more puzzling to paleontologists. Bobrovskiy and colleagues thought one style of fossil, known as “Flinders” after the place it was first found, might be the result of decaying tissue creating an open space.
They tested the concept using lumps of ice in the shape of the Death Star to represent the creature and placed it within sediments similar to those that surrounded the Ediacaran fossils all those years ago. The experiment was run at 1°C so the ice would melt slowly, mimicking the breakdown of a soft-bodied organism. The Death Star's surface features, such as the famous engineering bay, were not preserved, but the shape of an internal corrugated cardboard sheet was, indicating that the Dickinsonia patterns we are seeing are the shape of the tougher parts, rather than the softer outside.
"As the organisms decayed, softer sediment from below gradually flowed into the forming void, creating a cast," Bobrovskiy said in a statement. "Now we know that what we are looking at is an impression of a soft organic skeleton that may have been anywhere within Dickinsonia's body.”
The bits that were lost could include a digestive system and we would have no way of knowing. Previous ideas about Flinders fossilization explained it as the result of cementation from above, which would create a “death mask” that should have preserved the shape of organs like a mouth if they existed.
Bobrovskiy told IFLScience that it is still possible Dickinsonia were mouthless, sucking elements in through their membranes like some primitive animals today, but his work opens up the possibility of much greater complexity.