The Ediacaran creatures

The story

A geologist and an abandoned mine

In 1946 a young Australian geologist, Reg Sprigg, was working in the Flinders Ranges in South Australia. He had to check some old abandoned mines, to see whether it was worth reopening them. On a hill called Ediacara, walking among the rocks, he noticed some strange marks on slabs of sandstone: shapes like discs, leaves and jellyfish. He realised they were fossils. But they were older than any that were known at the time.

In those days it was thought that before the Cambrian, that is, before 539 million years ago, there was no visible animal life. Sprigg thought he had found Cambrian jellyfish. Later, studying the rocks more closely, geologists understood that they were even older. That hill was proof that complex life had begun earlier than people had believed. Today, in memory of the place, all this life is called the Ediacaran fauna (or biota), and the last period before the Cambrian is called the Ediacaran.

A strange world

The Ediacaran fauna looks like nothing we know. The creatures that lived in those seas, roughly between 575 and 539 million years ago, were soft: no shells, no bones, no legs. Some were disc-shaped, like Tribrachidium, with three spiralling arms. Others looked like fern fronds, like Charnia, which stood fixed to the seabed. Dickinsonia, one of the most famous, was a flat, segmented oval that could grow to more than a metre long, like a rippled doormat.

For a long time scientists wondered what they were. Some considered them early animals, others lichens, fungi or even a type of life in its own right, with no descendants. Today many scholars think they are a mixture of different groups. In 2018 a team of researchers analysed the fat molecules preserved in Dickinsonia fossils and found a type of cholesterol, which is characteristic of animals. It is a strong clue that Dickinsonia was an animal, but the debate is not closed.

Another example is Kimberella, an animal like a slug, which had a kind of hood and left parallel scratch marks on the seabed, as if it were scraping algae and microbes off the bottom. These are the first traces of animals that feed as they move.

The first animal with symmetry

In 2020 a discovery came that concerns the date of this very stage. Scholars at the University of California, Riverside described, in rocks at Nilpena, near Ediacara, a small worm called Ikaria wariootia, about 555 million years old. It was only a few millimetres long, the size of a grain of rice, and had an elongated body with a front and a back end, and a digestive tube running through the body. According to the researchers it is the oldest known animal with bilateral symmetry, that is, with a right side and a left side that match, like ours. Animals with this shape, which include insects, fish, reptiles and us, are among its distant descendants.

Where did they go?

At the end of the Ediacaran, about 539 million years ago, most of these forms disappeared. We do not know for certain why. Perhaps the amount of oxygen in the water changed. Perhaps the first more active animals arrived, which ate the mats of microbes on which many Ediacaran creatures lived, and which disturbed them by digging in the bottom. Perhaps all of these things together. In any case, this story is told with doubts: the fossils are few and fragile, and researchers go on arguing.

A curious fact is how they were preserved. Because they were soft, these creatures became fossils only in special conditions: they were buried quickly by sand, and a crust of minerals formed on the sand, like a mask, preserving the imprint. This is why the Ediacaran fossils look like casts, and not like real remains.