Dickinsonia: The Ediacaran Fossil Confirmed as an Early Animal

Dickinsonia is a flat, oval fossil organism from the Ediacaran Period, best known from rocks in South Australia and northern Russia that are about 560 to 550 million years old. For decades scientists argued over what it was, suggesting jellyfish, worms, lichens and even giant single-celled life. In 2018, traces of cholesterol preserved with fossils from the White Sea region showed that Dickinsonia was an animal, making it one of the oldest animals confirmed in the fossil record.

Background: life before the Cambrian

The Ediacaran Period ran from about 635 to 538.8 million years ago, just before the Cambrian Explosion. It was formally recognised by geologists in 2004, the first new geological period added to the timescale in more than a century, and its reference point lies in the Flinders Ranges of South Australia.

During this time, the first large, complex organisms appeared on the sea floor. Known collectively as the Ediacaran biota, they included fronds, discs, tubes and quilted forms with no obvious mouth, legs or shell. Many have no clear modern relatives, which is why classifying them has been so hard.

The first such fossil to be widely accepted as Precambrian was Charnia, found in 1956–57 at Charnwood Forest in England. Similar fossils had already been found a decade earlier in South Australia, where the Ediacara Hills gave the whole group its name.

Discovery and naming

In 1946 the geologist Reg Sprigg was surveying old mines in the Ediacara Hills, north of Adelaide, when he noticed impressions of soft-bodied organisms in the sandstone. At first many experts doubted that the rocks were older than the Cambrian, and Sprigg’s finds received little attention.

In 1947 Sprigg described several forms, including Dickinsonia, which he named after Ben Dickinson, the director of mines in South Australia. Later work, especially by Martin Glaessner from the 1950s onwards, established that the Ediacara fossils were indeed Precambrian.

Several species are now recognised, including Dickinsonia costata, D. tenuis, D. menneri, D. lissa and D. rex. They are known from South Australia and from the White Sea coast of Russia, with additional finds in the Ural Mountains and Ukraine. The Russian fossils near the White Sea are dated to roughly 558 million years ago.

What Dickinsonia looked like

A typical Dickinsonia fossil is an oval impression with a central groove running along its length and many narrow segments, or “isomers”, spreading out to each side. The segments on the left and right sides are slightly offset from one another rather than mirrored exactly, a feature that sets it apart from most later animals.

Sizes vary enormously. Small specimens are only a few millimetres long, while the largest, assigned to D. rex, are estimated to have reached about 1.4 metres. The body was very thin, probably only a few millimetres thick, and lacked any hard parts.

Studies of growth series, including a 2017 analysis by Renee Hoekzema and colleagues, suggest that Dickinsonia grew by adding new segments at one end of the body, while existing segments enlarged. That pattern of growth fits an animal better than a fungus or colony of cells.

How it lived

Dickinsonia appears to have lived on the shallow sea floor, which in the Ediacaran was often covered by thick microbial mats. Trace fossils found alongside it show chains of faint oval impressions that match the shape of the organism. They suggest that Dickinsonia could move short distances and fed by absorbing or digesting the mat beneath its body, rather than by eating with a mouth.

There is no evidence of a gut, eyes or limbs. Its way of life has often been compared to that of placozoans, very simple modern animals that glide over surfaces and digest food through their underside.

The 2018 cholesterol discovery

The key breakthrough came from chemistry rather than anatomy. Ilya Bobrovskiy, then a doctoral student at the Australian National University working with Jochen Brocks, collected Dickinsonia fossils from cliffs along the White Sea in the Arkhangelsk region of Russia. Unusually, these specimens still carried thin films of original organic matter.

The team analysed the steroid molecules in those films. Animals mainly produce cholesterol, while fungi, plants and many algae produce other kinds of sterols. The Dickinsonia films were overwhelmingly dominated by cholesterol-type steroids, in contrast to the surrounding sediment, which carried the signature of algae and bacteria.

The results were published in the journal Science in September 2018. They effectively ruled out the lichen and fungus interpretations and placed Dickinsonia among the animals. Researchers still debate exactly where it sits on the animal family tree, and whether it was a very early branch or related to simple groups such as placozoans.

Timeline

• c. 635 million years ago: The Ediacaran Period begins after the last great Cryogenian glaciation.
• c. 558 million years ago: Dickinsonia lives along the coast of what is now the White Sea region of Russia.
• c. 555–550 million years ago: Dickinsonia is common on sea floors preserved in the Flinders Ranges of South Australia.
• c. 538.8 million years ago: The Ediacaran ends and the Cambrian Period begins.
• 1946: Reg Sprigg discovers soft-bodied fossils in the Ediacara Hills.
• 1947: Sprigg names Dickinsonia after Ben Dickinson.
• 2004: The Ediacaran Period is formally added to the geological timescale.
• 2018, September: A study of preserved steroids identifies Dickinsonia as an animal.

Myths and facts

A common claim is that Dickinsonia was a type of jellyfish. Early researchers did suggest this, but its thin, segmented, mat-dwelling body and its cholesterol signature do not match a jellyfish.

In 2021 a Dickinsonia-like impression was reported from the Bhimbetka rock shelters in central India. Further study in 2023 showed it was the decayed remains of a beehive stuck to the rock, not a fossil, so India is not a confirmed Dickinsonia locality.

Why it matters

Dickinsonia shows that large animals lived on Earth before the Cambrian, roughly 20 million years before the main burst of diversity that followed. The 2018 study also showed how ancient molecules can settle questions that fossil shapes alone cannot, offering a new tool for exploring the earliest history of animal life.