Ediacaran Biota: Earth's First Large Complex Organisms

The Ediacaran biota is the name given to the earliest known communities of large, complex organisms, which lived in the world’s oceans from about 575 to 539 million years ago. Named after the Ediacara Hills of South Australia, where they were first recognised as Precambrian fossils, these soft-bodied creatures include fronds, discs, quilted mats and the first undoubted animals. They fill the gap between a world of microbes and the Cambrian Explosion, and scientists still debate what many of them were.

Background: life before the Ediacaran

For more than three billion years, life on Earth was overwhelmingly microscopic. Bacteria built mats and mound-like stromatolites in shallow seas, and single-celled eukaryotes, the group that includes the ancestors of plants and animals, appeared well over a billion years ago.

Between about 717 and 635 million years ago, the planet passed through the extreme ice ages known as Snowball Earth. The Ediacaran Period begins at the end of the last of these, about 635 million years ago, and runs to the start of the Cambrian, now dated to about 539 million years ago.

Oxygen levels in the oceans seem to have risen in steps during this period, although the details are debated. Large Ediacaran fossils appear only in its later part, from roughly 575 million years ago.

Discovery in Australia

The first Ediacaran fossils to be recognised as such were found in 1946 by Reg Sprigg, a young geologist working for the South Australian government. While inspecting old mine workings in the Ediacara Hills, he noticed jellyfish-like impressions on the undersides of sandstone slabs.

Sprigg believed the fossils were very ancient, but experts were sceptical. He thought they were early Cambrian, and his reports attracted limited attention. In the 1950s and 1960s the Austrian-born palaeontologist Martin Glaessner, based at the University of Adelaide, studied the growing collections and argued convincingly that the fossils were older than the Cambrian.

In fact, similar fossils had already been found elsewhere. The disc-shaped Aspidella was described from Newfoundland in 1872, and fossils from Namibia were described in the 1930s. Because the rocks were not then known to be Precambrian, their significance was missed. The discovery of the frond Charnia in England in 1957, in rocks that were clearly Precambrian, helped to settle the question.

The main types of organisms

Ediacaran organisms are strange to modern eyes. Most had no shells, bones or obvious mouths, and many look unlike any living group. Scientists usually group them by body plan rather than by firm family relationships.

Fronds and rangeomorphs

Rangeomorphs were frond-shaped organisms built from repeated, branching units, a pattern often compared to a fractal. Examples include Charnia, Fractofusus and Bradgatia. Many lived in deep water far below the reach of sunlight, so they could not have photosynthesised; they probably absorbed nutrients directly from seawater.

Dickinsonia and other quilted forms

Dickinsonia was a flat, oval organism divided into segment-like bands, ranging from a few millimetres to more than a metre long. In 2018 researchers found traces of cholesterol-type molecules preserved with specimens from Russia, strong evidence that it was an animal.

Possible early bilaterians

Kimberella, best known from the White Sea coast of Russia, is often interpreted as an early animal related to molluscs. It is found with scratch marks that look like feeding traces. Spriggina, named after Reg Sprigg, has a head-like region and a segmented body, but its relationships remain debated.

Early skeletons

Near the end of the Ediacaran, small tube-shaped organisms such as Cloudina began to build mineralised skeletons. Some Cloudina tubes show bore holes, which some researchers interpret as early evidence of predation.

Three assemblages

Palaeontologists often divide Ediacaran fossils into three broad assemblages, which differ in age, environment and typical organisms. The boundaries between them are approximate.

The deep-water Avalon assemblage is beautifully exposed at Mistaken Point in Newfoundland, where whole communities were buried by volcanic ash. The White Sea assemblage shows the greatest variety, including the first clear signs of animals moving across the seafloor.

How the fossils were preserved

Soft-bodied organisms rarely fossilise. Most Ediacaran fossils are impressions on the bases of sandstone beds, left after storms or underwater flows buried communities in sand. Microbial mats that covered much of the seafloor seem to have been crucial.

The Australian palaeontologist James Gehling proposed that as buried bodies decayed, minerals formed in the overlying mat and created a “death mask” that recorded their shape. Where volcanic ash fell, as in Newfoundland and England, entire living surfaces were captured almost like snapshots.

What were they?

The biggest question is how Ediacaran organisms fit into the tree of life. Glaessner saw many of them as early members of living animal groups, such as jellyfish, sea pens and worms.

In the 1980s and 1990s the German palaeontologist Adolf Seilacher challenged this. He argued that many shared a quilted, air-mattress-like construction and represented a separate, failed experiment in multicellular life, which he called the Vendobionta. The American geologist Gregory Retallack has gone further, suggesting some were lichens living on land, a view most specialists reject.

Today the picture is mixed. Dickinsonia and Kimberella are widely accepted as animals, and trace fossils show that some organisms moved and burrowed. Other forms, especially the rangeomorphs, may belong to extinct branches near the base of the animal tree or to groups with no living descendants.

Disappearance

Most Ediacaran forms disappear from the record around the boundary with the Cambrian, about 539 million years ago, although a few may have lingered. The cause is debated.

One idea is that new animals that burrowed into the seabed and grazed on microbial mats destroyed the habitat the Ediacaran organisms depended on. Another emphasises the rise of predators. Some researchers point to changes in ocean chemistry, while others argue that part of the apparent extinction reflects the loss of the special conditions that preserved soft bodies. The American palaeontologist Mark McMenamin popularised the image of a peaceful “Garden of Ediacara” before predators appeared, but this remains an interpretation rather than an established fact.

Myths and facts

A common misconception is that the Ediacaran organisms were simply jellyfish. Sprigg and Glaessner did interpret many of the round impressions as jellyfish, but most of these discs are now thought to be holdfasts, the anchoring bases of frond-like organisms, or colonies of microbes.

Another is that animals appeared suddenly in the Cambrian with no forerunners. Ediacaran fossils, together with trace fossils of crawling and burrowing, show that animal life had a longer prehistory. Molecular studies, which estimate when lineages split by comparing genes, suggest that the first animals may be older still, though early forms were probably tiny and seldom fossilised.

It is also sometimes said that the Ediacaran biota lived in a world without movement. In fact, trackways and burrows become more common through the period, and organisms such as Dickinsonia appear to have shifted position on the microbial mats where they fed.

Legacy and research today

The Ediacaran biota changed how scientists view the origin of animals. Before its recognition, complex life seemed to appear abruptly in the Cambrian, a puzzle Charles Darwin had noted. Ediacaran fossils show that large, multicellular organisms existed tens of millions of years earlier.

In 2004 the International Union of Geological Sciences ratified the Ediacaran Period, named after these fossils, as the first new geological period in more than a century. Its starting point is defined in the Flinders Ranges. In 2024 the Flinders Ranges were inscribed on the UNESCO World Heritage List, in part for their Ediacaran fossils, while Mistaken Point in Newfoundland had been listed in 2016.

New sites and techniques, from chemical biomarkers to digital models of fossil surfaces, continue to change interpretations. Excavations at Nilpena, where whole fossil beds have been uncovered and reassembled, have revealed how communities were arranged on the seafloor.

Timeline

• c. 635 million years ago: The Marinoan glaciation ends and the Ediacaran Period begins.
• c. 575 million years ago: Large Ediacaran organisms appear in the Avalon assemblage.
• c. 560-550 million years ago: The White Sea assemblage flourishes, with early mobile animals such as Kimberella.
• c. 539 million years ago: Most Ediacaran forms disappear near the start of the Cambrian Period.
• 1872: Elkanah Billings describes the disc fossil Aspidella from Newfoundland.
• 1946: Reg Sprigg finds fossil impressions in the Ediacara Hills, South Australia.
• 1957: Schoolboy Roger Mason reports the frond Charnia in Charnwood Forest, England.
• 2004: The Ediacaran Period is formally ratified, with its base defined in the Flinders Ranges.
• 2018: Biomarker evidence confirms Dickinsonia as an animal.

Why it matters

The Ediacaran biota is the oldest window onto large, complex life. It shows that animals and other multicellular organisms were already experimenting with new body plans before the Cambrian Explosion, and it helps scientists understand how the living world moved from microbes to the diversity of animals seen today.