The Ediacaran Hills That Hold the First Animals: South Australia's Flinders Ranges

A slab of sandstone in the Flinders Ranges preserves the outline of something that looks like a frond pressed in stone. It is not a plant. It is not an animal in any modern sense. It is Dickinsonia, a soft-bodied organism that lived 560 million years ago, and it is one of the oldest complex life forms ever found anywhere on Earth.

The Flinders Ranges of South Australia contain the richest and most complete record of the Ediacaran biota—the first large, multicellular organisms to appear in the fossil record. These hills, now dry and rust-coloured, were once the floor of a shallow, sediment-starved sea where microbial mats stabilised the seabed and strange, quilted creatures drifted or crawled.

A Seafloor Preserved in Quartz

The fossils occur in the Rawnsley Quartzite, a unit of the Ediacaran-age Wilpena Group, deposited between about 560 and 550 million years ago. At the time, the Flinders region lay on the passive margin of the supercontinent Rodinia, slowly subsiding under a quiet inland sea. Fine silt and sand rained down in thin layers, burying the soft-bodied organisms before scavengers or decay could erase them.

What makes the preservation exceptional is the microbial mats. These sticky films of cyanobacteria and other microbes coated the seafloor, binding sediment and creating a firm substrate. When a Dickinsonia or a Spriggina died and sank onto the mat, it was quickly covered by a pulse of sand. The sand moulded the underside of the organism in fine detail, and the mat itself prevented mixing. The result is a cast, preserved in quartz sandstone, that shows the precise shape and segmentation of creatures that had no bones, shells, or hard parts of any kind.

The Ediacaran fossils were so strange that when Reg Sprigg first described them in 1947, he called them "medusoid"—jellyfish-like—because he had no other vocabulary for what he was seeing.

More than 50 species have now been described from the Flinders Ranges, making it the type locality for the entire Ediacaran biota. The fossils are not all fronds. Some are discs, some are tubes, some are complex, branching networks. They represent a biological experiment that largely ended at the base of the Cambrian, when hard skeletons and active predation changed the rules of life.

The Ediacaran Period: A Window in Time

The Ediacaran Period (635–539 million years ago) is the only geological period defined by a fossil assemblage rather than a boundary in rock. The Flinders Ranges are central to this definition. The Global Stratotype Section and Point for the base of the Ediacaran is not here—it is in the Flinders Ranges, at the base of the Nuccaleena Formation, a cap carbonate that records the end of the Marinoan glaciation, often called Snowball Earth.

Above that cap carbonate lies the fossil-bearing sequence. The Ediacaran biota appears in three distinct assemblages in the Flinders Ranges: the Avalon-type (deep-water fronds), the White Sea-type (shallow-water discs and segmented forms), and the Nama-type (tubular and calcified organisms). The Flinders Ranges preserves mostly the White Sea assemblage, dated to around 555–545 million years ago, which is the most diverse and best understood.

What They Were, and What They Weren't

For decades, paleontologists debated whether Dickinsonia was a worm, a jellyfish, a lichen, or something unrelated to any living phylum. In 2018, a study of organic molecules preserved in a Dickinsonia fossil from the White Sea region of Russia found cholesterol-like lipids—evidence that it was an animal, not a fungus or protist. The Flinders Ranges specimens, though older, show the same morphology.

Yet the Ediacaran biota is not a simple precursor to modern animals. Many of its forms have no clear descendants. The frond-like Rangea and Charnia probably anchored to the seafloor and filtered particles from the water. The segmented Spriggina may have crawled across the mat. Tribrachidium had three-fold symmetry, a body plan almost unknown in later life. The Ediacaran was a time of experimentation, and most of those experiments ended.

The Landscape Today

The Flinders Ranges rise to about 1,100 metres above sea level, their ridges carved from folded Neoproterozoic and Cambrian sediments. The Brachina and Bunyeroo Gorges cut through the sequence, exposing the fossil beds in cross-section. Visitors can walk the same bedding planes where Dickinsonia rests in stone, the organism and the rock separated by half a billion years and nothing more than a millimetre of quartz cement.

The fossils are protected within the Nilpena Ediacara National Park, established in 2021. The park covers 60,000 hectares of the western Flinders Ranges, including the original Ediacara fossil fields discovered by Reg Sprigg in 1946. It is one of the few places on Earth where you can stand on a Precambrian seafloor and see the first stirrings of complex life, still in place, still preserved in the sand that buried them.

A Quiet Revolution

The Ediacaran biota changed how we think about the origin of animals. Before Sprigg's discovery, the fossil record of complex life began abruptly at the base of the Cambrian, around 539 million years ago. The Ediacaran fossils pushed that boundary back by nearly 30 million years and showed that the Cambrian explosion had a long, slow fuse.

The Flinders Ranges remain the best window into that fuse. The fossils are not spectacular in the way of dinosaur bones or ammonites. They are subtle—impressions in sandstone, often barely visible in raking light. But they record a world without predators, without burrowers, without hard shells. A world where life was flat, soft, and quiet. And then it changed.