The Ash That Held the First Animals: South Australia's Ediacara Hills

In 1946, a geologist named Reg Sprigg was mapping the Ediacara Hills, a low ridge of rust-coloured quartzite about 600 kilometres north of Adelaide. He split open a slab of sandstone and found something that should not have been there: a frond-like impression, soft-bodied and delicate, preserved in rock that was supposed to be older than animal life. He was looking at a garden of the earliest complex organisms on Earth, and no one believed him.

The Quiet Sea That Never Dried

The Ediacara Hills today are dry, dusty, and unremarkable. Five hundred and sixty million years ago, they lay beneath a shallow sea on the margin of a continent that had not yet become Australia. The water was calm, the seafloor covered in microbial mats—thin, leathery layers of cyanobacteria and other microbes that stabilised the sediment like living felt.

There were no predators. Nothing burrowed. Nothing had shells. The seafloor was a soft carpet, and onto it, something new began to grow.

The Ediacaran biota were the first large, complex organisms on Earth. They were not plants, not animals, not fungi—though some may have been distant ancestors of modern animal groups. They lived as fronds, discs, and ribbons, attached to the mat or lying flat upon it. Some grew to a metre across. They fed by absorbing nutrients from the water or by grazing on the microbial film. They had no mouths, no guts, no limbs. They were experiments in being alive.

The Storm That Cast Them in Stone

The preservation of these organisms required a series of unlikely events. A storm would stir up fine sand and silt from deeper water, carrying it as a turbid cloud across the shelf. The sediment settled slowly, smothering the microbial mat and the organisms upon it in a layer of quartz grains.

The key was the mat itself. The microbial layer acted as a seal, preventing the sand from mixing with the underlying mud. When the organisms decayed, their bodies left a cavity in the sand—a natural mould. Later, coarser sediment filled the cavity from above, creating a cast on the underside of the overlying sandstone bed.

This is why the best Ediacaran fossils in the Flinders Ranges are found not on the surface of the rock but on the base. You have to turn the slab over to see them. The fossils are three-dimensional impressions, preserved in the sole of a storm bed, like footprints in a layer of ash.

"The most remarkable thing about the Ediacara fossils is not their age, but the fact that they exist at all."

A Mistaken Identity That Lasted Decades

Sprigg published his discovery in 1947. The scientific establishment dismissed it. The fossils were too old to be animals, the argument ran—the Precambrian was supposed to be barren. The impressions were probably inorganic, or the tracks of worms that had burrowed down from younger rocks above.

It took two decades for the consensus to shift. In the 1960s, a British palaeontologist named Martin Glaessner re-examined Sprigg's collection and recognised the forms as genuine fossils. He named the assemblage the Ediacara biota, after the hills where they were found.

The Ediacara Hills are now a UNESCO World Heritage site. The fossils have been found on every continent except Antarctica, but the Flinders Ranges remain the richest and most diverse assemblage known. More than thirty species have been described from the site, including Dickinsonia, a ribbed oval that grew up to a metre long, and Spriggina, a segmented organism that may have been an early ancestor of the arthropods.

What the Sandstone Still Holds

The Ediacaran organisms vanished from the fossil record about 538 million years ago, just before the Cambrian explosion. No one knows why. Perhaps they were outcompeted by the new, mobile, shell-bearing animals that appeared in the Cambrian. Perhaps a change in ocean chemistry or the disappearance of the microbial mats destroyed their habitat.

The rocks of the Ediacara Hills still hold answers. New specimens are found every year, often by amateur collectors walking the same ridges that Sprigg walked. The fossils are fragile—the sandstone weathers quickly in the dry air, and once exposed, the impressions begin to flake away. There is no time to be careless.

The Ediacara Hills are not dramatic. They do not rise sharply from the plain. They are a low, worn ridge, the colour of rust and dust. But split open any slab of the Rawnsley Quartzite, and you may find a frond that has waited half a billion years to be seen.