The Seafloor That Gave Birth to Animals: South Australia's Ediacaran Trace Fossils

In 1929, a South Australian geologist named Reg Sprigg found something odd in the Flinders Ranges: what looked like worm burrows in 560-million-year-old rock. He sent photographs to a prominent British palaeontologist, who dismissed them as nothing more than sedimentary cracks. Sprigg put the specimens aside and nearly threw them out. Seventeen years later, he returned to those same hills and found something far stranger — the impressions of soft-bodied organisms no one had ever seen before. He had discovered the Ediacaran biota. But those earlier trace fossils, the ones dismissed as cracks, told an equally profound story: the story of animals learning to move.

The Tracks Before the Bodies

The trace fossils of the Flinders Ranges are older than the famous body fossils by several million years. They appear in the Brachina Formation and the Bunyeroo Formation, sedimentary layers deposited on an ancient seafloor between 560 and 550 million years ago. These are not impressions of bodies. They are burrows, feeding trails, and resting traces — evidence of behaviour preserved in stone.

The most common is Helminthoidichnites, a simple sinuous trail left by an organism grazing on microbial mats. Others are more complex: Treptichnus, a zigzag burrow that suggests a probing, searching motion; Skolithos, a vertical tube where an animal lived upright in the sediment. These trace fossils show that Ediacaran organisms were not passive blobs drifting in the current. They burrowed. They grazed. They moved with intention.

What the Burrows Reveal

A body fossil tells you what an animal looked like. A trace fossil tells you what it did. The Flinders Ranges trace fossils reveal a biological revolution underway.

The earliest burrows are shallow, horizontal, and simple — the trails of organisms that lived on the seafloor surface. Later traces become vertical and complex. This progression records an arms race between predators and prey, between burrowers and the microbial mats that held the seafloor together. As organisms learned to burrow deeper, they opened new ecological niches. They aerated the sediment. They created the first three-dimensional benthic ecosystems.

One trace fossil, Phycodes pedum, appears as bundled, branching burrows that radiate outward from a central point. This pattern suggests an organism systematically mining the sediment for food — complex foraging behaviour that required a nervous system and muscular coordination. These are not the traces of plants or fungi. These are the marks of animals.

The first animal to burrow into sediment changed the world as profoundly as the first animal to walk on land.

The Rock That Held the Record

The trace fossils of the Flinders Ranges are preserved in fine-grained sandstone and siltstone, deposited in a shallow marine basin that stretched across what is now South Australia. The key formation is the Rawnsley Quartzite, a 550-million-year-old unit that also contains the famous Ediacaran body fossils at Nilpena and the Ediacara Hills.

But the trace fossils appear lower in the section, in older rocks. They push the first evidence of animal behaviour back to at least 560 million years ago — 20 million years before the Cambrian explosion, and 10 million years before the first Ediacaran body fossils appear in the same region. The rocks of the Flinders Ranges contain, in a single geological sequence, the entire transition from microbial seafloors to complex animal ecosystems. No other place on Earth preserves this record so completely.

The Quiet Revolution

The trace fossils of the Flinders Ranges lack the drama of the Ediacaran body fossils. They are not photogenic. They do not appear in museum exhibits as often. But they answer a question the body fossils cannot: were these organisms animals, or something else?

The burrows prove they were animals. Only animals move with purpose. Only animals burrow. Only animals leave behind the structured, systematic traces that appear in these South Australian rocks. The Ediacaran trace fossils are the oldest unambiguous evidence of animal behaviour on Earth. They show that the first animals were not the strange, quilted fronds of the Ediacara Hills. They were smaller, simpler, and far more active — organisms that crawled across the seafloor, ate the microbial mats, and in doing so, changed the planet forever.