The 558-Million-Year-Old Fat That Solved an Animal Mystery

In the Flinders Ranges of South Australia, a sandstone cliff holds the outline of a creature that died 558 million years ago—and that outline still has fat in it. The fossil, Dickinsonia, looks nothing like modern life: an oval ribbed like a doormat, up to a metre long, with no mouth, no gut, and no organs. For decades, paleontologists argued over what it was. Then, in 2018, a team extracted organic molecules from a Dickinsonia fossil and found cholesterol—a lipid that only animals produce. The Ediacaran biota, it turned out, were not lichens, not colonies of single cells, not failed experiments. They were animals, and they left their chemistry behind in the rock.

The Chemistry of Soft Bodies

Fossils usually preserve hard parts: bone, shell, wood. Soft tissues rot or are eaten before they can mineralise. But in the Ediacaran, before predators or scavengers evolved, a dead body could lie on the seafloor for months. Microbial mats grew over it, sealing it from oxygen. Clay minerals replaced the tissues cell by cell. The result is a fossil that preserves not just the shape but the original organic carbon.

Dickinsonia fossils from the Flinders Ranges contain intact steranes—molecules derived from cholesterol. Cholesterol is a biomarker unique to animals; no plant, fungus, or bacterium produces it. The discovery proved that Dickinsonia belonged to the animal kingdom, settling a century-old debate. Other Ediacaran fossils, like the frond-like Charniodiscus, lack animal biomarkers. They may have been something else entirely.

The rock that preserves these molecules is the 558-million-year-old Ediacara Member of the Rawnsley Quartzite. It is a sandstone, deposited in shallow marine conditions during the final 15 million years of the Proterozoic Eon. Within it, the fossils are three-dimensional casts on the soles of sandstone beds—the impressions of organisms buried by sudden sand flows.

A World Before Movement

Ediacaran animals had no mouths, no guts, and no anuses. They absorbed nutrients directly through their skin, feeding on dissolved organic matter or symbiotic microbes. They did not move. The trace fossils of the Ediacaran are limited to resting impressions and the occasional scratch; there are no burrows, no tracks, no evidence of active hunting. The world was still and quiet.

Then, 541 million years ago, at the start of the Cambrian, animals evolved guts, eyes, and claws. They began to burrow, swim, and hunt. The Ediacaran soft-bodied forms vanished, replaced by the first hard-shelled organisms—the small shelly fauna that mark the Cambrian explosion. The Flinders Ranges preserve the boundary: a single bed of sandstone, the Ediacara Member, overlain by Cambrian limestone packed with burrows and shells.

The Ediacaran world was not a failed beginning. It was a stable ecosystem that lasted 30 million years—longer than the entire Cenozoic Era.

The Landscape That Holds Them

The Flinders Ranges today are a folded landscape of quartzite ridges and red valleys, eroded from sediments deposited in the Adelaide Rift Complex. The Ediacara Member outcrops in a narrow band that runs for hundreds of kilometres through the northern ranges. The best exposures are at Nilpena Ediacara National Park, where researchers have excavated more than a thousand individual fossils, mapped onto the original bedding surfaces.

Each surface records a single moment of burial: a sand flow, probably from a storm or slump, that smothered the seafloor and preserved it in place. The fossils are not transported or jumbled. They are exactly where they lived, frozen in the instant of their death. One surface, called the Alice Bed, contains 300 specimens of Dickinsonia, Spriggina, and Parvancorina—a snapshot of an Ediacaran community, complete and undisturbed.

The Flinders Ranges are still rising, still eroding. Every winter rain exposes new fossils. The Ediacaran biota is not a closed chapter; it is a deposit still being opened, molecule by molecule, bed by bed.