The 550-Million-Year-Old Seafloor That Still Moves Like Slime

In the Flinders Ranges of South Australia, a 550-million-year-old seafloor has been frozen in stone—and it looks alive. Sandstone beds of the Rawnsley Quartzite preserve not just the bodies of Ediacaran organisms but the very ground they lived on: microbial mats that once carpeted the seabed, now visible as wrinkled, rippled, and torn surfaces. These are not fossils of animals. They are fossils of a living skin.

The Mat That Held the World Together

Before animals evolved skeletons, shells, or burrowing limbs, the seafloor was held in place by microbial mats—dense communities of bacteria and algae that secreted sticky mucus, binding sediment into a firm, leathery surface. In the Ediacaran, these mats were everywhere. They acted as a biological pavement, stabilising the seabed and controlling how sediment moved across it.

The Flinders Ranges preserve some of the finest examples of these ancient mats. Beds at Brachina Gorge and Bunyeroo Gorge show delicate wrinkle structures, elephant-skin textures, and torn flaps of mat that rolled up like carpet before being buried by sand. These are not trace fossils made by animals. They are the fossilised behaviour of the mat itself—ripping, curling, and re-growing in response to currents and storms.

The Ediacaran seafloor was not a passive surface. It was a living membrane, breathing and flexing between the water and the sediment.

When the Ground Ripped Open

Some of the most striking structures in the Rawnsley Quartzite are "mat chips"—fragments of microbial mat that were torn loose by storms, transported, and re-deposited as flat pebbles of sand held together by organic slime. These chips can be found stacked like shingles, oriented by currents, preserving the direction of ancient storms that swept across the shallow sea.

In other beds, the mats show evidence of "mat roll-ups"—large sections that peeled away from the seabed and curled into tubes, later filled by sand. These structures are identical to those seen in modern tidal flats where cyanobacterial mats dry and crack, but in the Ediacaran, the process happened underwater. The mat was elastic enough to stretch, tear, and fold without disintegrating.

This elasticity is key. It tells us that Ediacaran microbial mats were thick, cohesive, and resilient—more like a sheet of rubber than a thin film. They could withstand currents that would rip apart a modern bacterial biofilm. And they created a seafloor environment unlike anything that came after.

The World the Mats Made

The microbial mat acted as a chemical and physical barrier between the seawater and the sediment. Beneath the mat, the sediment became anoxic, rich in sulfide, and soft as toothpaste. Above the mat, oxygenated water supported the strange quilted organisms of the Ediacara biota—Dickinsonia, Spriggina, and Tribrachidium—that fed on the mat itself or on the organic particles it trapped.

When the Cambrian explosion began, animals evolved burrowing limbs and hard skeletons that allowed them to dig through the mat, destroying it. Within a few million years, the matground ecosystem collapsed. The leathery seafloor was replaced by a mixed, churned sediment—the "mixed ground" that defines Phanerozoic seas.

The Flinders Ranges preserve the last great expanse of matground on Earth. The wrinkled surfaces, torn chips, and rolled-up carpets are the record of a world that existed for nearly a billion years and vanished in a geological instant. The mats did not die out. They were eaten. And the animals that ate them were the ones that inherited the planet.