Ancient Underwater Canyons in South Australia Helped Preserve Earth's Earliest Animal Fossils

In the Flinders Ranges of South Australia, a mountainous region known for one of the richest collections of Ediacaran fossils anywhere, geologists have been reading the rock for a different kind of clue: not the animals themselves, but the landscape that kept them. A new study maps what that landscape looked like about 550 million years ago, and finds it was cut by enormous canyons under the sea.

For the first few billion years of its history, life on Earth stayed mostly small, dominated by microbes. Then, around 600 million years ago, in the Ediacaran Period, larger and more complex organisms appeared. Collectively called the Ediacaran biota, they were mostly soft-bodied and often strange, with a size and complexity not seen before. Some had what the source describes, with some humor, as question-mark-shaped rear ends. Many looked like nothing alive today. The period itself ran from about 635 million to 538.8 million years ago.

They did not last. A mass extinction roughly 550 million years ago ended their run and set the stage for the Cambrian explosion, when new animals proliferated. The Ediacaran forms seemed to vanish without a trace. That we know anything about them is remarkable, given their age and how little hard material they left to fossilize.

The researchers, led by Karol Faehnrich, a geologist at Adelaide University, mapped geological features across the Flinders Ranges to reconstruct the terrain late in the Ediacaran and to understand how fossil-bearing sediments piled up. What they found were traces of colossal canyons oriented toward the southeast. In places they plunged 280 meters (919 feet) deep and stretched more than 6 kilometers (3.7 miles) across.

According to the team, the canyons formed largely through underwater erosion, which began with a major fall in sea level. When the seas rose again, the landscape flooded and the canyons filled with sediment. The characteristics of the deposits suggest powerful water flows capable of moving large volumes of material. Drawing on modern submarine canyons for comparison, the researchers conclude that underwater erosion was more likely the driver than river dynamics alone.

It was within this setting that a layer of Rawnsley Quartzite called the Ediacara Member preserved its rare record of the region's inhabitants. "We found evidence that huge underwater canyons cut across the continental shelf before being rapidly flooded as sea levels rose again," Faehnrich says. He adds that the flooding created the conditions that let one of the world's richest Ediacaran fossil assemblages survive.

Co-author Diego Garcia-Bellido, a biologist at Adelaide University, points to the delicacy of what was saved. Many Ediacaran fossils are impressions of soft-bodied organisms, he notes, and such fossils survive only when conditions are just right. In his reading, the rise in sea levels opened a window of exceptional preservation, and the evidence points to a mix of changing sea levels and local geological processes shaping the ancient basin.

Big questions remain open. Scientists still cannot say how the Ediacaran biota fit into the broader family tree, whether as ancestors of later animals or as evolutionary dead ends. Earlier research tied the biota to global sea-level changes, as a possible spur to evolutionary innovation and as a factor in preservation, but the authors acknowledge this is speculative. Why these creatures evolved, why they died out, and what part sea level played in their fate are still unanswered.