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

For the first few billion years of its history, life on Earth stayed mostly small. Then, around 600 million years ago, in the Ediacaran Period, strange large organisms began to show up. They were the planet's first animals, arriving after a very long stretch dominated by microbes.

A new study looks at that moment, but its focus is less on the creatures themselves than on a puzzle about how we know them at all. Collectively called the Ediacaran biota, they were mostly soft-bodied, and the source describes many of them as Seussian, some with question-mark-shaped rear ends. Many looked like nothing alive today. They vanished in a mass extinction about 550 million years ago, which cleared the way for the Cambrian explosion. As new animals spread, the Ediacaran forms seemed to disappear without a trace. Given their age and how little hard material they had to leave behind, it is remarkable that we know anything about them.

The researchers argue that quirks of Ediacaran landscapes, coasts, and climates combined to keep hints of these animals intact. Their work centers on rock formations in the Flinders Ranges of South Australia, a mountainous region known for a rare, diverse, well-preserved collection of Ediacaran fossils. The Ediacaran Period ran from roughly 635 million to 538.8 million years ago.

To reconstruct the setting, the team mapped geological features across the ranges. They wanted to know what the landscape looked like about 550 million years ago, how it changed late in the period, and how fossil-bearing sediments piled up. The maps revealed traces of enormous canyons oriented to the southeast. They plunged as deep as 280 meters (919 feet) and in places stretched more than 6 kilometers (3.7 miles) across.

According to the researchers, the canyons formed largely through erosion under water, beginning with a major fall in sea level. When the sea later rose, the landscape flooded and the canyons filled with sediment. In those conditions, a layer of Rawnsley Quartzite known as the Ediacara Member preserved rare evidence of the region's former inhabitants. "We found evidence that huge underwater canyons cut across the continental shelf before being rapidly flooded as sea levels rose again," says lead author Karol Faehnrich, a geologist at Adelaide University. In his words, that flooding created the conditions that let one of the world's richest Ediacaran fossil records survive.

The character of the sediment deposits in the canyons indicates powerful water flows that could move large volumes of material. By comparison with modern submarine canyons, the researchers conclude that underwater erosion was more likely to have driven the process than river dynamics alone. Co-author Diego Garcia-Bellido, a biologist at Adelaide University, said the evidence points to changing sea levels combined with local geological processes shaping the ancient basin. Many Ediacaran fossils, he noted, are delicate impressions of soft-bodied organisms, which survive only when conditions are just right. Their work, he said, shows the rise in sea level helped create a window of exceptional preservation.

Big questions remain open. Scientists still do not know how the Ediacaran biota fit into the broader family tree, whether as ancestors of animals or as evolutionary dead ends. They do not know why the group radiated so quickly or why it declined. Earlier research tied the biota to global sea-level changes, as a possible driver of evolutionary innovation and a factor in preservation, but the study stresses that this link is speculative. What it adds is a clearer picture of why the fossils survived. The study appears in Earth and Planetary Science Letters.