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About 574 to 539 million years agoReputable source · 2 sourcesWell documented

The Ediacaran biota: animals fill the world

On the timeline · around About 574 to 539 million years ago · Toward the Rise of AnimalsToward the Rise of AnimalsThe Ediacaran biota: animals fill the world775 Ma750 Ma725 Ma700 Ma675 Ma650 Ma625 Ma600 Ma575 Ma550 Ma

What happened

In the last tens of millions of years before the Cambrian, the fossil record fills with strange, soft-bodied organisms unlike anything alive today, first found in 1946 at the Ediacara Hills in South Australia and since recovered on every continent except Antarctica. NASA Astrobiology's account of the Nilpena fossil site in South Australia calls this the first time animals moved through open water, the first time they crawled or grazed the seafloor, and the first time some of them reproduced sexually. The best-known member, Dickinsonia, was a flattened, ribbed oval that could grow over a metre across. For decades its identity was debated: animal, fungus, giant single-celled organism, no one could say for certain. That changed when researchers led by Jochen Brocks analyzed molecules preserved in a Dickinsonia fossil from Russia's White Sea coast and found it held up to 93 percent cholesterol, a molecule that regulates animal cell membranes, while the sediment around the fossil held a completely different molecule associated with algae and fungi. Brocks described the contrast as black and white.

Why it matters

The Ediacaran biota is the first time the fossil record shows large, complex, moving organisms in any abundance, roughly 35 million years before the Cambrian explosion multiplies that complexity many times over. Whatever most of these organisms actually were, several were now confirmed animals, which makes the Ediacaran the true opening act of the animal story the Cambrian is famous for.

How we know

The organisms themselves are known from body impressions preserved in fine sediment, often on the underside of sandstone beds where storms buried microbial mats. Dickinsonia's animal identity specifically rests on molecular biomarkers, chemical residues of cholesterol extracted directly from the fossil tissue, not merely inferred from its shape.

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