Body fossil of Yilingia spiciformis

A fossil of Yilingia spiciformis and the track it left as it moved.Credit: Z. Chen et al./Nature

More than half a billion years ago, a strange, worm-like creature died as it crawled across the muddy sea floor. Both the organism and the trail it left lay undisturbed for so long that they fossilized. Now, they are helping to revise our understanding of when and how animals evolved.

The fossil, which formed some time between 551 million and 539 million years ago, in the Ediacaran period, joins a growing body of evidence that challenges the idea that animal life on Earth burst onto the scene in an event known as the Cambrian explosion, which began about 539 million years ago.

“It is just pushing things further and further back into the Ediacaran,” says Rachel Wood, a geoscientist at the University of Edinburgh, UK. The Cambrian explosion no longer appears to be such an abrupt event in the history of life on Earth, she says. An analysis of the fossil, along with a few dozen similar specimens found in the same rock sequence in southern China, is published in Nature1.

“What’s extraordinary about this paper is it’s three home runs in the same five-page manuscript,” says Simon Darroch, a palaeontologist at Vanderbilt University in Nashville, Tennessee. First, it’s exceptionally rare to find a dead animal preserved at the end of a trail it made when alive, he says. Second, the fossil dates to a crucial moment in the evolution of animal life.

And third: “It’s such a bizarre-looking organism,” says Darroch. The creature, which has been named Yilingia spiciformis and was up to 27 centimetres long, seems to

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