An extinct creature that looked like a cross between a millipede and an earthworm was one of the first animals that could move under its own power.
The animal has been named Yilingia spiciformis. It was up to 27 centimetres long and up to 2.6 cm wide. Its body was divided into many segments, each carrying two spiky appendages. It looked a bit like an ear of wheat.
The fossils were found in the Dengying Formation in southern China. They are between 551 and 539 million years old. This was the Ediacaran period, when the first confirmed multicellular animals appear in the fossil record. Before the Ediacaran, life on Earth seems to have been almost entirely single-celled, but after the Ediacaran, complex plants and animals flourished.
Alongside 35 fossils of Y. spiciformis, the rocks also yielded 13 trace fossils: tracks that were left behind by the animals as they moved along the sediment on the seabed. One body fossil was actually found right next to its tracks, offering hard evidence that Y. spiciformis was able to move.
“It is the first segmented animal that has been shown to be capable of directional movement,” says Shuhai Xiao at Virginia Tech.
Xiao says that Y. spiciformis isn’t quite the oldest animal that could move from A to B. “The first mobile animal is probably about 565 million years old,” he says. One such creature was the slug-like Kimberella, which could slither across the sea floor. Other Ediacaran animals like Dickinsonia could probably move in a less directed way, by letting the water current take them, says Xiao.
The new fossils will help us understand how animals evolved the ability to move, says Xiao. “When we look at the animal family tree, clearly animals started as non-motile organisms,” he says. These early, stationary animals may have resembled modern sponges. “The question then becomes, when did animal motility evolve, and whether it evolved once or several times among animals.”
Answering this question will mean understanding what sort of animal Y. spiciformis was. It could be an early arthropod – the group that includes insects and millipedes – or an annelid, like a modern earthworm. It could also be ancestral to both groups. “If it’s a more early animal, it would suggest that perhaps locomotion evolved once in the common ancestor of annelids and arthropods,” says Xiao.