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A peculiar worm-like creature from 518 million years ago is the oldest animal known to have lost body parts it no longer needed. Just as modern cave-dwelling organisms have evolved reduced eyes, the ancient animal evolved to lose its back legs.

“This is the earliest example in the fossil record of an organism undergoing secondary loss, losing body parts it no longer needed,” says Richard Howard at the University of Exeter, UK.

Facivermis yunnanicus was a worm-like creature from the Cambrian period, when the first complex animals evolved in the seas. It was under 10 centimetres long and had five pairs of spiny legs on its front half, while its rear end was swollen.

It has previously been hard to determine exactly what kind of animal F. yunnanicus was. “People have said it’s all kinds of things over the years,” says Howard.

He and his colleagues re-examined known specimens and studied new ones, which all came from a collection of fossils from China known as the Chengjiang biota.

The researchers discovered that some fossils were accompanied by a tube, which F. yunnanicus evidently made and then lived inside. This implies that it was a filter feeder, similar to some modern tube worms, that would have anchored its tube to a surface and caught passing fragments of food with its front limbs.

Its closest relatives had long and feathery appendages for catching food, but they still had rear legs, which they used to anchor themselves in place.

“Facivermis took that to an extreme, where it’s just lost its back legs altogether and built a tube to live in,” says Howard. “It’s a specialised member of an already specialised group.”

The analysis also changes our ideas on F. yunnanicus’s place in the evolutionary story.

Some of the most primitive animals were essentially worms. Early on, some of these worms split into two groups. One group, the Cycloneuralia, remained limbless: they gave rise to some modern worms like nematodes. The other, the Lobopodians, evolved legs. They gave rise to three groups with legs: the arthropods, which include insects and spiders, velvet worms and tardigrades, also known as “water bears”.

The idea was that F. yunnanicus was “a missing link” from which those two fossil groups split off. But Howard says this is wrong, because the animal had distinctive eyes that mark it as a Lobopodian.

“Our study shows this worm wasn’t a missing link,” he says. “It was a simplified Lobopodian, a Lobopodian that had lost its legs. Its appearance made it look more primitive than it was.”

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