The early worm gets the legs – that’s not how the saying goes but it is how this new study goes. The fossil creature in question is, in fact, the earliest evidence of an animal having limbs and then evolving to lose them from lack of use.

The worm-like creature, called Facivermis, lived around 518 million years ago during the Cambrian period, an ancient time when major groups of animals first emerged in the fossil record. The Cambrian explosion was a time of rapid diversification of complex, multicellular lifeforms.

Facivermis lived an underwater existence – its body described by scientists as having “a swollen posterior end,” a long, slender body, and five pairs of spiny arms. Previous work suggests it was the middle-man between the legless cycloneuralian worms and the limbed lobopodians. 

Not so, say a team from the University of Exeter, Yunnan University, and the Natural History Museum. The new study suggests Facivermis lived a tube-dwelling life anchored to the sea floor and lost its lower limbs over time from disuse. A crucial piece of evidence was a fossil with a tube surrounding the lower portion of Facivermis.

"This has been a fun fossil to work on! It’s a specialized member of an already specialized group and the secondary loss of limbs is a very interesting story in such a surreal looking animal," lead author Richard Howard, whose study is published in the journal Current Biology, told IFLScience.

Its back limbs would not have helped the critter under the sea. The Facivermis fossil is a rare early Cambrian example of secondary loss, in this case losing its legs helped to better modify it to a tube-dwelling lifestyle. Facivermis was likely most similar to feather duster worms, though still distantly related. These feather dusters are also tube-dwelling and use a suspension feeding technique.

A puzzling part of the history is the preservation of the tube. For now, the team is uncertain whether the tube was built from sediment or produced by the creature itself. About 30 Facivermis specimens are recorded in the literature but only two show the tube, so it's "very rare" say the team. It's possible scientists have missed the tubes in the past if they lacked the body fossil association.

"Modern tube worms (i.e. sabellid polychaetes) build their tubes themselves by gluing grains of sediment together – which gives the tube a distinctive irregular appearance," said Howard. "We don’t see this in Facivermis. It’s a more regular structure with a consistent shape and texture – but ultimately we don’t know much more about it!"