Bizarre Worm-like Creature Lost Its Legs 518 Million Years Ago Because They Were No Longer Useful

An alien-like species of lobopodian is the first known example of evolutionary reversion. Scientists writing in Current Biology explained the creature discarded its back limbs for a simpler, and typically more primitive, body shape.

Facivermis was a worm-like species with a long, thin shape and five spiny arms on the top part of its body. Its bottom half was limbless, melding into a swollen bottom that helped anchor the animal to the seafloor. The species inhabited the planet approximately 518 million years ago, during the Cambrian period, when it would filter particles of food in the water.

The creature has mystified scientists since its discovery 30 years ago. Some professionals have suggested it was a "missing link" creature bridging the gap between legless cycloneuralian worms and lobopodians, which are worm-like creatures with pairs of limbs sandwiching its body. Only now do scientists know where this bizarre-looking creature fits into the evolutionary tree thanks to well-preserved fossils collected from the Chengjiang Biota in Yunnan Province, southwest China.

Analysis of these fossils puts facivermis firmly in the Cambrian lobopodian group—one that preceded three modern animal groups, including Arthropoda (i.e. including insects, shrimps and spiders), Tardigrada (i.e. water bears) and Onychophora (i.e. velvet worms).

Facivermis illustration
An illustration of a facivermis, the first species known to have lost its limbs. Franz Anthony

Crucially, facivermis display the earliest known example of a phenomenon called "secondary loss," whereby it lost its legs in a similar fashion to modern-day snakes. A study recently published in Science Advances described as species of ancient snake that existed with hind limbs for some 70 million years. As Newsweek reported at the time, it was likely just one of many that went through such a transformation.

The style of evolution is antithetical to what we expect. In most instances, organisms evolve to have more complex body structures—not simpler.

"Occasionally we see the opposite occurring," senior author Xiaoya Ma said in a statement. "What excited us in this study is that even at this early stage of animal evolution, secondary-loss modifications— and in this case, reverting 'back' to lose some of its legs—had already occurred."

Lead author Richard Howard explained that an important clue to its development was a particular fossil where the creature's lower half was encased in a tube.

"We don't know the nature of the tube itself, but it shows the lower portion of the worm was anchored inside by a swollen rear end," he said in a statement. "Living like this, its lower limbs would not have been useful, and over time the species ceased to have them."

It appears to be unique among its relatives, who tended to have somewhere between three and nine pairs of lower legs made for walking. In contrast, facivermis may have been stuck in one place.

Illustration of Facivermis
The status of the facivermis has puzzled scientists for centuries. Franz Anthony