Somewhere between the iron maiden and Pinhead, you’ll find a misunderstood worm named Hallucigenia. For decades, researchers couldn’t make heads or tails (literally) of these bizarre-looking fossils, calling it an “evolutionary misfit” because they couldn’t be neatly linked to modern animals. But now, this legged worm has finally found its place in the evolutionary tree of life -- as ancestors to modern-day velvet worms, according to work published in Nature this week. 

Between five and 35 millimeters long, Hallucigenia lived on the ocean floor around 505 million years ago during the Cambrian Explosion -- a burst in biodiversity when most major animal groups first appeared in the fossil record. It has seven or eight pairs of legs that end in claws. Their fossils were first identified in the 1970s, but early interpretations placed it backwards and upside-down: The row of spines on its back were thought to be its legs, which themselves were thought to be tentacles along its back, and its head was mistaken for its tail. This view was overturned and the parts put in their proper place in the 1990s. 

In search of “smoking gun” characteristics linking this bizarre worm with modern animals, Martin Smith and Javier Ortega-Hernández from University of Cambridge examined the organization of Hallucigenia sparsa claws in fossils uncovered from the Burgess Shale in Canada’s Rocky Mountains. 

“It’s often thought that modern animal groups arose fully formed during the Cambrian Explosion,” Smith says in a news release. “But evolution is a gradual process: today’s complex anatomies emerged step by step, one feature at a time. By deciphering ‘in-between’ fossils like Hallucigenia, we can determine how different animal groups built up their modern body plans.” 

After magnifying the claws 1,000 times, the duo discovered an important connection to modern velvet worms, also known as onychophorans. These legged worms live in tropical rainforests and fire jets of slime at their prey. Layers of cuticle (similar to our fingernails) are nestled one inside another -- like an onion or a Russian nesting doll -- in both Hallucigenia claws and the velvet worm’s claws and jaws, where they are basically legs modified for chewing. 

“Because it did have onion-layer claws, Hallucigenia was not an evolutionary dead end,” Smith writes in The Conversation. “Rather, it represents an early pit stop on the way to the velvet worm body plan, which arose gradually over time.”

In this new family tree, velvet worms and Hallucigenia are distant cousins separated by half a billion years, with other legged worms (called lobopodians) belonging elsewhere. “It turns our current understanding of the evolutionary tree of arthropods -- the group including spiders, insects and crustaceans -- upside down,” Ortega-Hernandez says. “Most gene-based studies suggest that arthropods and velvet worms are closely related to each other; however, our results indicate that arthropods are actually closer to water bears, or tardigrades, a group of hardy microscopic animals best known for being able to survive the vacuum of space and sub-zero temperatures -- leaving velvet worms as distant cousins.” 

Images: Elyssa Rider via ScienceDaily (top), M. R. Smith/Smithsonian Institute via Cambridge CC BY-NC-SA 3.0 (middle)