One of the strangest fossils ever found has finally found its place in evolutionary history.

In the 1970s, researchers found the fossil of the worm Hallucigenia, although the perplexed researchers weren’t really sure what they were looking at due to the creature’s odd spines that shoot off of its wormy body.

However, modern scientists have now discovered that those strange spines are actually the worm’s legs, while the things thought to be the worm’s legs are actually spines. The mix-up had confused researchers for years, until they were able to liken the fossilized worm to the modern-day velvet worm.

The realizations have made Hallucigenia a part of evolutionary history, claims study researcher Javier Ortega-Hernandez, a paleobiologist at the University of Cambridge. Prior to these discoveries, it was widely believed that velvet worms are closely related to spiders, crustaceans, and insects. Now, researchers are realizing that the velvet worms are actually only distant cousins to arthropods.

“An exciting outcome of this study is that it turns our current understanding of the evolutionary tree of arthropods–the group including spiders, insects, and crustaceans–upside down,” said Dr. Javier Ortega-Hernandez.

“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.”

The ancient Hallucigenia lived during the Cambrian Explosion, which happened around 542 million years ago. Researchers believe that the worms were sea dwellers, and were only a tiny 1.3 inches long. By studying the fossil and looking at today’s velvet worms, scientists were able to learn a bit more about the evolutionary process.

“Its often thought that modern animal groups arose fully formed during the Cambrian Explosion,” said study researcher Martin Smith, an earth scientist at the University of Cambridge. “But evolution is a gradual process: Todays 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.”

The new information was published online in the journal Nature.

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