For over a century, the Cambrian arthropod Helmetia expansa remained a mystery. Discovered by paleontologist Charles Doolittle Walcott in 1918, it was initially classified as a crustacean.

The fossil, dated to about 444 million years ago, contained a new species of arthropod that fossilized inside-out, according to a paper published in the journal Palaeontology last week.