Hallucigenia was not an arthropod. It did not have jointed legs or a segmented body covered in a hard exoskeleton.
However scientists have had a difficult time figuring out exactly what it was. In the 1990s, after studying newly discovered fossils, scientists finally identified Hallucigenia as a close relative of a rare modern animal known as an onychophoran. Onychophorans are probably closely related to arthropods.
Long live the onychophoran
If the hypothesis that Hallucigenia was an onychophoran is correct, it means that Hallucigenia‘s lineage is a long-lived one. Oncychophorans have weathered all of Earth’s mass extinctions, but they have never become a very diverse group — there are fewer than 100 species of onychophoran alive today.
An Almost Arthropod: The Onychophoran
Onychoporans share certain characters with arthropods, but are lacking a hard exoskeleton or jointed legs. Onychophorans are probably closely related to arthropods and branched off the tree just before a fully hardened exoskeleton and jointed legs evolved.
Although not officially part of our arthropod story, they certainly deserve a closer look. These candy-colored animals can be bright blue, purple, orange, and green, and are covered in warty bumps. These carnivorous rain-forest dwellers have an unusual hunting style. They pursue small snails, worms, and insects and then shoot a glue-like substance at the intended victim, which quickly hardens and traps the prey.
Read the surprising story of how paleontologists eventually identified Hallucigenia: Which way is up? Reconstructing Hallucigenia.