Opabinia was not an arthropod — it lacked the namesake trait of arthropods: jointed legs. However, at various times, scientists have hypothesized that it was a crustacean or a trilobite — both arthropods.
Now it’s clear why Opabinia has been so hard to fit into any familiar group. With its five mushroom-shaped eyes, clawed proboscis, and unusual body, Opabinia is simply very different from any animal alive today. Opabinia evolved alongside arthropods, chordates, and echinoderms in the Cambrian — but unlike these groups, Opabinia‘s lineage went extinct by the end of the Cambrian. Because it shares some traits with arthropods, researchers hypothesize that Opabinia might be closely related to the ancestral arthropod.