Nice try, but maybe we should have shown you the hint first:
Arthropods have segmented bodies and jointed legs. Opabinia had a clearly segmented body — but did it have jointed legs? Look closely at the fossils and reconstructions above. Each body segment has two pairs of flaps. One pair was a simple flap and was probably used for swimming — but these were not jointed. The second pair looks striped in the reconstruction above. These stripes are the imprints of gill layers and were used for getting oxygen from the water.
In fact, 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.
