The Cambrian arthropod from the Burgess Shale with prominent paired head appendages, once interpreted as a possible trilobite ancestor, was Marrella.
Marrella splendens was first collected from British Columbia’s Burgess Shale by Charles Doolittle Walcott in 1909. Its unusual head shield carries two pairs of long spines, while its body bears numerous biramous limbs with walking and gill-bearing branches. The combination made it difficult to place among familiar arthropod groups.
Early interpretations treated Marrella as an unusual trilobite or as a form close to the ancestry of trilobites. A detailed 1971 redescription by Harry Blackmore Whittington showed that its legs, gills, and head appendages differed significantly from those of trilobites, crustaceans, and chelicerates. Marrella is now classified within Marrellomorpha, an extinct arthropod group with uncertain deeper relationships.
Marrella was probably a small swimmer living near the seafloor. Its unusual spines may have helped stabilize it while swimming or deter predators. It is also one of the most abundant Burgess Shale fossils, making it especially valuable for studying Cambrian arthropod diversity. Anomalocaris is the common mix-up: it had large frontal grasping appendages, but it was a different radiodont predator.