Exceptionally preserved fossils from the Palaeozoic era provide crucial insights into arthropod evolution, with recent discoveries bringing phylogeny and character homology into sharp focus1,2,3,4. Integral to such studies are anomalocaridids, a clade of stem arthropods whose remarkable morphology illuminates early arthropod relationships5,6and Cambrian ecology7,8,9. Although recent work has focused on the anomalocaridid head6,7,8,9,10, the nature of their trunk has been debated widely5,11,12,13,14,15,16,17,18. Here we describe new anomalocaridid17specimens from the Early Ordovician Fezouata Biota of Morocco19, which not only show well-preserved head appendages providing key ecological data, but also elucidate the nature of anomalocaridid trunk flaps, resolving their homology with arthropod trunk limbs. The new material shows that each trunk segment bears a separate dorsal and ventral pair of flaps, with a series of setal blades attached at the base of the dorsal flaps. Comparisons with other stem lineage arthropods16,20,21,22indicate that anomalocaridid ventral flaps are homologous with lobopodous walking limbs and the endopod of the euarthropod biramous limb, whereas the dorsal flaps and associated setal blades are homologous with the flaps of gilled lobopodians (for example,Kerygmachela kierkegaardi,Pambdelurion whittingtoni) and exites of the ‘Cambrian biramous limb’23. This evidence shows that anomalocaridids represent a stage before the fusion of exite and endopod into the ‘Cambrian biramous limb’5,16,23, confirming their basal placement in the euarthropod stem4,5,6, rather than in the arthropod crown24or with cycloneuralian worms14. Unlike other anomalocaridids, the Fezouata taxon combines head appendages convergently9adapted for filter-feeding with an unprecedented body length exceeding 2 m, indicating a new direction in the feeding ecology of the clade. The evolution of giant filter-feeding anomalocaridids may reflect the establishment of highly developed planktic ecosystems during the Great Ordovician Biodiversification Event25.