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 relationships5,6 and 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 anomalocaridid17 specimens 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 arthropods16,20,21,22 indicate 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 crown24 or with cycloneuralian worms14. Unlike other anomalocaridids, the Fezouata taxon combines head appendages convergently9 adapted 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.
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M. and B. ‘Ou Said’ Ben Moula discovered the specimens and provided support in the field. S. Beardmore, D. Field, A. Little, R. Racicot and O. E. Tetlie assisted with fieldwork in 2011. L. Ben Moula and B. Tahiri provided practical assistance. J.-B. Caron loaned specimens of Hurdia from ROM collections. S. Butts and J. Utrup, K. Hollis and D. Erwin curated specimens and facilitated access to the collections at the YPM and USNM respectively. N. Utrup built custom cradles to support the articulated specimens. J. Vinther and P. Hull discussed filter-feeding and gigantism, and J. Lamsdell eurypterid book gills and phylogenetic methodology. E. Martin made available precise biostratigraphical data for the localities. G. Edgecombe provided unpublished observations on Cucumericrus. M. Collins discussed appendage function and prepared the digital reconstruction (Fig. 3). M. Fox advised on preparation and consolidation techniques and provided equipment. C. Graham and J. Slawski provided support and access to digital imaging facilities. The initial part of the research was done while P.V.R. was at Ghent University and in receipt of a mobility grant from the Biology Commission of the Research Foundation – Flanders (FWO). A.C.D. was financially supported by the Swedish Research Council (Vetenskapsrådet) and the Oxford University Museum of Natural History. The research was supported by National Science Foundation Grant EAR-1053247 and by the Division of Invertebrate Paleontology, YPM.
Extended data figures
Video clarifying the position of the separate block and the relationship of dorsal and ventral flaps in the complete holotype YPM 237172, and the relationship between the lateral and central carapace elements in paratype YPM 525437.
This video clarifies the position of the separate block and the relationship of dorsal and ventral flaps in the complete holotype YPM 237172, and the relationship between the lateral and central carapace elements in paratype YPM 525437.
About this article
Scientific Reports (2016)