Despite being among the most celebrated taxa from Cambrian biotas, anomalocaridids (order Radiodonta) have provoked intense debate about their affinities within the moulting-animal clade that includes Arthropoda. Current alternatives identify anomalocaridids as either stem-group euarthropods1,2,3, crown-group euarthropods near the ancestry of chelicerates4, or a segmented ecdysozoan lineage with convergent similarity to arthropods in appendage construction5. Determining unambiguous affinities has been impeded by uncertainties about the segmental affiliation of anomalocaridid frontal appendages. These structures are variably homologized with jointed appendages of the second (deutocerebral) head segment, including antennae and ‘great appendages’ of Cambrian arthropods, or with the paired antenniform frontal appendages of living Onychophora and some Cambrian lobopodians. Here we describe Lyrarapax unguispinus, a new anomalocaridid from the early Cambrian Chengjiang biota, southwest China, nearly complete specimens of which preserve traces of muscles, digestive tract and brain. The traces of brain provide the first direct evidence for the segmental composition of the anomalocaridid head and its appendicular organization. Carbon-rich areas in the head resolve paired pre-protocerebral ganglia at the origin of paired frontal appendages. The ganglia connect to areas indicative of a bilateral pre-oral brain that receives projections from the eyestalk neuropils and compound retina. The dorsal, segmented brain of L. unguispinus reinforces an alliance between anomalocaridids and arthropods rather than cycloneuralians. Correspondences in brain organization between anomalocaridids and Onychophora resolve pre-protocerebral ganglia, associated with pre-ocular frontal appendages, as characters of the last common ancestor of euarthropods and onychophorans. A position of Radiodonta on the euarthropod stem-lineage implies the transformation of frontal appendages to another structure in crown-group euarthropods, with gene expression and neuroanatomy providing strong evidence that the paired, pre-oral labrum is the remnant of paired frontal appendages1.
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This published work and the nomenclatural acts it contains have been deposited in ZooBank under accession number http://zoobank.org/urn:lsid:zoobank.org:pub:189DCAFF-0DD6-49C2-BE80-E999DDF059C1.
This work was supported by the National Natural Science Foundation of China (U1302232, 41372031 and 40962001), a Leverhulme Trust Research Project Grant (F/00 696/T), by the Center for Insect Science, University of Arizona, and by a grant from the Air Force Research Laboratory (FA86511010001) to N.J.S. We thank T. Goral for assistance with energy dispersive X-ray spectroscopy. We acknowledge A. Daley’s advice about anomalocaridid anatomy.
Extended data figures
Dataset of characters for phylogenetic analysis in Nexus format (editable in Mesquite or NDE).