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Brain structure resolves the segmental affinity of anomalocaridid appendages



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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Figure 1: L. unguispinus from the Chengjiang Lagerstätte.
Figure 2: L. unguispinus.
Figure 3: Comparison of onychophoran and L. unguispinus brain.
Figure 4: Evolutionary shift of frontal appendage and its ganglia.

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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.

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Fossil data were analysed by all authors, all of whom contributed to the text.

Corresponding authors

Correspondence to Xianguang Hou or Nicholas J. Strausfeld.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Comparison of frontal appendages of Lyrarapax and Amplectobelua.

a, L. unguispinus, YKLP 13304a. bd, Immature specimens of A. symbrachiata. Scale bars: a, 2.5 mm; bd, 2 mm.

Extended Data Figure 2 Eyes of L. unguispinus and onychophoran brain.

a, Horizontal aspect of right eye of YKLP 13304b. The fracture plane has exposed radiating reliefs suggestive of rhabdomeres (rh) in a clear zone that terminate as pale bases (rhb) within a darker area interpreted as pigment withdrawn to level of basement membrane (bm) lying above paler bluish areas corresponding to expected locations of optic neuropil (opn). Occasional lenses (le) are resolved. b, Inset of same eye in the counterpart of YKLP 13304a, reversed for direct comparison with the part, radiating reliefs arrowed (rh). c, Cartoon of a, depicting identified components in a and b. d, Left eye of YKLP 13304b, with profiles (arrowed) suggestive of lenses. Scale bars: c, 0.5 mm; d, 0.5 mm. e, Frontal ganglion (frg) of E. rowelli (Onychophora) showing its supply by frontal appendage nerve (fan). Lateral protocerebrum, lpr; optic tract, opt. Scale bars: ac, 200 μm; d, 500 μm; e, 100 μm.

Extended Data Figure 3 L. unguispinus.

a, Dorsal view of whole specimen YKLP 13306a with intact head shield (hs), eyestalks (eys), neck (ne) and eight exposed trunk segments. Rectangles refer to insets bh. b, Right eyestalk and retina (re). c, Enlargement of the ‘shoulder’ (sh) of greatly extended flap (fl) from first trunk segment. d, Flaps of trunk segments 6 and 7 with enlargement (in e) to show setal blades (sb) and muscle (m). f, Head shield showing characteristic oval anterior margin. g, left eye stalk (eys) of counterpart (YKLP13306b) showing brown corneal layer (o) overlying darker photoreceptor layer. Scale bars: a, 2 cm; b, 3 mm; cg, 1 cm.

Extended Data Figure 4 Relationships of Radiodonta.

Strict consensus of 108 shortest cladograms (96 steps) under equal character weights, based on modified data set from ref. 6 (Supplementary Information).

Supplementary information

Supplementary Information

This file contains details of specimen preservation, morphological description of specimens, description of phylogenetic analysis, and supplementary references. This file was replaced on 21 July 2014. (PDF 247 kb)

Supplementary Data

Dataset of characters for phylogenetic analysis in Nexus format (editable in Mesquite or NDE). (TXT 11 kb)

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Cong, P., Ma, X., Hou, X. et al. Brain structure resolves the segmental affinity of anomalocaridid appendages. Nature 513, 538–542 (2014).

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