Brain structure resolves the segmental affinity of anomalocaridid appendages

Nature volume 513, pages538–542(2014)Cite this article

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Abstract

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.

Main

Arthropoda von Siebold, 1848

Radiodonta Collins, 1996

Amplectobeluidae Vinther et al., 2014

Lyrarapax unguispinus gen. et sp. nov.

Life Science Identifier (LSID). urn:lsid:zoobank.org:act:BFE89E0E-CEB2-406A-B516-FEC5EC5A7C2A

Etymology. lyra (Latin): referring to an overall lyre-like body shape; rapax (Latin): predator; unguis (Latin): claw; spinus (Latin): thorn, alluding to the spinose, claw-like frontal appendages.

Holotype. Holotype YKLP 13304a, b (Fig. 1 and Extended Data Figs 1a and 2a–d), part and counterpart.

Referred material. Paratypes YKLP 13305 (part only, Figs 2 and 3b, c), YKLP 13306 (part and counterpart, Extended Data Fig. 3).

Locality. Ercaicun (YKLP 13304, 13306) and Mafang (YKLP 13305) in Haikou, Yunnan Province, China.

Horizon. Heilinpu Formation, Cambrian Series 2, Stage 3, Yu’anshan Member (Eoredlichia–Wutingaspis assemblage zone).

Diagnosis. Small anomalocaridid, body length to 8 cm, with a pronounced, four-segmented neck; frontal appendage short, one proximal podomere bearing a stout, blade-shaped endite with pectinate, sclerotized spines that gradually become larger distally, the successive podomeres with alternating small and larger endites; body flap of first trunk segment hypertrophied, paddle-shaped, succeeding body flaps narrowing relatively strongly; tail fan composed of three pairs of blades.

L. unguispinus is known from three largely complete specimens (Supplementary Information). It is assigned to Radiodonta based upon the following diagnostic characters6: large, club-shaped, compound eyes, having the same relative size, shape and position as in Anomalocaris canadensis7; frontal appendages, the only appendages in the head or trunk, arthropodized and bearing serially repeated ventral spines on each podomere (Fig. 1e and Extended Data Fig. 1a); a large mouth cone opening ventrally in the head (Fig. 1a–d, f), its size being typical for a radiodontan8; short, narrow segments defining a neck region (as in A. canadensis and Peytoia nathorsti); segmental body flaps, a tripartite tail fan (Fig. 1a, b) and dorsal bands of setal blades (‘gills’) (Extended Data Fig. 3e) having the usual proportions for the group. A pigmented rim at the anterior margin of the head (best seen in YKLP 13304a; Fig. 1d, g) is interpreted as the margin of a dorsal carapace, as known from other radiodontans7. Phylogenetic analysis resolves L. unguispinus within Radiodonta6 as the closest relative of Amplectobelua (Extended Data Fig. 4) based on shared derived characters of the pincer-like frontal appendages (Supplementary Information).

Figure 1: L. unguispinus from the Chengjiang Lagerstätte.
figure1

a, b, Ventral view of YKLP 13304b (counterpart) showing mouth cone (arrowed mo), eye stalks (eys), retinal pigmentation (rep), four neck segments (ne), traces of putative vascular system (vs) and its branches (open arrows) leading to metameric muscle blocks (m) aligned with lateral flaps, including oar-like first pair (fl, between arrowheads), and partly overlapping blades of tail fan (tf). c, d, Ventral view of YKLP 13304a (part) showing one frontal appendage (fa), part of the dorsal cowl (cw) and pigmented head shield anterior rim (hs). e, Frontal appendage: large proximal podomere (pp) equipped with serial spines allowing apposition against inner teeth of more distal podomeres. f, Enlarged oral cone showing concentric ridges and triangular areas (stars) suggestive of denticles. g, Pigmented rim of head shield. Scale bars: a, also for b–d, 5 mm; e, f, 1 mm; g, 3 mm.

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In all three specimens (Figs 1, 2 and Extended Data Fig. 3) the eyestalks contain terminal areas that are black, sometimes surmounted by dark blue colouration. Ommatidia (Extended Data Fig. 2a, b) are indicated as long, slightly curved troughs that extend through pale blue-pigmented areas in the right eye of YKLP 13304b, terminating in a darker zone lying distal to a roughly oval area of black pigmentation that further extends as a broken tract directed towards the head (Extended Data Fig. 2a–c). A similar trace is observed in the left eye of YKLP 13304b where there is evidence for faceted organization (Extended Data Fig. 2d). The observed relationship between photoreceptor terminals and underlying structure is typical of euarthropod eyes equipped with compound facets, which have been demonstrated as the eye structure typifying anomalocaridids9. The black domains in YKLP 13304 are thus interpreted as the first optic neuropil of the eye. Corresponding black domains in YKLP 13305 are connected via black tracts to further broadened pigmented areas disposed centrally, lateral to and confluent with an approximately symmetric domain extending across the head in front of the mouth, compared with YKLP 13304 (Fig. 2a). Anterior to this dark area, and attached to each side of it anteriorly, are two approximately almond-shaped areas located at a position corresponding to the point of origin of the frontal appendages. Employment of a digital blue filter accentuates these dark structures as well as contiguous fragments comprising cord-like traces that extend a short distance caudally, indicative of two parallel tracts from the main pre-oral mass (Fig. 2f). Energy-dispersive X-ray spectroscopy resolves the dark areas as carbon concentrations (Fig. 2g). Allowing for some distortions probably due to compression, bilateral symmetry demonstrates that these structures represent an organ system. Comparison of YKLP 13304 and 13305 suggests these structures are not contiguous with the mouth cone, or any regions caudal to it (Fig. 2h), and the confluence of these carbon-rich areas with tracts leading from the compound eyes identifies the observed areas as flattened brain and cerebral ganglia (Fig. 2f–h). These structures correspond to the profile of a flattened horizontal view of a compressed onychophoran brain (Fig. 3a and Extended Data Fig. 2e, both compared with Fig. 3b, c). The trunk of YKLP 13305 resolves other carbon-rich organ systems that are structurally distinct from those in the head. These other systems are indicative of a large convoluted gut, gut-associative muscle and variously oriented lamellate body-wall muscle blocks (Fig. 2c–e). These are similar, respectively, to gut muscle and striated muscle traces described from Pambdelurion whittingtoni10 and A. canadensis7.

Figure 2: L. unguispinus.
figure2

a, b, Dorsal view of YKLP13305 (left side slightly tilted downwards) resolving straight midgut (mg) and sinusoidal alimentary tract (alt). Four neck and eleven trunk segments, the first providing paired oar-like flaps (fl between arrowheads), the last providing the tail fan (tf). Dark areas in the head indicate paired frontal appendage ganglia (frg), optic tract (opt) linking retinas (re) in eyes (ey) to flattened lateral protocerebral lobes (lpr in h) flanking an approximately bilaterally symmetric protocerebrum (pr). Metameric striate areas indicate muscle (m). c–e, Raised and indented grooves of muscle blocks (enlargements of boxed areas in b). f–h, Neural traces: blue digital filter (f) cancels colours in fossil except dark neural regions (for example, medial protocerebrum, mpr) that are resolved by scanning electron microscopy and energy-dispersive X-ray spectroscopy (g), as carbon-rich domains, and shown by oblique illumination relative to eye and head margins (h); bm, basement membrane and first optic neuropil. Raised neck segments gradually obscure caudally directed descending tracts (dt). Scale bars: a, b, 1 cm; c–e, 0.5 mm; f (also for g) and h, 5 mm.

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Figure 3: Comparison of onychophoran and L. unguispinus brain.
figure3

a, Horizontal section of hemibrain of E. rowelli stained with osmium–ethyl gallate, showing frontal appendage ganglion (frg) anterior to optic tract (opt) and second optic neuropil (on2) connected to medial protocerebral neuropil (mpr; lateral protocerebrum, lpr). b, c, Both sides of brain of L. unguispinus YKLP 13305 aligned to match orientation of a. Corresponding areas (and retinas, re) indicated, as is one root of descending tracts (dt). d, e, Comparison of E. rowelli and L. unguispinus brains. Nervous extensions into the frontal appendages (fa; on subsequent sections of E. rowelli; Extended Data Fig. 2e), not visible in the fossil, are added (paler blue). Incomplete distal part of frontal appendage of L. unguispinus reconstructed (paler grey). Eye, ey. Scale bars: a, 100 μm; c (also for b), 2 mm.

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Carbon-rich cephalic structures of YKLP 13305 (Fig. 2g) resolve the disposition of brain centres that substantially differ from ground pattern organization expected of a euarthropod brain, but resolve specific correspondences with the cerebral ground pattern of Onychophora. Mandibulates and chelicerates ( = Euarthropoda) possess segmental ganglia connected by paired nerve cords to tripartite brains, the latter a consequence of developmental fusion of three neuromeres rostral to or flanking the gut. This ground pattern has existed at least since the Lower Cambrian, as exemplified by the mandibulate-like brain of Fuxianhuia protensa11 and the chelicerate-like central nervous system of the megacheiran Alalcomenaeus sp.12.

Such segmental arrangements contrast to the organization of the central nervous system in Onychophora13, where nerve cords from paired pre-ocular frontal appendages supply a brain that lies in front of the mouth and which provides paired lateral cords comprising continuous synaptic neuropil connected by orthogonal connections down its length14. No segmental ganglia are resolved13, an absence interpreted as an ancestral trait of Arthropoda15. In Onychophora, localization of engrailed protein in the brain resolves only a single posterior segmental boundary16, thereby refuting its proposed tri- or bipartite segmental organization17,18. Resolving the brain and nerve cords of the onychophoran Euperipatoides rowelli with osmium–ethyl gallate staining19 demonstrates that bundled axons from the frontal appendages supply a large ganglion-like neuropil that lies anterior to the optic tract, the latter supplying neuropil of a heterolateral brain lying anterior to and above the mouth (Fig. 3a). Carbon deposits in the head of L. unguispinus likewise resolve paired, approximately almond-shaped, pre-ocular domains situated above the origin of the frontal appendages (Figs 2f–h and 3b, c). These structures are here interpreted as frontal appendage ganglia that are contiguous with the anterior brain and, as in Onychophora, are anterior to optic tracts extending into the brain from the eyes. The absence of any suboesophageal neuropil mass, but the origin of paired tracts directed caudally (Fig. 3b, c), offers further evidence for the correspondence of brain organization in onychophorans and anomalocaridids (Fig. 3d, e), whereas none of the putative brain regions of L. unguispinus correspond to either of the two ground patterns of euarthropod brains. Enlargements at two points along the anomalocaridid optic nerve suggest two successive optic neuropils (Fig. 3c), one distal within the eyecup, as would be expected of a highly visual predator. Onychophora also possess two neuropils, albeit smaller, one distal beneath the photoreceptor layer, the other central20.

As inferred from their position on the head1, the association of anomalocaridid frontal appendages with ganglia anterior to the eye and optic nerves identifies frontal appendages as pre-protocerebral and thus unambiguously distinct from postocular euarthropod appendages such as chelicerae, chelifores or first antennae (crustacean antennules) that originate at the deutocerebrum21,22. The assignment of frontal appendages to the deutocerebrum by forcing their homology with euarthropod antennae23 or a transformation series that proposes anomalocaridid frontal appendages as homologues of megacheiran great appendages and chelicerae4 are thus contradicted.

A pre-protocerebral appendage in L. unguispinus with an associated ganglion that is connected to the anterior surface of the protocerebrum (Figs 2f–h and 3e) is shared with Onychophora but not Euarthropoda. Although the stem-group euarthropod Isoxys has been interpreted as possessing an anomalocaridid-like appendage24, this is likely to originate posterior to the eyestalks and, if so, implies that similarities are convergent. Accommodating Radiodonta on the euarthropod stem lineage demands a transformation of the ancestral pre-protocerebral frontal appendages into another structure. The labrum has been suggested as the most logical candidate for the euarthropod homologue of the anomalocaridid frontal appendages1. Support for this hypothesis is provided by studies of labral development, particularly in the amphipod Parhyale hawaiensis25 where appendicular gene expression is resolved during the embryonic differentiation of the labrum in front of the eyes, followed by a subsequent migration to a tritocerebral location during the development and caudal migration of the stomodeum. Molecular evidence from Tribolium shows the labrum composed of appendicular endites26. Expression of appendage gap genes in mandibulates and chelicerates demonstrates homologous morphogenetic expression in the labrum across all Euarthropoda22. Tracing sensory neurons from the odonate labrum demonstrates a sensory organization typical of jointed limbs27. A pre-protocerebral appendicular ancestry of the labrum1 is reflected across extant Euarthropoda by the presence of discrete paired neuropils that receive its afferents and which are directly connected to the anterior roof of the protocerebrum via an ascending axon tract28. This organization supports an evolutionary shift of the placement of the frontal appendages accompanied by a corresponding shift of their cognate ganglia, which, throughout, have retained their ancestral connection to the most anterior surface of the brain (Fig. 4).

Figure 4: Evolutionary shift of frontal appendage and its ganglia.
figure4

a–c, Proposed transformation of frontal appendages (fa, orange) and cognate ganglia (frg, purple) in Radiodonta (a) to labrum (orange) and cognate ganglia (frg) in a crustacean (c), via a hypothetical stem euarthropod (b) where the mouth has migrated from the first to second cephalic segment (in a segments coloured 1–3; correspondingly in b and c). Axon tracts linking frg and roof of protocerebrum (open arrow) shown in purple. a1, a2: crustacean first and second antennae.

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Methods Summary

All studied specimens of L. unguispinus (YKLP 11304–11306) are deposited at the Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming, China. Digital images of the specimens were captured with Nikon D3X or Leica DFC5000 cameras attached to a Leica M205C photomicroscope and were processed in Adobe Photoshop CS5 using, where appropriate, digital filter functions to absorb specific colours. High-resolution images and geochemical maps were obtained using a LEO 1455VP scanning electron microscope and energy-dispersive X-ray spectroscopy, at 12 kV accelerating voltage and 12 Pa chamber pressure. The final carbon maps were montaged from individual maps with Adobe Photoshop CS5.

Accession codes

Data deposits

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.

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Acknowledgements

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.

Author information

Affiliations

  1. Yunnan Key Laboratory for Palaeobiology, Yunnan University, Kunming, 650091, China

    • Peiyun Cong
    • , Xiaoya Ma
    •  & Xianguang Hou

  2. Department of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, UK

    • Xiaoya Ma
    •  & Gregory D. Edgecombe

  3. Department of Neuroscience, University of Arizona, Tucson, 85721, Arizona, USA

    • Nicholas J. Strausfeld

  4. Center for Insect Science, University of Arizona, Tucson, 85721, Arizona, USA

    • Nicholas J. Strausfeld

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Contributions

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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Extended data figures and tables

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

a, L. unguispinus, YKLP 13304a. b–d, Immature specimens of A. symbrachiata. Scale bars: a, 2.5 mm; b–d, 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: a–c, 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 b–h. 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; c–g, 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) doi:10.1038/nature13486

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