The molecularly defined clade Ecdysozoa1 comprises the panarthropods (Euarthropoda, Onychophora and Tardigrada) and the cycloneuralian worms (Nematoda, Nematomorpha, Priapulida, Loricifera and Kinorhyncha). These disparate phyla are united by their means of moulting, but otherwise share few morphological characters—none of which has a meaningful fossilization potential. As such, the early evolutionary history of the group as a whole is largely uncharted. Here we redescribe the 508-million-year-old stem-group onychophoran Hallucigenia sparsa2,3,4,5,6 from the mid-Cambrian Burgess Shale. We document an elongate head with a pair of simple eyes, a terminal buccal chamber containing a radial array of sclerotized elements, and a differentiated foregut that is lined with acicular teeth. The radial elements and pharyngeal teeth resemble the sclerotized circumoral elements and pharyngeal teeth expressed in tardigrades7,8,9, stem-group euarthropods10,11,12 and cycloneuralian worms13. Phylogenetic results indicate that equivalent structures characterized the ancestral panarthropod and, seemingly, the ancestral ecdysozoan, demonstrating the deep homology of panarthropod and cycloneuralian mouthparts, and providing an anatomical synapomorphy for the ecdysozoan supergroup.
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We thank D. Erwin, M. Florence and P. Fenton for access to material and assistance with collections, G. Kretschmann and S. Whittaker for assistance with electron microscopy, and K. Meechan for assistance with data collection. ROM specimens were collected under Parks Canada Research and Collection permits to D. Collins. TNT is made available with the sponsorship of the Willi Hennig Society. Funding for this research was provided by Clare College, Cambridge (M.R.S.), NSERC Discovery Grant no. 341944 (J.-B.C.) and the ROM DMV Acquisition and Research Fund and Endowment Fund (J.-B.C.). This is ROM Burgess Shale project number 60.
The authors declare no competing financial interests.
Extended data figures and tables
a, b, largest (a, ROM 57169) and smallest (b, ROM 62093) specimens, to the same scale; c, ROM 57168, with enlargements of the anterior (d) and mid-trunk (e). Acronyms as in Fig. 1. Scale bars, 5 mm.
a, ROM 63139, showing posterior body termination; b, c, NMNH 198658, showing posterior termination (see also Fig. 1e); d–g, ROM 63143: e, enlargement of region marked in d; f, g, backscatter SEMs of regions marked in e. Acronyms as in Fig. 1. Scale bars, 5 mm (a–d), 1 mm (e), 0.5 mm (f), 0.1 mm (g).
a, c, ROM 63142: a, composite image incorporating part and counterpart of the entire specimen; c, claw pair. b, d–g, ROM 63051: b, composite image incorporating part and counterpart of the entire specimen; d, anterior section; e, f, eyes; g, claw pair. c–e are backscatter electron micrographs. Acronyms as in Fig. 1. Scale bars, 5 mm (a, b), 500 μm (d), 50 μm (c, f, g), 20 μm (e).
a–d, ROM 61513: a, entire specimen; b–d, enlargements of anterior region, showing mouth opening, aciculae and eyes; mouth opening to right in b, to left in c, d. e, f, ROM 61143: anterior region marked in e is enlarged in f. Acronyms as in Fig. 1. Scale bars, 5 mm (a, e), 1 mm (b, f), 200 μm (c), 20 μm (d).
a, part; b, counterpart, anterior section, showing eyes; c, d, eyes and mouthparts (backscatter SEM); e, f, detail of eyes (counterpart). Acronyms as in Fig. 1. Scale bars, 1 mm (a), 400 μm (b), 200 μm (e), 100 μm (c, d), 20 μm (f).
a–d, NMNH 83935 (holotype): in contrast to body tissue, decay fluids lack a sharp margin and are non-reflective; e, f, ROM 57776, showing full length of appendage one. Acronyms as in Fig. 1. Scale bars, 5 mm.
Acronyms as in Fig. 1. Scale bar, 5 mm.
a–d, NMNH 193996: b, c, enlargements of area boxed in a; c, secondary electron micrograph; d, backscatter electron micrograph of region marked in c. e–g, ROM 63141, showing position of mouth. h–j, ROM 63144: i, secondary electron image of region marked in h; j, backscatter electron image of region marked in i, showing eyes and mouthparts, with interpretative diagram. k–m, ROM 63140: l, backscatter SEM of head, showing right eye and mouthparts (enlarged in m, with interpretative diagram). Acronyms as in Fig. 1. Scale bars, 10 mm (k), 5 mm (a, e, h), 1 mm (b, c, l), 0.5 mm (i), 0.1 mm (d, j, m).
a, ROM 43045, cluster of dissociated specimens; b, ROM 63145, dissociated specimen showing spines in close anatomical position. Acronyms as in Fig. 1. Scale bars, 10 mm.
This file contains Supplementary Notes 1-2, which detail the character coding for phylogenetic analysis, and transformations implied by our most parsimonious tree. (PDF 691 kb)
This table details examined specimens of Hallucigenia sparsa. (XLS 49 kb)
This table details and tabulates measurements recorded from Hallucigenia specimens. (XLSX 181 kb)
This zipped file contains the Phylogeny data files comprising character matrix in Nexus and Excel formats, most parsimonious trees for all values of k and TNT script used to generate trees. (ZIP 188 kb)
Rotating three-dimensional model of Hallucigenia sparsa, depicting anatomical interpretations. By Lars Fields. (MOV 24882 kb)
Reconstruction of Hallucigenia soft anatomy, showing a possible interpretation of walking style and trunk flexibility. Reconstruction by Lars Fields. (MOV 33554 kb)
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Smith, M., Caron, JB. Hallucigenia’s head and the pharyngeal armature of early ecdysozoans. Nature 523, 75–78 (2015). https://doi.org/10.1038/nature14573
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