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Hallucigenia’s head and the pharyngeal armature of early ecdysozoans

Smith, M.R.; Caron, J.-B.

Hallucigenia’s head and the pharyngeal armature of early ecdysozoans Thumbnail


J.-B. Caron


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.


Smith, M., & Caron, J. (2015). Hallucigenia’s head and the pharyngeal armature of early ecdysozoans. Nature, 523(7558), 75-78.

Journal Article Type Article
Acceptance Date May 18, 2015
Online Publication Date Jun 24, 2015
Publication Date Jun 24, 2015
Deposit Date Oct 6, 2015
Publicly Available Date Dec 14, 2016
Journal Nature
Print ISSN 0028-0836
Electronic ISSN 1476-4687
Publisher Nature Research
Peer Reviewed Peer Reviewed
Volume 523
Issue 7558
Pages 75-78
Public URL
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