Letter | Published:

Hallucigenia’s onychophoran-like claws and the case for Tactopoda

Nature volume 514, pages 363366 (16 October 2014) | Download Citation

Abstract

The Palaeozoic form-taxon Lobopodia encompasses a diverse range of soft-bodied ‘legged worms’ known from exceptional fossil deposits1,2,3,4,5,6,7,8,9. Although lobopodians occupy a deep phylogenetic position within Panarthropoda, a shortage of derived characters obscures their evolutionary relationships with extant phyla (Onychophora, Tardigrada and Euarthropoda)2,3,5,10,11,12,13,14,15. Here we describe a complex feature in the terminal claws of the mid-Cambrian lobopodian Hallucigenia sparsa—their construction from a stack of constituent elements—and demonstrate that equivalent elements make up the jaws and claws of extant Onychophora. A cladistic analysis, informed by developmental data on panarthropod head segmentation, indicates that the stacked sclerite components in these two taxa are homologous—resolving hallucigeniid lobopodians as stem-group onychophorans. The results indicate a sister-group relationship between Tardigrada and Euarthropoda, adding palaeontological support to the neurological16,17 and musculoskeletal18,19 evidence uniting these disparate clades. These findings elucidate the evolutionary transformations that gave rise to the panarthropod phyla, and expound the lobopodian-like morphology of the ancestral panarthropod.

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Acknowledgements

The authors are supported by Research Fellowships at Clare College (M.R.S.) and Emmanuel College (J.O.-H.), University of Cambridge, UK. Thanks to J.-B. Caron and T. Harvey for images, access to material and discussions. D. Erwin, K. Hollis and P. Fenton facilitated access to museum specimens, and S. Whittaker assisted with electron microscopy. E. kanangrensis were collected from the Blue Mountains, New South Wales, with assistance from G. Budd and N. Tait and funding from an H.B. Whittington Research Grant (Paleontological Society). N. Butterfield and R. Janssen provided additional material. Parks Canada provided research and collection permits to Royal Ontario Museum teams led by D. Collins. The software TNT is funded by the Willi Hennig Society.

Author information

Affiliations

  1. Department of Earth Sciences, Downing Site, University of Cambridge, Cambridge CB2 3EQ, UK

    • Martin R. Smith
    •  & Javier Ortega-Hernández

Authors

  1. Search for Martin R. Smith in:

  2. Search for Javier Ortega-Hernández in:

Contributions

M.R.S. conceived the project; dissected, described and interpreted specimens; and ran the phylogenetic analysis. J.O.-H. led the integration of developmental data into phylogenetic analysis and the interpretation of results. Both authors contributed equally to data analysis, discussion of results and manuscript preparation.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Martin R. Smith.

Extended data

Supplementary information

PDF files

  1. 1.

    Supplementary Information

    This file contains details of character coding for phylogenetic analysis, analytical methodology, and transformations implied by our most parsimonious tree.

Zip files

  1. 1.

    Supplementary Data

    Zipped archive containing character matrix in Nexus and Excel formats, most parsimonious trees for all values of k in Nexus format (all individual trees, plus MPTs) and human-readable PDF format (MPTs only), and TNT script used to generate trees.

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https://doi.org/10.1038/nature13576

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