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A palaeontological solution to the arthropod head problem

Abstract

The composition of the arthropod head has been one of the most controversial topics in zoology, with a large number of theories being proposed to account for it over the last century1. Although fossils have been recognized as being of potential importance in resolving the issue2,3, a lack of consensus over their systematics4,5 has obscured their contribution. Here, I show that a group of previously problematic Cambrian arthropods from the Burgess Shale and Chengjiang faunas form a clade close to crown-group euarthropods, the group containing myriapods, chelicerates, insects and crustaceans6. They are characterized by modified or even absent endopods, and two pre-oral appendages. Comparison with reconstructions of the crown-group euarthropod ground plan6 and recent investigations into onychophorans7,8 demonstrates that these two appendages are the first antenna (of extant crustaceans) and a more anterior appendage associated with an ocular segment. The latter appendage has been reduced in all crown-group euarthropods. Its most likely relic is as a component of the labrum9. These fossils thus tie together results from disparate living groups (onychophorans and euarthropods).

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Acknowledgements

This work grew out of a collaboration with R. A. Dewel, whom I thank for discussions that helped shape many of the ideas contained herein. D. Waloszek and J. Eriksson have also provided much help and insight. I also thank J. Bergström, H. Xianguang, D. Collins and D. Erwin for allowing access to material. The comments of G. Edgecombe substantially improved the paper. This work was funded by the Swedish Research Council (VR).

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Supplementary information

Character analysis and data matrix including supplementary table 1 (PDF 24 kb)

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Further reading

Figure 1: Cladistic analysis (see Supplementary Information for details) of upper stem-group euarthropods, with schematic drawings of the pre-oral region of selected taxa.
Figure 2: Scheme for arthropod head segmentation implied by this analysis.

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