Cephalic and appendage morphology of the Cambrian arthropod Sidneyia inexpectans |
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Affiliation: | 1. Early Life Institute, The Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi''an 710069, China;2. Museum für Naturkunde, Leibniz Institute for Research on Evolution and Biodiversity at the Humboldt University Berlin, D-10115 Berlin, Germany;1. Division of Earth Sciences, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia;2. Department of Earth Sciences, The Natural History Museum, London SW7 5BD, UK;3. School of Natural and Built Environments, University of South Australia, Mawson Lakes, SA 5095, Australia;1. Palaeoscience Research Centre, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia;2. Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada |
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Abstract: | Sidneyia inexpectans Walcott, 1911 from the Cambrian Series 3 Burgess Shale of British Columbia is largely accepted as a representative of the artiopodans, an assemblage of Paleozoic arthropod taxa, including trilobites and their immediate relatives. Its appendage morphology was never fully understood, but the exopod seemed to differ from that of other artiopodans, except for the shared presence of lamellae. The head was considered to comprise only the ocular and antennular segments, these being covered entirely on the ventral side by a large doublure. This short head was often taken as an evidence for variability of head segment counts in Cambrian arthropods, and to falsify the hypothesis of a head with three postantennular segments in the euarthropod ground pattern. Restudy of a substantial amount of material of S. inexpectans shows that previous interpretations of a short head were based on taphonomically deformed specimens, where the head was either partly folded, or entirely flipped under the thorax, resulting in the dorsal shield being mistaken for an extensive doublure. Rather than an extensive doublure, there is a broad hypostome, and the head comprises ocular, antennular, and at least two postantennular appendage bearing segments. The appendage morphology is shown to be consistent with artiopodan affinities. The exopod is of the bilobate flap-like type with lamellae inserting on the proximal portion, earlier proposed as a potential autapomorphy of Artiopoda. Reinforcement of artiopodan affinities for S. inexpectans and reinterpretation of its head reconciles this species with current understanding of arthropod phylogeny and head segmentation. |
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Keywords: | Euarthropoda Head segmentation Appendage morphology Cambrian Burgess Shale |
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