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Palaeoanatomy and biological affinities of a Cambrian deuterostome (Stylophora)

Abstract

Stylophora are a peculiar extinct group of asymmetrical deuterostomes whose biological affinity has been fiercely debated1,2,3,4,5,6,7,8,9,10,11,12,13,14,15. Disarticulated skeletal elements of a ceratocystid stylophoran recovered from the earliest Middle Cambrian of Morocco are not only the oldest stylophorans in the fossil record, but their exceptional preservation provides crucial data on the microstructure of its skeleton. Stylophoran plates are constructed of a three-dimensional mesh, termed ‘stereom’, identical to that of living echinoderms in which stereom microstructure provides a reliable guide to the nature of the investing soft tissues16,17,18. Using modern echinoderm anatomy to interpret stereom microstructure of stylophoran elements, here we show that the large proximal lumen of their appendage was filled with muscle and that ligamentary tissues bound distal elements firmly together. We find no evidence for a mouth in the proximal lumen and no evidence that the covering plates of the appendage were articulated. Thus, although skeletal structure suggests that stylophorans are echinoderms, their appendage was not a feeding arm but a muscular locomotory organ.

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Figure 1: The stylophoran Ceratocystis.
Figure 2: The stylocone of Ceratocystis sp.
Figure 3: Stereom of the stylocone and distal appendage ossicle of ? Ceratocystis.
Figure 4: Stereom fields in modern echinoderms.

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Acknowledgements

The authors thank J. J. Álvaro for comments on an earlier draft of this paper. Funding was provided by High Lat resources through the European Commission's Access to Research Infrastructure action of the Improving Human Potential Programme and the Swedish Museum of Natural History. Author Contributions The two authors contributed equally to this paper.

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Correspondence to Sébastien Clausen.

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Supplementary Table

Stereoms present in the stylocone and distal ossicles of Ceratocystis. (DOC 24 kb)

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Clausen, S., Smith, A. Palaeoanatomy and biological affinities of a Cambrian deuterostome (Stylophora). Nature 438, 351–354 (2005). https://doi.org/10.1038/nature04109

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