Abstract
The systematic affinities of several Palaeozoic skeletal taxa were only resolved when their soft-tissue morphology was revealed by the discovery of exceptionally preserved specimens. The conodonts provide a classic example, their tooth-like elements having been assigned to various invertebrate and vertebrate groups for more than 125 years until the discovery of their soft tissues revealed them to be crown-group vertebrates1. Machaeridians, which are virtually ubiquitous as shell plates in benthic marine shelly assemblages ranging from Early Ordovician (Late Tremadoc) to Carboniferous2, have proved no less enigmatic. The Machaeridia comprise three distinct families of worm-like animals, united by the possession of a dorsal skeleton of calcite plates that is rarely found articulated. Since they were first described 150 years ago3 machaeridians have been allied with barnacles4,5, echinoderms6,7, molluscs3,8,9,10 or annelids9,11,12. Here we describe a new machaeridian with preserved soft parts, including parapodia and chaetae, from the Upper Tremadoc of Morocco, demonstrating the annelid affinity of the group. This discovery shows that a lineage of annelids evolved a dorsal skeleton of calcareous plates early in their history; it also resolves the affinities of a group of problematic Palaeozoic invertebrates previously known only from isolated elements and occasional skeletal assemblages.
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Acknowledgements
S. Bengtson, D. Eibye-Jacobsen and K. J. Peterson commented on the manuscript and cladistic analysis. T. A. Hegna and A. Seilacher are thanked for discussions. E. Lazo-Wasem and C. MacClintock provided access to annelid and machaeridian specimens from the collections of the Yale Peabody Museum of Natural History. Specimen YPM 221134 was provided by M. Ben Said Ben Moula, and YPM 220639 by R. Reboul and V. Reboul. These last three persons, with W. De Winter, B. MacGabhann, B. Van Bocxlaer and D. Van Damme, assisted with fieldwork.
Author Contributions P.V.R. performed field work and acquired the specimens. All authors contributed to the interpretation of the fossils and writing the paper.
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Vinther, J., Van Roy, P. & Briggs, D. Machaeridians are Palaeozoic armoured annelids. Nature 451, 185–188 (2008). https://doi.org/10.1038/nature06474
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DOI: https://doi.org/10.1038/nature06474
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