1. Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520, USA
2. Department of Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Ghent, Belgium
3. Present address: School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland.

Correspondence to: Derek E. G. Briggs¹ Correspondence and requests for materials should be addressed to D.E.G.B. (Email: derek.briggs@yale.edu).

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 barnacles^{4, 5}, echinoderms^{6, 7}, molluscs^{3, 8, 9, 10} or annelids^{9, 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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