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 vertebrates(1). Machaeridians, which are virtually ubiquitous as shell plates in benthic marine shelly assemblages ranging from Early Ordovician ( Late Tremadoc) to Carboniferous(2), 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 ago(3) machaeridians have been allied with barnacles(4,5), echinoderms(6,7), molluscs(3,8-10) 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.