Machaeridians are marine Palaeozoic fossils, known from sediments of early Ordovician to mid Permian age (Bengtson, 1978; Cooper and Grant, 1972; Kaasa, 1992). Rare, completely articulated specimens reveal that they were long, slender, bilaterally symmetrical animals, covered by a dorsal scleritome comprising longitudinally arranged series of calcite sclerites (e.g., Adrain et al., 1991; Högström and Taylor, 2001a, 2001b). However, most known machaeridians, including those described in this paper, comprise isolated disarticulated sclerites. Until recently, the taxonomic affinities of the group were uncertain, and historically they were assigned to a number of different phyla (e.g., see Adrain, 1992). However, the recent account of an exceptionally preserved specimen with soft tissue anatomy, from the Lower Fezouata Formation (Lower Ordovician, Tremadocian) of Morocco, now indicates that machaeridians were annelid worms (Vinther et al., 2008).