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).

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 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.

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).

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