Last week I briefly mentioned a new paper in Nature about an "armored worm" called Plumulites bengtsoni, and I've finally gotten a chance to read a bit more about this strange Ordovician creature. Previously, fossils determined to belong to a group called the machaeridians were found in great abundance, but like conodont teeth before the discovery of a more complete organism, no one was sure what sort of animal the machaeridian parts belonged to. Well, until now, anyway. The discovery of Plumulites bengtsoni reveals that animals once hypothesized to be aberrant barnacles or echinoderms are actually annelid worms, the hard parts (the biomineralized armor) being attached to preserved soft parts that have finally allowed for a resolution of the enigmatic fossils.
While the position of machaeridians within annelids is difficult, the fact that the group falls within the Annelida is clear. Plumulites bengtsoni bears a number of parapodia with chaetae, or (if you're unfamiliar with the terms) pairs of unjointed feet on either side that branch with bristles on the end. For those of you who have seen a sand worm (or other member of the Polychaeta), Plumulites bengtsoni would almost look like an armored version of the green worms that make bubbles at tide line at the shore (this isn't to say that machaeridians were polychaetes, just that they might have had some resemblance to them). Unfortunately the head of P. bengtsoni is missing, but the soft parts that have been preserved have definitely helped to finally resolve their phylogenetic position. Why there are no armored worms today and why these annelids evolved armor in the first place are questions that are still up in the air and will require the study of more exquisitely-preserved fossils (if such wonderful specimens can be found), P. bengtsoni perhaps raising more questions than it has answered.
References; Vinther, J.; Van Roy, P.; Briggs, D. (2008) "Machaeridians are Palaeozoic armoured annelids." Nature, Vol 451, pp. 185-188 doi:10.1038/nature06474
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Fantastic, isn't it? I'm not so sure that machaeridians are crown-group annelids - the paper finds them as such if the sclerites are regarded as homologous with chitinous elytra in Recent aphroditids, but if that state is coded as uncertain in machaeridians the annelids collapse into polytomy.
I've covered this discovery myself, and actually had the first author turn up to comment!