Hi all,
I confess ignorance of the Machaeridia fossil group till yesterday. Historically, Machaeridians have been most often regarded as echinoderms or possibly descendants of echinoderm ancestors. But, according to a paper in Nature earlier this month, these disarticulated calcitic shell plates which are common in ancient marine deposits seem to be parts of annelids. The authors thus have added a new (fossil) Class to Phylum Annelida, an event which seems to be worth a mention on this list!
Caron, J.-B. 2008: Palaeontology: Ancient worms in armour [News and Views]. Nature 451: 133-134.
http://dx.doi.org/10.1038/451133ahttp://www.nature.com/nature/journal/v451/n7175/full/451133a.html
Vinther, J.; Van Roy, P. ; Briggs, D. E. G. 2008: Machaeridians are Palaeozoic armoured annelids. Nature 451(7175) 10 January: 185-188. Reprint requests to: derek.briggs from yale.eduhttp://www.nature.com/nature/journal/v451/n7175/full/nature06474.htmlhttp://dx.doi.org/10.1038/nature06474http://www.nature.com/nature/journal/v451/n7175/suppinfo/nature06474.html
"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. Machaeridians, which are virtually ubiquitous as shell plates in benthic marine shelly assemblages ranging from Early Ordovician (Late Tremadoc) to Carboniferous, 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 machaeridians have been allied with barnacles, echinoderms, molluscs or annelids. 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."
Inter alia Vinther et al state that "The inner shell plates in machaeridians are offset and overlap along the median line, as do the elytra of some scale worms. The elytra of scale worms, however, occur only on every other segment, whereas the shell plates of machaeridians appear to be present on every segment." However, this comment is not correct. Some Pholoididae and Sigalionidae have many consecutive elytra in the posterior body, and all scale worms have adjacent elytra on segments 4-5.
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Geoff Read <g.read from niwa.co.nz>
http://www.annelida.net/http://www.niwascience.co.nz/ncabb/