The morphology and fine structure of the Ordovician Cephalodiscus−like genus Melanostrophus

The holotype and a new specimen from the type locality, as well as a few new specimens of Melanostrophus fokini Öpik,1930, an enigmatic invertebrate from the Ordovician of the Baltic region, have been examined using combined LM, SEMand TEM techniques. This form is reinterpreted as a ?cephalodiscid hemichordate. Its skeleton or coenecium is an en−crusting assemblage of uniform zooidal tubes, forming a circular or subcircular palisade−like structure.The zooidal tubesare long (up to 50 mm) and slender, similar to zooidal tubes of the extant pterobranch hemichordate Cephalodiscus (Orthoecus). The fine structure of the skeleton wall is similar to that in graptolites and four components have been recog−nized within periderm: (i) thick, outer cortical layer, (ii) very thin fusellar layer, constructed of annular growth bands,with their oblique sutures arranged randomly, resembling the fusellar layer of some pterobranchs and primitive graptolites, (iii) inner cortical layer, and (iv) thin, enamel−like inner lining. The periderm is abundantly perforated by pitsand holes of different diameters; some of them were probably caused by saprophytic or parasitic borers, but the largestones (up to 100 μm) are probably primary and mark a tube bifurcation. It is concluded that cortex formation is not asynapomorphy for graptolites.