The morphology and fine structure of the Ordovician
Cephalodiscus-like genus  Melanostrophus
Acta Palaeontologica Polonica 2004, 4, 519-528
The holotype and a new specimen from the
type locality, as well as a few new specimens
Melanostrophus fokini Opik, 1930, an
enigmatic invertebrate from the Ordovician of
the Baltic region, have been examined using
combined LM, SEM and TEM techniques. This
form is reinterpreted as a ?cephalodiscid
hemichordate. Its skeleton or coenecium is an
encrusting assemblage of uniform zooidal
tubes, forming a circular or subcircular
palisade-like structure. The zooidal tubes are
long (up to 50 mm) and slender, similar to
zooidal tubes of extant pterobranch
Cephalodiscus (Orthoecus). The
fine structure of the skeleton wall is similar to
that in graptolites and four components have
been recognized 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
abundatly perforated by pits and holes of
different diameters; some of them were
probably caused by saprophytic or parasitic
borers, but the largest ones (up to 100
micrometers) are probably primary and mark a
tube bifurcation. It is concluded that cortex
formation is not a synapomorphy for