Vesicular virgular apparatus in the graptolite Orthograptus calcaratus

A pyritized virgular apparatus (nematularium) has been observed in Orthograptus calcaratus s.l. (Lapworth) from the Pleurograptus linearis Zone in the Ordovician Dicellograptus Shale on Bornholm, Denmark. Specimens preserved in full relief show the free part of the nematularium to be circular in cross section, 0.5 mm in diameter, surrounded by a carbon film, and infilled with pyrite. The surface is covered by rounded elongated hummocks that generally are irregularly arranged but in a few cases lie in one row or are paired. SEM investigations of cross sections reveal that the nematularium has internal carbonized membranes surrounding vesicular tissue. These virgular structures form in the distal part of the thecae portion of the rhabdosome. This new type of vesicular nematularium is unique as earlier observations on other diplograptids showed the nematularia to be single-walled, with two or three vanes. The formation of the vesicular virgular apparatus and its life function is discussed: it most likely had a floating and/or stabilizing function, and may have contained buoyant material, such as vacuolated tissue with fat globules or gas inclusions.