Bohemograptus papilio sp. nov. three-dimensionally preserved monograptid (Graptolithina) with an adaptation to retard sinking, from the Upper Silurian Poland

An unexpected new form of Bohemograptus with unusual morphological structure has been found in the upper part of the Bohemograptus praecornutus Biozone, Upper Ludlow of the Mielnik borehole, Poland. Bohemograptus papilio sp. nov. has the characteristic rhabdosome for Bohemograptus, in addition to extraordinary paired latero-dorsal additions, not comparable to any processes typical for Bohemograptus thecal openings. The new structure, a patagium, is located in the proximal part of the rhabdosome, and is built from main rods dividing repeatedly into thinner lists with significant remnants of membranes spread between them. Another new structure is the virgellarium, a membranous addition to the virgella. Membranes of the patagium and virgellarium are not built from microfusellar tissue as additions in other bohemograptids. It is suggested that the membranous additions were orientation devices, constructed to retard the sinking of the colony in the water column.