Siphuncular structure in the orders Tarphycerida and Barrandeocerida (Cephalopoda: Nautiloidea)

The siphuncular structure is described in two Silurian taxa, Boionautilus tyrannus and Cumingsoceras complanatus, currently placed in the Tarpycerida. Tarphycerids have the Nautilus type of connecting ring that is composed of an outer, thick, spherulitic-prismatic layer and an inner glycoprotein layer, the latter was destroyed by diagenesis. However, both Silurian specimens have the connecting ring of the calcified-perforate type, previously known to occur in orthocerids, actinocerids, plectronocerids and now also in barrandeocerids. In this type, the inner layer of the connecting ring is calcified and perforated by pore canals. Boionautilus and Cumingsoceras are therefore classified with barrandeocerids and not with tarphycerids