The “rostrum”-problem in coleoid terminology - an attempt to clarify inconsistencies

In the coleoid literature, the terminology of shell elements that are deposited on the external surface of the primary shell wall is inconsistent and confusing. Morpho- and phylogenetic interpretations have been therefore controversial. A strict layer-by-layer comparison of seventeen species from seven coleoid subgroups suggests that the coleoid shell is covered by either one (Diplobelida, Spirulida, Sepiida, Vampyropoda) or two (Aulacocerida, Belemnitida, Belemnoteuthidida) outer shell formations. The confusion has been caused mainly by the often ignored presence of a primordial rostrum in the Aulacocerida, Belemnitida, and Belemnoteuthidida. The primordial rostrum is a secondary shell formation, which covers the entire primary shell and, which is itself enveloped by a tertiary shell formation, the rostrum proper. In the Diplobelida, Spirulida, and Sepiida, the primary shell is invested by a single outer formation, the sheath; a rostrum proper is absent in the latter groups. As a secondary shell formation, the aulacocerid, belemnitid and belemnoteuthidid primordial rostrum on the one hand, and the diplobelid, spirulid and sepiid sheath on the other hand are considered to represent homologues. Accordingly, the rostrum proper of the Aulacocerida, Belemnitida, and Belemnoteuthidida are homologues as tertiary shell formations. Outer shell formations in gladius-bearing vampyropods (and teuthids?) might be represented by a single layer. The clarification of the homology of secondary and tertiary shell formations, however, cannot resolve phylogenetic relationships within the Coleoidea.