Protoconch morphology of Conoteuthis (Diplobelida, Coleoidea) and its implications on the presumed origin of the Sepiida

The phylogenetic origin of the Sepiida is still unclear. The presumed derivation from diplobelid belemnoids can be tested by comparison of their initial chambers (protoconchs). The first investigation of a juvenile phragmocone of the diplobelid Conoteuthis hayakawai from the Cenomanian of Hokkaido (northern Japan) yielded surprising insights into the internal protoconch morphology of the Diplobelida. The Conoteuthis protoconch exhibits in many aspects a mosaic of the belemnitid and the spirulid protoconch types. Whereas the sepiid protoconch represents most probably a modification of the spirulid type, the Conoteuthis protoconch is unique in having a closed (imperforated) proseptum. Apart from this, Conoteuthis differs in many other characters from the earliest sepiids (Ceratisepia, Belosepia), which makes a closer relationship unlikely.

► The first record of diplobelid Conoteuthis hayakawai from the Cenomanian.
► The protoconch provides first insights into the morphology of the proseptum.
► Contrary to belemnitids and spirulids, the proseptum lacks a siphuncular opening.
► The Conoteuthis protoconch probably represents a new type of coleoid protoconch.
► The presumed origin of early sepiids from Conoteuthis is unlikely.