An early Eocene Antarcticeras nordenskjoeldi: the analysis of the ‘oegopsid coleoid’ hypothesis

The Ypresian Antarcticeras Doguzhaeva has an internal conch and a central siphuncle that have been recently found also in the Late Devonian longicone. The Antarcticeras-related concept of twofold origin of endocochleates - Coleoidea and Paracoleoidea - is relied on the assessment that dissimilar morphology, first of all, marginal or central siphuncle, indicates their bactritoid or orthoceroid ancestries The hypothesis has been however suggested that the conch of Antarcticeras presents a conus of a gladius and, hence, Antarcticeras is a coleoid. This view has necessitated several assumptions: migration of marginal siphuncle to a central position; lifetime absence of connecting rings; rudimentary septa; the unable for buoyancy regulation conch; presence of siphuncle and septa in the apical conuses of gladii. It is shown in the present paper that the conuses of the present day squid Berryteuthis lack siphuncles and septa. In the light of new data on inner structure of the conus and lack of the reliable morphological data for the interpretation of the Antarcticeras conch as a conus of a gladius, the hypothesis on the teuthid affiliation of Antarcticeras is highly impossible.