Obinautilus

Obinautilus is an extinct genus of shelled cephalopod that has been variously identified as an argonautid octopod^{[3][4][5][6][7][8]} or a nautilid.^{[1][9][10][11][12]} It is known from the Late Oligocene to Pliocene of Japan.^[4] The shell is discoidal and very involute, with rapidly expanding and compressed whorls, fine radial ribs, a rounded venter with a shallow furrow, and almost closed umbilicus.

Obinautilus Temporal range: Oligocene – Pliocene
Obinautilus pulchra
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Order:	Octopoda
Family:	Argonautidae
Genus:	†Obinautilus Kobayashi, 1954[1]
Species
†Obinautilus awaensis (Tomida, 1983)[2] †Obinautilus pulcher Kobayashi, 1954[1]

Based on the examination of O. pulcher fossils from the Oligocene, the Tremoctopus–Argonauta divergence has been estimated to have occurred at least 29 million years ago.^[5]

Species

Two species are recognised in the genus.^[4]

• †Obinautilus awaensis (originally described as Argonauta awaensis^[2]) — Pliocene (possibly also latest Miocene^[4]) Upper Senhata Formation of Bōsō Peninsula, Japan^[6]
• †Obinautilus pulcher (sometimes spelled O. pulchra^[5][11]) — Late Oligocene Nichinan Formation of Japan^[6]