Obinautilus is an extinct genus of shelled cephalopod that has been variously identified as an argonautid octopod or a nautilid. It is known from the Late Oligocene to Pliocene of Japan. 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.

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.

Species

Two species are recognised in the genus.

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

Further reading

  • Noda, H., K. Ogasawara & R. Nomura (1986). Systematic and paleobiogeographic studies on the Japanese Miocene argonautid “Nautilus” izumoensis. University of Tsukuba, Institute of Geoscience, Science Reports, Section B, Geological Sciences 7: 15–42.
  • (in Japanese) Yanagisawa, Y. (1990). (PDF). Archived from (PDF) on 2007-09-27. Bulletin of the Geological Survey of Japan 41(3): 115–127.