Obinautilus Explained

Obinautilus is an extinct genus of shelled cephalopod that has been variously identified as an argonautid octopod^{[1] [2] [3] [4]} or a nautilid.^{[5] [6] [7] [8]} It is known from the Late Oligocene to Pliocene of Japan.^[9] 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.^[10]

Species

Two species are recognised in the genus.

• †Obinautilus awaensis (originally described as Argonauta awaensis^[11]) — 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.
• Yanagisawa, Y. (1990). Web site: Age of fossil Argonautidae (Cephalopoda) from Hokuriku Province (central Honshu, Japan) based on diatom biostratigraphy. . dead . https://web.archive.org/web/20070927010041/http://www.gsj.jp/Pub/Bull/vol_41/41-03_02.pdf . 2007-09-27 . Bulletin of the Geological Survey of Japan 41(3): 115–127.

Notes and References

1. Ozawa, T. & S. Tomida (1996). Occurrence of Aturia coxi (Cephalopoda; Nautilida) from the uppermost Miocene of Japan and its implication for late Miocene marine climate in the northwestern Pacific. Journal of Paleontology 70(5): 795–798.
2. Martill, D.M. & M.J. Barker (2006). A paper nautilus (Octopoda, Argonauta) from the Miocene Pakhna Formation of Cyprus. Palaeontology 49(5): 1035–1041.
3. Strugnell, J.M., A.D. Rogers, P.A. Prodöhl, M.A. Collins & A.L. Allcock (2008). The thermohaline expressway: the Southern Ocean as a centre of origin for deep-sea octopuses. Cladistics 24(6): 853–860.
4. Strugnell, J. & A.L. Allcock (2010). Co-estimation of phylogeny and divergence times of Argonautoidea using relaxed phylogenetics. Molecular Phylogenetics and Evolution 54(3): 701–708.
5. Ward, P. (1980). Comparative shell shape distributions in Jurassic–Cretaceous ammonites and Jurassic–Tertiary nautilids. Paleobiology 6(1): 32–43.
6. Teichert, C. & T. Matsumoto (2010). The Ancestry of the Genus Nautilus. In: W.B. Saunders & N.H. Landman (eds.) Nautilus: The Biology and Paleobiology of a Living Fossil. Springer. pp. 25–32.
7. http://umdb.um.u-tokyo.ac.jp/DKoseibu/specimens/en/08493_.html UMUT CM 08493: Obinautilus pulchra Kobayashi, 1954
8. Eyden, P. (2010). Fossil Octopuses. TONMO.
9. Saul, L.R. & C.J. Stadum (2005). Fossil argonauts (Mollusca: Cephalopoda: Octopodida) from Late Miocene siltstones of the Los Angeles Basin, California. Journal of Paleontology 79(3): 520–531.
10. Strugnell, J., J. Jackson, A.J. Drummond & A. Cooper (2006). Divergence time estimates for major cephalopod groups: evidence from multiple genes. Cladistics 22(1): 89–96.
11. Tomida, S. (1983). Two new fossil Argonauta and firstly discovered Aturia coxi Miller from the late Tertiary of Boso Peninsula, Japan. Muzunami-shi Fossil Museum Bulletin 10: 107–116.