Izumonauta is an extinct genus of shelled octopods from the Mid to Late Miocene of Japan and New Zealand.Kobayashi, T. (1954). Izumonauta, a new genus of the Argonautinae, with a note on their rare but gregarious fossil occurrence. Japanese Journal of Geology and Geography 25: 21-34.
The keels of Izumonauta eggcases lack tubercules and are intermediate in morphology between those of the earlier Obinautilus and later Argonauta.
The New Zealand fossil material was described from the Kapitean Stage (uppermost Miocene).Marshall, B.A. (1971). Izumonauta (Argonautidae, Cephalopoda, Coleoida) from the Kapitean Stage (Uppermost Miocene) of New Zealand. New Zealand Journal of Geology and Geophysics 14: 288-292.
References
Further reading Yanagisawa, Y. 1990. Bull. Geol. Surv. Japan 41 (3): 115-127.