First post-Miocene Argonauta from Japan, and its Palaeontological Significance

A fossil of Argonauta hians Lightfoot, 1786, an epipelagic octopod species living now in tropical and subtropical oceans, was obtained from the upper Pliocene Dainichi Formation of the Kakegawa Group, central Japan. Its occurrence accompanied by abundant tropical and subtropical benthic molluscs indicates the influence of the warm ocean current in the mid-latitude of NW Pacific during the Late Pliocene warm episode (c. 2 Ma). This is the first record of argonautid species from a post-Miocene Japanese formation and marks the first appearance of a living Argonauta species in the NW Pacific. In the Middle Miocene, many argonautid species flourished around the Japanese Islands, but none are known from the post-Miocene as a result of the Late Miocene intensified Antarctic glaciation (Robin, 1988) and the terminal Miocene cooling event (Beu, 1990). The genus Argonauta is the only surviving genus known from the Pliocene to the present. The fossil occurrence of Argonauta hians from the Japanese Pliocene suggests that this surviving lineage expanded its distribution into the mid-latitude NW Pacific during the Pliocene warm climatic event.