Map showing Japanese archipelago, Sea of Japan and surrounding part of continental East Asia in Middle Pliocene to Late Pliocene (3.5-2 Ma, 3.5-2 million years ago).
Kameda Y. &amp; Kato M. (2011). "Terrestrial invasion of pomatiopsid gastropods in the heavy-snow region of the Japanese Archipelago". BMC Evolutionary Biology 11: 118. doi:10.1186/1471-2148-11-118. Figure 5C. Modified: added color of ocean.
Yuichi Kameda &amp; Makoto Kato