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Molluscan and pollen fossils in the mangrove swamp are known from the upper Lower-lower Middle Miocene sediments in Southwest Japan. Pollen analysis was performed for the purpose of finding a fossil mangroves in Northeast Japan. Sixteen samples were collected from two sites of the Middle Miocene Myozawabashi Formation in the Oguni area, Yamagata Prefecture, Northeast Japan. As a result, evergreen Quercus occurred dominantly in the pollen composition of both sites, which is characteristic for the NP-zone of the Neogene pollen zones. In particular, mangrove pollen Kandelia sp. fossils were found in an oyster fossil bed. This is the northernmost record of fossil mangroves in Japan. The living mangrove Kandelia candel L. has a northern limit at South Kyushu Kiire, Kagoshima Prefecture , and so the average winter January temperature of Oguni town in the early Middle Miocene period is estimated to have been at least higher than today. Since a mangrove swamp-dwelling bivalve Geloina was recently found in northern Iwate Prefecture, the mangrove pollen may be found in more northern places than the Oguni area.

Molluscan and plant fossils from ancient mangrove swamps are known from upper Lower to lower Middle Miocene sediments in Southwest Japan. Given that the mangrove-swamp-dwelling bivalve Geloina was recently found in the Tate Sandstone Member of the Kadonosawa Formation in the Ninohe district, northeastern Japan, it was expected that mangrove pollen would also be found at this site. To investigate this possibility, samples were collected from the Tate Sandstone Member at two sites Tate and Kitadate . The pollen composition at both sites is characteristic of the NP-zone of Neogene pollen zones Yamanoi a . Mangrove pollen Kandelia sp. fossils were first found in the uppermost part of the Tate Sandstone Member at the Kitadate site, representing the northernmost record of Miocene fossil mangrove in Japan. Based on presentday mangrove assemblages and distribution, the average wintertime temperature in the Ninohe area at the time represented by the pollen fossils is estimated to have been about higher than that of today.CThe Geological Society of Japan 2010 114

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