Anatomically well-preserved permineralized seeds of a new species, Decodon mosanruensis sp. nov. (Lythraceae), were obtained from the late Middle Miocene Shimokawa Group, Shimokawa, central Hokkaido, Japan. Based on the seed shape and three well-developed integumentary zones, the fossil seeds were compared with fossil and extant taxa and were found to most closely resemble those of the only extant species, Decodon verticillatus, which is restricted to the subtropical and temperate regions of eastern North America. A cladistic analysis using seven seed characters of the fossil and living representatives of Decodon indicate that D. mosanruensis is most closely related to the extant D. verticillatus. The occurrence of D. mosanruensis in lacustrine sediments, together with Glyptostrobus, Alnus, Osmunda, and plants commonly growing in mesic habitats, provides compelling evidence that this species grew in wetlands associated with a lake and represents part of a fairly autochthonous flora. Based on the distribution patterns of fossil representatives of the genus, it appears that Decodon migrated from western North America into Europe via the more southern North Atlantic Thulian route sometime prior to late Eocene. By Oligocene, the genus had become widely distributed throughout eastern Siberia. It appears that D. mosanruensis probably evolved from the North American population and migrated into northern Japan through Beringian Corridor sometime prior to the Miocene.
During the later half of the Last Glacial Age (about 30,000-10,000 years ago), boreal conifer forests mainly consisting of Picea dominated in Japan, and much amounts of fossil cones of Picea have been discovered from peaty sediments at many sites. Many of these cones have been identified to extant species, i.e., Picea glehnii (Fr. Schm.) Masters, P. koyamae Shirasawa or P. shirasawae Hayashi, or to extinct taxa, i.e., P. cf. shirasawae Hayashi or P. tomizawaensis Suzuki by various authors. To clarify direct derivatives of fossil taxa and changes in their morphology and distribution after the Last Glacial Age, we studied and analyzed the variation in cone morphology of an extant taxon, P. glehnii, because this species has much morphological variation over a wider distribution range of Sakhalin, Hokkaido and Northern Honshu. Cone morphology of P. glehnii varies widely in size of cones, and in size and shape of cone scales: 1) cone size is 37.7-77.3 (mean 57.5) mm long and 14.5- 22.1 (mean 18.3) mm wide, 2) scale size is 11.0-16.2 (mean 13.6) mm long and 8.8-12.8 (mean 10.8) mm wide, 3) shape of scales varies from fan-shape to rhomboidal, and 4) undulation of the top margin of cone scales varies from smooth (grade I) to heavily undulate (grade V). These variations in cone morphology are consistent within individuals but quite variable among different individuals within each population. No variational gradients were found along the distribution range.