化石

Permineralization is a particular fossilization process that preserves the anatomical details of plants from the past, sometimes with external features. In extremely well preserved material, even motile sperm released from pollen tubes can be found in the ovule of Permian gymnosperms. Outcrops bearing permineralized plants are not unusual in Japan because of the presence of various mineral sources that enhance permineralization, such as volcanic ash and calcium-rich marine deposits. Among various localities producing permineralized plant remains in Japan, Hokkaido is a well-known and studied area. This paper introduces selected permineralized plant fossils mostly from the Cretaceous Yezo Supergroup of Hokkaido in order to summarize the scientific contributions of earlier studies on these materials, and also to expand general interest in permineralized fossils and promote further studies. The Cretaceous materials of Hokkaido not only preserve a wide range of vascular plants, but also contain various symbionts such as fungi and tissue-boring insects. Plant permineralized fossils present an avenue to the diverse biology of the past.

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Foraminiferal fossils from the Pliocene Tsukushimoriyama Formation and lower Pleistocene Daishaka Formation in the southeastern Tsugaru Peninsula, Northeast Japan, are examined. Twelve species of planktonic foraminifers were identified among 13 samples. The occurrence of Neogloboquadrina pachyderma (dextral) and a radiometric age of the Otakizawa Tuff Member in the Tsukushimoriyama Formation suggest that the geological age of the formation is the Late Pliocene. The Daishaka Formation is assigned to the Lower Pleistocene on the basis of coiling direction of N. pachyderma and K-Ar ages of the formation. One hundred and sixteen species of benthic foraminifers were identified among 20 samples. Q-mode cluster analysis shows that these samples are divided into four clusters. The samples from the Tsukushimoriyama Formation belong to Clusters 1 and 2. The stratigraphic distributions of these clusters suggest that the middle and upper parts of the Tsukushimoriyama Formation deposited on upper bathyal zone in cool- and mild-temperate areas, respectively. The samples from the Daishaka Formation belong to Clusters 2, 3 and 4. Their stratigraphic distributions indicate that the formation changed in the depositional depth from middle to outer sublittoral zone to upper bathyal zone. Although the most of the formation was deposited under cool- or mild-temperate condition, a short interval in the upper part was affected by warm current. This influence might represent one of the intermittent inflows of the paleo-Tsushima Warm Current to the Japan Sea.

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This paper reports on a faunal change of Middle Jurassic (Bajocian) radiolarians from manganese carbonate nodules in the Inuyama area, Mino Terrane. Five radiolarian assemblages, distinguished from five horizons (IN-16, IN-10, IN-7, IN-3 and IN-1 in ascending order) of the Unuma section, are well-preserved and extremely diversified in specific composition. About 300 species of radiolarian fossils are obtained from each manganese carbonate nodule. Approximately two thirds of radiolarian species are composed of common species between two horizons. The value of Spumellaria/Nassellaria (S/N) ratio is high in the lowermost horizon (IN-16) and low in the uppermost horizon (IN-1). Although the extinction and origination rates of radiolarian species are totally constant through the section, the origination of nassellarian species in the uppermost horizon shows a high rate. On the basis of these faunal analyses, the radiolarian faunal change was constant during Bajocian time except for a little change of the S/N ratio and the origination rate of nassellarian species. It is suggested that there was not the large oceanic environmental change during Bajocian time in the western part of the Panthalassa.

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Various-sized and -shaped calcareous concretions occur abundantly in the Lower Miocene Kokozura Formation of the Takaku Group which crops out on the Izura coast and its nearby north, Kita-Ibaraki City, north of Ibaraki Prefecture. The Kokozura Formation is mainly composed of intensely bioturbated, very fine-grained muddy sandstone, which may have been deposited on the outer shelf of the Joban forearc basin. The calcareous concretions are divided into the following six types on the basis of their shapes, sizes and modes of occurrence : Type I (large-sized concretions with undulatory surface, 1 to 6m in diameter), Type II (smaller, less than a few decimeters in diameter, and irregular-shaped concretions closely associated with Type I), Type III (pipe-shaped hollow concretions, a few decimeters in diameter and less than 2m in length), Type IV (sheet to lenticular concretions, 10 to 20cm in thickness and a few meters in lateral length), Type V (burrow-fill sand concretions concentrated and connected to each other in very fine-grained sandstone) which grades upward into Type VI (concreted sandstone bed, 2 to 3m in thickness, including abundant burrows). In the lower part of the outcrop, the Types I and II are dominant and commonly associated with Type III, whereas the uppermost part consists of Types V and VI which developed into a huge concreted bed, 2 to 3m thick, over 50m wide and 100m long. ^<13>C-depleted carbon isotopic compositions (-20.9 to -29.2‰ vs PDB) of Types I and II suggests that these concretions were formed under the influence of methane seepage. Molluscan assemblages dominated by Lucinoma acutilineatum are associated with the Type I, II, and III concretions. Most of L. acutileatum shells keep their life position. The mode of fossil occurrence indicates that L. acutileatum formed a chemosynthetic community and used methane seepage through chemosynthetic bacteria for their energy source.

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