化石

Recent whale-fall communities from lower sublittoral to bathyal sea environments have been studied in the Pacific and Atlantic Oceans. In contrast, a few fossil whale-fall communities were found from the middle Oligocene to middle Miocene deep-water deposits in Japan and Washington State, USA. It is certain that these communities have acted as dispersal stepping stones for the chemosynthetic species. On the other hand, the hypothesis that the communities acted as evolutionary stepping stones from shallow water to hot vent via cold seep site are controversial. The ecosystem around the whale-bones appeared in the Oligocene. The whale-fall communities are characterized by small mussels as Adipicola and Idas through time. However, other taxonomic composition have changed from the Oligocene to the Recent. The Oligocene communities include some articulate brachiopods and no vesicomyids, while the Miocene communities in Japan share many common genera including vesicomyids with the Recent communities, other than small limpets and other gastropods. For making clear the evolutionary change of whale-fall communities, it is necessary to find more Recent and fossil wood-fall communities as well as whale-fall communities.

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A partial skeleton of Mesodermochelys undulatus Hirayama and Chitoku, 1996 (Testudines: Chelonioidea: Dermochelyidae), with carapace estimated as 1.3m long, was collected from the Late Cretaceous (early Late Campanian) Izumi Group at Shionoe, Takamatsu City, Kagawa Prefecture, Western Japan. This occurrence supports the presence of highly localized sea turtle fauna dominated by Mesodermochelys in the sea adjacent to Japan during the Late Cretaceous.

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Planktonic foraminifera can record the upper ocean environments. For the purpose of the better understanding of the subpolar species Neogloboquadrina pachyderma, plankton tows, IMAGES core and sediment trap samples were analyzed. Plankton tow results indicate that N. pachyderma mainly inhabits below the pycnocline (>20m). On the other hand, N. incompta prefers shallower and warmer waters than N. pachyderma, and its abundance is correlated with the chlorophyll-a concentration around the Japanese Islands. Based on these spatial distributions, the changes in mass accumulation rates (MAR), and coiling ratio of N. pachyderma, the palaeo-environment and water mass structure off Shimokita area (41°33.9'N, 141°52.1'E) were reconstructed during the last 26,900 years. The only Oyashio Current affected the surface and subsurface waters in 26.9-10.6 thousand calendar years before present (cal. kyr BP) with relatively subsurface warming at〜14 cal. kyr BP. The Tsugaru Current started to enter the Pacific about 11.2-10.6 cal. kyr BP and led to the baroclinic conditions that surface and subsurface layers were under the influence of the Tsugaru and Oyashio currents, respectively. Stratification of water column and surface layer warming developed with increasing of inflow of the Tsushima Current and finally the subsurface layer started to warm at about 6 cal. kyr BP. Time-series sediment trap studies were also conducted in the northwestern North Pacific in 1998-2000. Seasonal flux profile of right-coiling N. pachyderma was consistent with that of the left-coiling one at the relatively northern area (50°N), while they were different at the southern area (40°N). It would be caused by the genetic variation of right-coiling N. pachyderma observed in the North Atlantic, and this boundary seemed to be 5% of right-coiling ratio of N. pachyderma.

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Dinoflagellates are one of major microplankton groups in marine coastal environments. Most dinoflagellates have several different stages in their life cycle consisting of motile and resting periods. Since chemical composition of the cyst is sporopollenin-like and resistant to physical, chemical, and biological decay, these cysts are preserved as fossils and are used as biostratigraphic index markers. Under these circumstances, motile cells of dinoflagellates have been mainly studied by biologists, and resting cysts by paleontologists. Therefore, many species which can produce resting cysts have two different names for different stages. Some of their relationships, however, are still unclear. In this study molecular analytical method for dinoflagellate cysts is applied for elucidating the relationships between cysts and motile cells, and to identify uncertain cysts. Through this study, the following two subjects are conducted; (1) The efficiency of molecular analytical method using a single cyst was examined. Pyrophacus steinii of which motile cells and cysts relationships has been clarified by the cyst (= Tuberculodinium vancampoae) incubation experiment was provided. The small subunit rDNA gene sequences of P. steinii cyst was detected and compared with that of the motile cell provided from the gene database. In the result, the base sequences of this cyst corresponded to that of the motile cell except for one undeterminable base. (2) Aspherical Alexandrium-like cyst without any prominent morphological features was identified by this method. The base sequence of this cyst corresponded to these of Alexandrium hiranoi motile cells provided from the gene database expecting undeterminable 14 bases. These results suggest that molecular analytical method is applicable for identification on modern uncertain dinoflagellates cysts. It could be particularly useful for discrimination of morphologically similar cysts such as toxic Alexandrium species.

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