Fossils

Five Miocene relict species were found in the Pliocene embaymental fauna including many boreal species from the Joshita Formation in the northern part of Nagano Prefecture. The occurrences of these species are confined to the upper sublittoral Anadara-Neogenella association. The upper sublittoral zone of cold-water embayment acts as one of the ecological refuges due to the high primary production rate and the lower predation pressure than in the warmer seas.

View full abstract

Stratigraphic ranges of four species Anadara amicula, A. ommaensis, Dosinia japonica and D. tomikawensis in the early Pleistocene Omma Formation were examined in order to evaluate their utility as regional stratigraphic indicators. The results indicate that D. japonica and D. tomikawensis can not be used as stratigraphic indices in the Omma Formation. On the other hand, the last occurrence (LO) of A. amicula and the first occurrence (FO) and LO of A. ommaensis are recognized within the depositional sequences 1, 2 and 6, respectively, in the middle part of the formation. These datums are dated by nannofossils at between 1.45 Ma (the age of the last appearance datum (LAD) of Helicosphaera sellii) and 1.24 Ma (the age of the LAD of Gephyrocapsa large) and may be used as regional biostratigraphic indicators.

View full abstract

The occurrence of Early Triassic radiolarians in black siliceous shale and black chert intercalated in siliceous claystone of the Paleozoic and Mesozoic sedimentary complex in the Kuzu area of the Ashio Terrane is reported in this paper. This radiolarian fauna is characterized by such Spumellarians as Palaeoscenidiidae, Pantanelliidae, and Sponguriidae. The following species are typical representatives in the present fauna, namely; Archaeosemantis venusta Sashida, Parentactinia nakatsugawaensis Sashida, Protopsium sp., Pantanellium (?) virgeum Sashida, Cryptostephanidium longispinosum (Sashida), and Oertlispongus sp. This fauna is correlative with that of the Parentactinia nakatsugawaensis (Pn) Assemblage of Sugiyama (1992) and is assigned to Spathian in age.

View full abstract

Show abstractHide abstract

Based on the stratigraphic investigations of the selected areas of Hokkaido (Fig.1), the successive occurrences of the following ten species of Mytiloides in the mid-Upper Cretaceous sequence have been clarified. The geological ages are determined by the associated ammonites and other well-known species of Inoceramus. They are here abbreviated as UC=Upper Cenomanian, LT, MT and UT=Lower, Middle and Upper Turonian, although the substages are defined tentatively. As to the definition of a species, we depend on current views of authors. In case of an unsettled taxon, we define the species on a number of specimens, admitting a considerably wide range of variation. (1) M. mikasaensis Matsumoto & Noda : UC Zone of Euomphaloceras septemseriatum; (2) M. cf. sackensis (Keller) : low LT; (3) M. columbianus (Heinz) : LT Zone of Pseudaspidoceras flexuosum; (4) M. goppelnensis (Badillet & Sornay) : LT (between 3 and 5); (5) M. mytiloides (Mantell) : mid-LT zone with Mammites sp.; (6) M. labiatus (Schlotheim) : upper LT (maybe lower or higher than 7); (7) M. subhercynicus (Seitz) : high LT; (8) M. hercynicus (Petrascheck) : isolated (highest LT or low MT); (9) M. teraokai (Matsumoto & Noda) : upper MT, Zone of Inoceramus hobetsensis, so far in Southwest Japan; (10) M. incertus (Jimbo) : UT Zone of Subprionocyclus neptuni-I. teshioensis. Some of the above species, e.g. (2), (6) and (8), are rare in the available material and need further hunting. (1) and (9) are so far endemic and should be globally searched for. However, the biostratigraphic succession of the Mytiloides species in our province is generally well correlated with that in the United States. Evolutionary and ecological problems are left for further studies.

View full abstract

Download PDF (6349K)

Register with J-STAGE for free!