Hiroshi Kurita

Paleocene dinoflagellates from the turtle mountain formation, southwestern Manitoba, Canada

Shallow marine deposits of the Paleocene Turtle Mountain Formation on the northeastern flank of the Williston Basin, southwestern Manitoba, yielded dinoflagellate assemblages which include Senegalinium microgranulatum, S. obscurum, Senegalinium sp., Spinidinium densispinatum, S.? pilatum, Cerodinium speciosum, Isabelidinium viborgense and Phelodinium magnificum. The assemblages indicate that the Turtle Mountain Formation is Early to early Late Paleocene in age. Low species diversity and high dominance of a few species are characteristics of the assemblages and suggest a nearshore depositional environment for the formation. Additional investigation of dinoflagellates of the Cannonball Formation in southcentral North Dakota confirms an early Late Paleocene age which permits correlation of the Cannonball Formation with the upper part of the Turtle Mountain Formation. Distinct similarities are present in dinoflagellate assemblages of Paleocene marine deposits within the Williston Basin. The assemblages also have marked similarities with Paleocene assemblages of the eastern Gulf Coastal Plain, U.S.A., which suggests extension of the Gulf sea into the Williston Basin during the Paleocene.

Trinovantedinium boreale Bujak-dominated dinoflagellate assemblages in Eocene-Oligocene stratified water in northern Japan

Abstract Dinoflagellate assemblages in marine late Middle Eocene to Early Oligocene formations in northern Japan are exceptionally limited in diversity and generally dominated by Trinovantedinium boreale . Other species in the assemblages include Bellatudinium hokkaidoanum Kurita and Matsuoka, sp. nov., Paralecaniella indentata , and Selenopemphix spp. The “restricted flora” developed particularly in Hokkaido, where earlier studies indicated a paleoenvironment of relatively shallow embayment with saline, stratified water. The persistent restriction in species composition was presumably influenced by the stratification and relatively low water temperature. The assemblages were also possibly influenced by high phytoplankton productivity which enhanced the dominance of peridiniacean forms. B. hokkaidoanum Kurita and Matsuoka, sp. nov. and the wide morphological variation of T. boreale are described.

Eocene Mollusca from the Tainohata Formation in Kôbe City, Southwest Japan

Abstract. The Tertiary Tainohata Formation of the Kôbe Group has yielded 10 species of Gastropoda and 19 species of Bivalvia. The formation is of late Middle Eocene age on the basis of the dinoflagellate cyst assemblage and radiometric data. The occurrences of the genera Trinacria, Vicarya, and Sulcobuccinum strongly support an Eocene age for the Tainohata Formation. The molluscan fauna shows high endemism at the species level, whereas it is characterized by Tethyan—Indo-West Pacific genera. Two indigenous molluscan assemblages, Potamides-Cyclina and Corbicula, were discriminated. The composition of these assemblages compares with those of the Neogene intertidal assemblages in embayments. This fact indicates that “temporal parallelism” in the shallow embayment assemblages can be traced back to the late Middle Eocene age in the Japanese Islands. The following 10 new species are proposed herein: Potamides huzitai sp. nov., Cerithideopsilla hondai sp. nov., Crepidula tainohataensis sp. nov., Cantharus (s.l.) andoi sp. nov., Barbatia nodal sp. nov., Trinacria nipponica sp. nov., Trapezium (Neotrapezium) kobe sp. nov., Tellina (Gastranopsis?) sumaensis sp. nov., Corbicula (Corbicula) uejii sp. nov., and Pitar ozakii sp. nov.

New and Mesozoic-relict mollusks from Paleocene wood-fall Communities in Urahoro Town, eastern Hokkaido, northern Japan

Abstract. Five species of bivalves and two species of gastropods are described from late Selandian to earliest Thanetian wood-fall communities from the Katsuhira Formation in Urahoro Town, eastern Hokkaido, northern Japan. Three bivalves and two gastropods are new to science: Thyasira (Thyasira) oliveri Amano and Jenkins, new species, Astarte (Astarte) paleocenica Amano and Jenkins, new species, Poromya katsuhiraensis Amano and Jenkins, new species, Neverita majimai Amano and Jenkins, new species, and Biplica paleocenica Amano and Jenkins, new species. Poromya katsuhiraensis n. sp. and Neverita majimai n. sp. are the earliest records of their genus. Astarte paleocenica n. sp. is the last species before the genus disappeared from the northern Pacific region during the Eocene, only to reappear with the opening of the Bering Strait during the latest Miocene. Moreover, two bivalve species and one gastropod genus are Cretaceous relict forms: Propeamussium yubarense (Yabe and Nagao, 1928), Myrtea ezoensis (Nagao, 1938), and Biplica Popenoe, 1957. These species and other relict protobranch bivalves had wide geographical ranges in the deep sea during the Cretaceous, which helped them to survive the end-Cretaceous mass extinction. The chemosynthesis-based species Bathyacmaea? sp., Myrtea ezoensis, and Thyasira oliveri n. sp. were recovered, but small bathymodioline mussels have not been found. This confirms that the small deep-sea mussels did not appear in the wood-fall communities at least by the earliest Thanetian.