Kazutaka Amano

Bivalves from Cretaceous cold-seep deposits on Hokkaido, Japan

Cretaceous cold-seep deposits of the Yezo Group on Hokkaido, Japan, yield a rich and well-preserved mollusk fauna. The systematics of nine bivalve species previously reported from these deposits can now be reevaluated using newly collected fossils. The fossils include a Cenomanian specimen of Nucinella gigantea with a drill hole possibly made by a naticid, by far the oldest record of a drill hole from a cold seep site. In Japan, Cretaceous seep bivalve assemblages are characterized by (i) the unique occurrence of large specimens of Nucinella (Manzanellidae), (ii) the commonly present nuculid Acila (Truncacila), and (iii) a high diversity of lucinids, possibly as many as four distinct genera. Two new species described are the Albian Acharax mikasaensis (Solemyidae) and the Albian to Campanian Thyasira tanabei (Thyasiridae), of which the former had previously been misidentified as the oldest vesicomyid, the latter as the oldest Conchocele.

Biogeography and the Pleistocene extinction of neogastropods in the Japan Sea

During the middle to late Pleistocene glacial lowstands, when the Japan Sea was almost isolated geographically, low-salinity surface water and anoxic deep water were formed. Pliocene to early Pleistocene buccinid gastropods which mainly lived deeper than the lower sublittoral zone underwent extinction owing to anoxic deep water. On the other hand, upper^sublittoral muricid and buccinid species suffered from extinction due to decrease of surface salinity. However, fossil records indicate that species in the Pacific Ocean east of Japan having shallow minimum depths reinvaded the Japan Sea through shallow, northern straits at least in the latest Pleistocene. The fossil and Recent records of two Buccinum species restricted to the Japan Sea side indicate normal salinity and oxic water at ca. 100^400 m during glacial lowstands. / 2003 Elsevier B.V. All rights reserved.

The Earliest Bathymodiolin Mussels: An Evaluation of Eocene and Oligocene Taxa from Deep-Sea Methane Seep Deposits in Western Washington State, USA

Abstract Bathymodiolin mussels are a group of bivalves associated with deep-sea hydrothermal vents and other reducing deep-sea habitats, and they have a particularly rich early Cenozoic fossil record in western Washington State, U.S.A. Here we recognize six species from middle Eocene to latest Oligocene deep-water methane seep deposits in western Washington. Two of them are new: Vulcanidas? goederti from the middle Eocene Humptulips Formation and Bathymodiolus (sensu lato) satsopensis from the late Oligocene part of the Lincoln Creek Formation. Very similar to the latter but more elongate are specimens from the early Oligocene Jansen Creek Member of the Makah Formation and are identified as B. (s.l.) aff. satsopensis. Bathymodiolus (s.l.) inouei Amano and Jenkins, 2011 is reported from the Lincoln Creek Formation. Idas? olympicus Kiel and Goedert, 2007 was previously known from late Eocene to Oligocene whale and wood falls in western Washington and is here reported from Oligocene seep deposits of the Makah and Pysht Formations. Vulcanidas? goederti occurs at a seep deposit from a paleodepth possibly as great as 2000 m, suggesting that its living relative, Vulcanidas insolatus Cosel and Marshall, 2010, which lives at depths of only 150–500 m, is derived from a deep-water ancestor. The bathymodiolins in western Washington indicate that the group originated at least in the middle Eocene and underwent a first diversification in the late Eocene to Oligocene. Early ontogenetic shells of all fossil species investigated so far, including the middle Eocene Vulcanidas? goederti, reflect planktotrophic larval development indicating that this developmental mode is an ancestral trait of bathymodiolins.

EVOLUTIONARY ADAPTATION AND GEOGRAPHIC SPREAD OF THE CENOZOIC BUCCINID GENUS LIRABUCCINUM IN THE NORTH PACIFIC

Abstract The Early Oligocene to Recent genus Lirabuccinum Vermeij, 1991, is a North Pacific clade of rocky-bottom predatory buccinid gastropods. A re-examination of all available material from eastern Asia and comparison of this material with western American species leads us to recognize four northwestern Pacific species: L. fuscolabiatum (Smith, 1875) from the Pliocene to Recent; L. japonicum (Yokoyama, 1926) from the Pliocene and Early Pleistocene; L. branneri (Clark and Arnold, 1923) from the early Middle Miocene, also known from the Oligocene in the eastern Pacific; and Lirabuccinum sp. from the late Middle Miocene. The genus originated in the eastern Pacific and subsequently spread to the western Pacific by late Early Miocene to early Middle Miocene time. Lirabuccinum exemplifies a common pattern among rocky-bottom North Pacific gastropods in that the early species have a thick, internally strongly ribbed or denticulate outer lip. As they adapted to the colder boreal realm during the Pliocene and Pleistocene, Lirabuccinum and such other clades as Nucella, Ceratostoma, and Ocinebrellus (all Muricidae) evolved thinner, less heavily reinforced outer lips.

An attempt to estimate the surface temperature of the Japan Sea in the Early Pleistocene by using a molluscan assemblage

Abstract In order to estimate the surface temperature of the Japan Sea in the Early Pleistocene, the shallow water molluscan assemblage (Mizuhopecten-Glycymeris assemblage) of the Omma-Manganji fauna was examined by using the HDM characteristic curve ( Ida, 1956 ) and climato-geographic zoning methods. Consequently, the average ocean temperatures of the Japan Sea borderland in Early Pleistocene were estimated to be 0.5-3.0°C lower than the Recent ones. During this time, intermittent invasions of warm current into the Japan Sea frequently took place with the opening of the paleo-Tsushima Strait. However, the obtained data in this study show that the Early Pleistocene climate of Japan Sea was less strongly affected by the warm current than the Recent one.

Miocene abyssochrysoid gastropod Provanna from Japanese seep and whale-fall sites

We describe three Miocene species of Provanna from Japan, two new and one in open nomenclature, that represent the only known fossil examples from whale-falls and a considerable increase in the Miocene diversity of the genus. Provanna hirokoae sp. nov. comes from the latest Middle Miocene Kuroiwa seep site in central Honshu. The shells of this species are mostly recrystallized, but contain relict crossed lamellar microstructures. Provanna alexi sp. nov. is from the early Middle Miocene Shosanbetsu whale-fall site in northwestern Hokkaido, and has well preserved shells comprising an outer simple prismatic layer and an inner crossed lamellar layer. The two Provanna specimens from the Middle Miocene Rekifune whale-fall site, in eastern Hokkaido, are preserved as external moulds only, so are left in open nomenclature. Based on current knowledge, the presence of an outer prismatic layer and an underlying crossed lamellar layer seems to be a common feature in the shells of Provanna, as well as in other genera belonging to the family Provannidae and the superfamily Abyssochrysoidea. Although the oldest occurrence of Provanna was in the Late Cretaceous, the genus did not spread geographically and ecologically until the Miocene (with four, or possibly five species), a date concordant with some molecular estimates. However, this could be an artefact of the fossil record because the known pre-Miocene seep and whale-falls are more geographically restricted than those from the Miocene.

Migration and adaptation of late Cenozoic cold-water molluscs in the North Pacific

Cold-water molluscan fauna originated in the North Pacific in accordance with the worldwide cooling events around the latest Eocene. The westward trans-Pacific migration of the cold-water molluscs occurred during the early to early middle Miocene, owing to shifting climatic belts and “ecological opportunity” rather than current direction. In contrast, the eastward migration of the cold-water molluscs occurred in cool climate ages from the early Oligocene to Holocene. As a result of the Plio-Pleistocene cooling, cold-water species spreaded to the Yellow and the East China Sea through the Japan Sea. Shifting climatic belts thus affected the zoogeographic range of cold-water species.

A New Paleocene Species of Aporrhaidae (Gastropoda) from Eastern Hokkaido, Japan

Abstract. We describe one new aporrhaid species, Kangilioptera inouei sp. nov., from the Paleocene Katsuhira Formation in Urahoro Town, eastern Hokkaido, Japan. This is the first record of a Cenozoic Aporrhaidae (Anchurinae) gastropod in Japan. Occurrences of Kangilioptera are confined to Paleocene deposits in western Greenland and Japan. With the addition of the bivalve Conchocele, the new find requires a reappraisal of the marine connection through the Bering Strait between Japan and Greenland during the Paleocene.

Taxonomy of Large Nuttallia (Bivalvia: Psammobiidae) in the Northwestern Pacific, with Remarks on the Evolution of the Genus

Abstract. The genus Nuttallia can be distinguished from other psammobiids by having a wide hinge plate, a subumbonal pit and the lower limb of the pallial sinus detached from the pallial line. It consists of the Nuttallia petri and N. nuttallii groups, based on the outline and size of shell. Among the large-sized N. petri group, Nuttallia commoda can be separated from the extant species N. petri by having a thin-walled shell, a compressed right valve, a gently sloping dorsal margin, and a lower shell. Moreover, it has been elucidated that N. commoda is one of the characteristic species of the Omma-Manganji fauna, which was confined to the Pliocene and lower Pleistocene deposits in the Japan Sea borderland. From the fossil records, the N. petri group appeared earlier than the N. nuttallii group geologically. Probably it evolved from the Eocene Soletellina, which spread northward taking advantage of the warm climate during the Eocene and adapted to the cooler temperature of the Oligocene in the northwestern Pacific. On the other hand, the N. nuttallii group flourished in the northeastern Pacific during the Miocene to Pliocene and migrated to the northwestern Pacific by the Pliocene.

A new Paleocene species of Bentharca (Bivalvia; Arcidae) from eastern Hokkaido, with remarks on evolutionary adaptation of suspension feeders to the deep sea

Abstract. We describe a new deep-sea arcid species, Bentharca steffeni sp. nov., from the Paleocene Katsuhira Formation in Urahoro Town, eastern Hokkaido. This is the oldest certain record of this genus. Shell microstructure of this new species is similar to the Recent species, B. asperula (Dall, 1891) in having an outer layer composed of thin simple lamellar fibrous prismatic and crossed lamellar structures and an inner layer composed of irregular complex crossed lamellar and irregular prismatic structures. Of these, the simple lamellar fibrous prismatic and the irregular prismatic structures have never previously been recognized even in B. asperula. From the occurrence of the new species, it has been elucidated that the genus Bentharca adapted to the deep sea as a refuge at least in the Paleocene.