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Yoshida K.,Shinshu University | Iba Y.,The National Museum of Nature and Science | Taki S.,Fuji Consul. Ltd. | Sugawara M.,Shinshu University | And 2 more authors.
Sedimentary Geology | Year: 2010

The discovery of serpentinite-bearing conglomerate including detrital blue amphibole grains in Cretaceous forearc basin sediments in the northern part of Japan supports large tectonic movement in the forearc basin during the Early Cretaceous. Conglomerate and sandstone are intercalated in the Lower Cretaceous offshore mudstone sequence and contain serpentinite clasts, chromian spinels, blue amphiboles, and oolitic grains with chromian spinel nuclei. The conglomerate and sandstones were deposited as a debris flow consisting of an admixture of siliciclastic terrigenous sediments and calcareous biogenic material with serpentinite-derived sediments. The sandstones consist of feldspathic-lithic arenites and wackes, and their composition suggests that most of the terrigenous material was derived from the Cretaceous volcanic terrane and the Jurassic accretionary complex located to the west of the depositional basin. However, the serpentinite body correlated with the provenance of the sediments is not distributed in these terranes. Because the oolitic grains in the matrix of the conglomerate include only angular chromian spinels and serpentine clasts without quartz and feldspar grains, most of the serpentinite-derived sediments are considered to have been supplied primarily from a serpentinite body exposed along the shallow marine environments near the sedimentary basin. Therefore, the serpentine-derived sediments and feldspathic and lithic materials have essentially different sources. Thus, we may infer that the examined sediments are due to large slope failure affecting a relatively wide area of shallow coast involving the place where the serpentine body was emplaced and the place where river-fed terrigenous siliciclastic sediments accumulated. The stratigraphic range of the serpentinite-derived sediments, which are sporadically distributed in the Cretaceous forearc basin of northern Japan, suggests that the serpentinite-derived material was actively supplied to the forearc basin in Albian times. Recent tectono-stratigraphy of the Late Mesozoic accretionary complex in Japan reveals a strike-slip movement in the subduction zone around northeastern Asia. Large earthquakes affected by active strike-slip motion of the oceanic plate are inferred to be the trigger of tectonic movement, such as large slope failure following serpentine intrusion in the forearc region. Another possibility is that subduction of a large number of seamount on the oceanic plate might also cause a large change in basin morphology, bringing about large slope failure. © 2010 Elsevier B.V.

Murakami M.,Waseda University | Shimada C.,Akita University | Shimada C.,Geological Survey of Japan | Hikida Y.,Nakagawa Museum of Natural History | Hirano H.,Waseda University
Journal of Vertebrate Paleontology | Year: 2012

Two new extinct porpoisesArchaeophocaena teshioensis, gen. et sp. nov., and Miophocaena nishinoi, gen. et sp. nov.are described from the upper Miocene Koetoi Formation (5.5-6.4 Ma) of Hokkaido, Northern Japan. The holotype of the former is composed of a partial skull, whereas the holotype of the latter is composed of a partial skull, right periotic, right stylohyoid, and pelvis. Both species are assigned to Phocoenidae on the basis of a unique combination of phocoenid characters: presences of frontal bosses, nasal protuberances, premaxillary eminences, and fossae for the inferior vestibule. However, they do not have dorsally developed preorbital sinus fossae or high premaxillary eminences, unlike derived phocoenids. Furthermore, their premaxillae contact the nasals (or the right premaxilla alone contacts the right nasal), unlike all other known phocoenids except Pterophocaena. A comprehensive morphological cladistic analysis indicates that the two new extinct species are the second-most basal phocoenids next to Pterophocaena and that Phocoenidae is the sister group of Delphinidae. The cranial morphology of the two new extinct species is intermediate in form between that of phocoenids and delphinids, supporting the hypothesis of a sister relationship for the two groups. Thus, these new species fill not only the morphological gap between Phocoenidae and other families of Delphinoidea but also a temporal gap in the fossil record of phocoenids in the western North Pacific. © 2012 Taylor & Francis Group, LLC.

Nishida K.,University of Tokyo | Nakashima R.,Geological Survey of Japan | Majima R.,Yokohama National University | Hikida Y.,Nakagawa Museum of Natural History
Paleontological Research | Year: 2011

We described the shell microstructure of the cold seep-associated bivalve Conchocele bisecta using the acetate peel method and scanning electron microscopy, and revealed complex microstructural changes with ontogeny. The shell of the bivalve has outer, middle, and inner layers that consist of aragonite. The outer layer consists of spherulites and spindles of various sizes and shapes; these microstructures are identified as spherulitic, planar spherulitic, spherulitic prismatic, and spindle-like structures. The middle layer is characterized by cone complex crossed lamellar structure in the outer part and crossed lamellar structure in the inner part. The inner layer is composed of cone complex crossed lamellar, fine complex crossed lamellar, and irregular prismatic structures. On the basis of the observations from the umbo to the ventral margin of each shell layer, we recognized two growth stages that are divided by microstructural distributions in the outer and inner layers and the positions of disturbance rings. The early growth stage is characterized by spherulitic structure in the outer layer, cone complex crossed lamellar structure in the inner layer, and scarce disturbance rings. The late growth stage, on the other hand, has specific microstructures (planar spherulitic, spherulitic prismatic, and spindle-like structures) that lie as sheeted layers within the spherulitic structure of the outer layer, and the disturbance rings are situated close to specific microstructures in the outer layer. In addition, alternations between areas that are cone complex crossed lamellar, thin-layered irregular prismatic, and fine complex crossed lamellar structures are observed in the inner layer. The characteristics of both growth stages indicate that shell growth rate is more variable in the late stage than in the early stage, and that growth rate decreases from the early to late stage, possibly caused by a physiological change. This study suggests that it is important to examine shell microstructural distribution to reveal shell growth and life history. © 2011 by the Palaeontological Society of Japan.

Murakami M.,Waseda University | Shimada C.,Akita University | Shimada C.,Geological Survey of Japan | Hikida Y.,Nakagawa Museum of Natural History | And 2 more authors.
Journal of Vertebrate Paleontology | Year: 2014

The oldest reported fossil record of Delphinidae is from the late Miocene (11 Ma) of California. Reliable Miocene fossil delphinids, however, are few. Eodelphis kabatensis from the upper Miocene Mashike Formation (8.5-13.0 Ma), Hokkaido, northern Japan, is the oldest described Miocene delphinid including a skull. Therefore, this species is a significant clue to understanding the early evolutionary history of Delphinidae. The original taxonomic assignment of this species within the genus Stenella is questionable; thus, we propose a new combination for the species, Eodelphis kabatensis Horikawa, 1977. Eodelphis is a basal delphinid, and comprehensive morphological cladistic analysis, including molecular topological constraints, supported this taxonomic revision. Paleobiogeographic analyses based on the present morphological cladistic analysis and analysis under the molecular constraints suggest that the origin and early diversification of Delphinidae occurred in the middle Miocene Pacific Ocean or elsewhere, respectively. © Mizuki Murakami, Chieko Shimada, Yoshinori Hikida, Yuhji Soeda, and Hiromichi Hirano.

Tanabe K.,University of Tokyo | Hikida Y.,Nakagawa Museum of Natural History
Paleontological Research | Year: 2010

A new Late Cretaceous vampyromorphid coleoid, Nanaimoteuthis yokotai sp. nov. is described on the basis of two lower jaws recovered from the Turonian of Hokkaido, Japan. The lower jaw of this new species consists of equally sized outer and inner lamellae, and the crest portion of the latter is wholly covered by the former. The maximum length of the outer lamella in the holotype attains 63.7 mm, suggesting a remarkably large body size of the living animal. The new specimens reported herein and other occurrences of cephalopod jaws and gladii which were previously described from Hokkaido (Japan) and Vancouver Island (Canada) demonstrate the existence of a comparatively highly diverse coleoid fauna including -in some cases exceptionally large- specimens of the Teuthida, Vampyromorphida, Cirroctopodida, Octopodida, and Sepiida in the Late Cretaceous North Pacific. © by the Palaeontological Society of Japan.

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