Yoshinori Hikida

An exceptionally well-preserved fossil seep community from the Cretaceous Yezo Group in the Nakagawa area, Hokkaido, northern Japan

Abstract A well-preserved fossil seep community has been found in a carbonate lens in the Santonian to Campanian Omagari Formation, Upper Yezo Group in the Nakagawa region, Hokkaido, north Japan. The carbonate lens (roughly ellipsoidal in plan view with a diameter of 10 m × 6 m, and a thickness of about 5 m) is composed mainly of various types of high-Mg calcite containing several to 10 mol% magnesium and little iron or manganese. The carbonate lens is divided into an upper tube worm-dominated boundstone and a lower carbonate breccia facies. In the boundstone facies, concentric cements occur in the vestimentiferan tubes, indicating that the worm tubes were conduits for seepage. Layered to veinlike precipitates of high-Mg calcite occur in the boundstone facies. The carbonate breccia facies contains clast-supported carbonate breccia with sideritic, silty and tuffaceous matrices. Chemosynthetic bivalves occur in the upper zone of the carbonate breccia. The most common of these is the lucinid Miltha sp. Others include the lucinid Thyasira sp., and vesicomyid Calyptogena. Many small molluscs occur in the matrices of the carbonate breccia. The most common of these are trochid archaeogastropods; the others are two acmaeid limpets, mesogastropods and nuculacean bivalves. Small terebratulid brachiopods are also common. The carbonate lens, with its chemosynthetic bivalves and vestimentiferan worm tubes, may have been formed by bacterial sulfate reduction and anaerobic methane oxidation, as it shows extreme 13C-depletion (δ13C = −41 to −45‰). The Omagari community resembles the modern cold-seep communities along the landward slope of the subduction-zone complex off the Pacific coast of Japan.

Two new extinct basal phocoenids (Cetacea, Odontoceti, Delphinoidea), from the upper Miocene Koetoi Formation of Japan and their phylogenetic significance

ABSTRACT Two new extinct porpoises—Archaeophocaena 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.

A new basal porpoise, Pterophocaena nishinoi (Cetacea, Odontoceti, Delphinoidea), from the upper Miocene of Japan and its phylogenetic relationships

ABSTRACT Pterophocaena nishinoi, gen. et sp. nov. (Phocoenidae), from the upper Miocene Wakkanai Formation (9.2–9.3 Ma) in Hokkaido, northern Japan, is described. This is the oldest fossil phocoenid in the western North Pacific, thus extending the fossil record of Phocoenidae an additional approximately 4 million years in the region compared with the previous record. The holotype, composed of a partial skull, with right periotic and tympanic bulla and a postcranial skeleton, has the following apomorphic characters: absence of the premaxillary eminence; posterior half of the hamular process of the pterygoids not separated by palatines; prominent dorsolaterally projecting premaxilla in the facial area; supraorbital process of the maxilla steeply sloping ventrolaterally; posteriorly protruding temporal crest; anteroventrally narrowed and inverted teardrop-shaped temporal fossa; and several short ridges on the tympanosquamosal recess of the squamosal. A comprehensive cladistic analysis (66 ingroup taxa and 278 morphological characters) indicates that Pterophocaena is the most basal phocoenid yet discovered, and that Phocoenidae and Delphinidae are closely related. The discovery of Pterophocaena leads to a review of previous hypotheses for the origin of Phocoenidae, and indicates that phocoenids had already diversified in the Pacific Ocean by late Miocene.

Gastropods from Late Cretaceous Omagari and Yasukawa Hydrocarbon Seep Deposits in the Nakagawa Area, Hokkaido, Japan

Sixteen gastropod species from two Campanian (Upper Cretaceous) hydrocarbon seep localities in Hokkaido, Japan are described. Seven species are new: the acmaeid limpet Serradonta omagariensis; three turbinids: Homalopoma abeshinaiensis, Cantrainea yasukawensis, and C. omagariensis; the trochid Margarites sasakii; the seguenzioid Cataegis nakagawensis; and the provannid Provanna nakagawensis. The most common species in the investigated localities are acmaeid limpets (S. omagariensis), tiny turbinids (H. abeshinaiensis, C. yasukawensis, C. omagariensis), and provannids/hokkaidoconchids (P. nakagawensis and Hokkaidoconcha hikidai). The Upper Cretaceous associations described here do not resemble Lower Cretaceous associations known from other regions but are composed of species similar to gastropods from Recent hydrocarbon seeps and hydrothermal vents in the Northwestern Pacific. This strongly suggest that this region possesses a regional pool of gastropods in chemosynthesis-based communities at least since Late Cretaceous time. The only group of gastropods described here which has no Recent counterpart is the Hokkaidoconchidae. A comparison to gastropods from plesiosaur falls and sunken wood of the same age and region strongly suggest that these invertebrate communities do not differ significantly from the coeval hydrocarbon seep communities described herein.

Eodelphis kabatensis, a new name for the oldest true dolphin Stenella kabatensis Horikawa, 1977 (Cetacea, Odontoceti, Delphinidae), from the upper Miocene of Japan, and the phylogeny and paleobiogeography of Delphinoidea

ABSTRACT 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.

TWO COLEOID JAWS FROM THE UPPER CRETACEOUS OF HOKKAIDO, JAPAN

Abstract Two isolated cephalopod jaws recovered from the middle Turonian of the Obira area and the Campanian of the Nakagawa area, Hokkaido, Japan, consist of short outer, and large and posteriorly elongated inner “chitinous” lamellae, with a sharply pointed rostrum in the outer lamella. These features are common with the upper jaws of Recent cephalopods. Comparison with the upper jaws of ammonoids and Recent cephalopods indicates that the two Cretaceous upper jaws are attributed to the Coleoidea other than the Octopodida. This assignment is also suggested by the cladistic analysis of the Nakagawa specimen compared with five upper jaw characters on 22 Recent cephalopod species. The Obira specimen differs from the Nakagawa specimen in having a much smaller jaw and a larger jaw angle, but its order-level assignment could not be determined because of imperfect preservation. The Nakagawa specimen shares several common features with the upper jaws of Recent Oegopsina; thus we assigned its higher systematic position to this suborder. Based on the extremely large upper jaw (97 mm maximum length), a new genus and species (Yezoteuthis giganteus) is proposed. This new taxon would have been as large as the modern giant squid Architeuthis, which commonly exceeds more than 5 m in body length. Our study postulates that studies of jaws are important to reconstruct the phylogeny of the Coleoidea.

Late Cretaceous Octobrachiate Coleoid Lower Jaws from the North Pacific Regions

Abstract Eight well-preserved cephalopod jaw fossils were discovered from the Upper Cretaceous (Santonian and Campanian) deposits of Vancouver Island, Canada, and Hokkaido, Japan. They occur individually in calcareous concretions and retain their three-dimensional architecture. Seven of them consist of a widely open outer lamella and a posteriorly projected inner lamella with a pointed rostrum. Both lamellae are made of fluorapatite, which may represent diagenetically altered chitin, and lack a calcareous element. Based on these diagnostic features, the seven jaw fossils are identified as lower jaws of the Coleoidea. Comparison with the lower jaws of modern coleoids allows us to distinguish the following new genera and species among them; Nanaimoteuthis jeletzkyi of the Order Vampyromorphida, and Paleocirroteuthis haggarti and P. pacifica of the Order Cirroctopodida. The lower jaws of these new taxa are clearly distinguished by having a much less projected inner lamella from those of modern and extinct species of the Superorder Decabrachia and the Order Octopodida. The maximum lengths of their outer lamellae (35.0–67.1 mm) are much larger than those of most modern vampyromorph and cirroctopodid species, indicating the large body size and weight of their owners. One of the other three lower jaws examined, characterized by a posteriorly extended outer lamella, may be assigned to the Octopodida. This study clearly demonstrates that large octobrachiate coleoids existed in the Late Cretaceous North Pacific.

A monospecific assemblage of terebratulide brachiopods in the Upper Cretaceous seep deposits of Omagari, Hokkaido, Japan

The Campanian (Upper Cretaceous) seep carbonate at Omagari (Hokkaido, Japan) yields a monospecific association of the terebratulide brachiopod Eucalathis methanophila Bitner sp. nov. The association is the only occurrence of brachiopods known from the post-Early Cretaceous history of chemosynthesis-based communities. Unlike many earlier rhynchonellide-dominated hydrocarbon seep associations—which disappeared in Aptian times—this association is composed of chlidonophorid terebratulides. It is hypothesised here that large rhynchonellide brachiopods have been outcompeted from chemosynthesis-based associations by large chemosymbiotic bivalves (especially lucinids) and that this seep association containing numerous terebratulide brachiopods originated as a result of immigration from the background fauna settling in a seep that lacked numerous large bivalves but offered some hard substrates for brachiopod attachment. Some living chlidonophorids are known to settle around seep/vent localities or more generally in deep-water hard-substrate settings. We review occurrences of brachiopods in chemosynthesis-based associations and show that brachiopods immigrated repeatedly to seep/vent environments. Eucalathis methanophila Bitner sp. nov. represents the oldest and single Mesozoic record of the genus. The new species is similar in ornamentation to three living species, Indo-Pacific E. murrayi, eastern Atlantic E. tuberata, and Caribbean E. cubensis but differs in having a higher beak and wider loop. Additionally the studied species is nearly twice as large as E. tuberata.

Orbitolina-bearing limestone pebbles from the lowermost part of the Lower Yezo Group (Lower Cretaceous) in the Nakagawa area, northern Hokkaido, Japan and its significance

り下部蝦夷層群,中部蝦夷層群,上部蝦夷層群,函淵層群に 区分されている(Okada, 1983).このうち夕張山地周辺の下 部蝦夷層群中部には,“オルビトリナ石灰岩”と呼ばれる石 灰岩体が古くから知られている(矢部, 1901).この石灰岩体 は下部蝦夷層群中の異地性岩体,すなわちオリストリスであ ると解釈されている(高嶋ほか, 1997など).“オルビトリナ 石灰岩”には大型有孔虫 Orbitolina をはじめ,厚歯二枚 貝,ネリネア,造礁サンゴ,石灰藻など白亜紀当時の熱帯~ 亜熱帯地域に特有な化石群集が含まれている(橋本, 1936; 佐 野, 1995).このことから“オルビトリナ石灰岩”の分布は, 北西太平洋における白亜紀の古気候,古生物地理を考える上 で重要であると考えられる.また,“オルビトリナ石灰岩” を含むオリストストロームは単調な岩相を示す下部蝦夷層群 において鍵層として有効である.さらに Orbitolina属は, 種のレンジが短いため地層の年代決定にも用いられている. これまで“オルビトリナ石灰岩”は,夕張山地を中心に南 は じ め に 北約 30 kmという限られた範囲に知られ,その分布の北限 は北海道中央部の比布地域であった(鈴木, 1957).筆者らは 今回,それよりも約 100 km北方に位置する北海道北部中川 地域の下部蝦夷層群最下部の礫岩層から“オルビトリナ石灰 岩”に類似する石灰岩礫を多数発見した.礫岩層とその中に 含まれる石灰岩礫の産状および石灰岩礫の微岩相の観察,石 灰岩礫に含まれる大型有孔虫 Orbitolinaの分類学的検討を 行った結果,この地域の下部蝦夷層群最下部の地質年代,お よび下位の空知層群との時間間隙について新知見を得た.ま た,下部蝦夷層群堆積時における後期 Aptianの浅海域での 石灰岩の形成とそれを起源とする堆積物の深海域への供給が 広範囲にわたる現象であったことが確かめられたので,ここ に報告する.

Ontogenetic changes in shell microstructures in the cold seep-associated bivalve, Conchocele bisecta (Bivalvia : Thyasiridae)

Abstract. 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.