Kodama, S., Takayanagi, H., Yoshii, K., Ha, T.T.N, Asami, R., Abe, O. and Iryu, Y., in press: Carbon and oxygen isotope records of Tridacna squamosa shells from two different latitudes in the Ryukyu Islands. Paleontological Research. doi:10.2517/2020PR003. Available online 25 Feb 2020. PDF
We report carbon (δ13C) and oxygen (δ18O) isotope records of two modern giant clam (Tridacna squamosa) shells from two sites (Ishigaki-jima and Okinoerabu-jima) at different latitudes in the Ryukyu Islands, Japan. The δ13C profiles of samples from the inner shell layer on cross-sections along the maximum growth axis display no ontogenetic trends or seasonal variations. This finding suggests that the calcification site is likely to be unaffected by CO2 uptake and release resulting from the metabolic activity of the molluskan host and algal symbionts. The δ18O profiles show distinct seasonal cycles. After accounting for the influence of seawater δ18O, the time-series variations are consistent with variations in sea surface temperature, and the temperature dependency of oxygen isotope fractionation is nearly identical to previously published δ18O–temperature relationships for biogenic and synthetic aragonite. We conclude that δ18O records from pristine fossils of this species will enable accurate paleoenvironmental reconstructions at high temporal resolution.
Sano, S., in press: Boreal molluscan records around the Jurassic–Cretaceous boundary in East Asia provide clues for the paleobiogeographical reconstruction in the mid-latitudes of the Northwest Pacific . Paleontological Research. 10.2517/2019PR023. Available online 28 Oct 2019. PDF
Studying marine paleobiogeographical conditions in the mid-latitudes of the Northwest Pacific around the Jurassic–Cretaceous boundary probably contributes to better understandings of the paleoclimatic and/or paleoenvironmental background of the evolution of the Late Mesozoic terrestrial ecosystem in East Asia. However, the uncertainty of paleogeography of the eastern margin of the Asian Continent has caused the difficulties for the paleobiogeographical discussion. In this paper, the strata containing Boreal faunal elements, Buchia, and cylindroteuthidid belemnites in East Asia (Far East Russia, Heilongjiang in northeastern China, and Japan) and their tectonic settings are reviewed. The Uda and Torom (northern Sikhote-Alin), Suibin (Heilongjiang), and Tetori (northern Central Japan) regions were located from north to south in the eastern margin of the already amalgamated Asian Continent around the Jurassic–Cretaceous boundary and can be considered the “fixed points for paleobiogeographical reconstruction. On the other hand, ”Buchia-bearing strata in the Komsomolsk (northern Sikhote-Alin) and Dong’an (Heilongjiang) regions can be considered to be deposited in the fore-arc basin or trench slope basin on the accretionary complex along the East Asian continental margin. The stratum around the Jurassic–Cretaceous boundary in the Partizansk Basin (southern Sikhote-Alin) contains both Buchia and Tethyan ammonoids and was deposited on the Paleozoic continental basement or block (Sergeevka Belt). The paleo-position of these three regions around the Jurassic–Cretaceous boundary is highly debated. The Tethyan–Pacific ammonoids, Boreal belemnites, and Tetori bivalve fauna, showing some similarities with those in the Boreal Realm and Early Cretaceous strata in Heilongjiang, are present in the late Tithonian–Berriasian Mitarai Formation of the Tetori Group in the Tetori Region. This stratum deposited in the eastern margin of the North China Block, provides evidence that the Boreal faunal elements reached the mid-latitudes of the Northwest Pacific. The position of the ecotone of the Boreal and Tethys realms in the Northwest Pacific can be discussed based on the comparison of the faunal elements among almost coeval strata in the Tetori Region (fixed point), the Sergeevka Belt, and the South Kitakami Belt (Pacific side of Northeast Japan), which is usually correlated with the Sergeevka Belt but contains only the Tethyan faunal elements. Further studies of the records of Tethyan and Boreal taxa in the “fixed points” and other localities could provide clues to reveal the paleoclimatic and/or paleoenvironmental background of the evolution of the terrestrial and marine ecosystems in East Asia around the Jurassic–Cretaceous boundary.
Shiino, Y., Kurihara, T., Ichinohe, R., Kishimoto, N., Yoshino, T. and Matsuoka, A., in press: A morphological analysis of the flat-shaped spumellarian radiolarian Dictyocoryne: morpho-functional insights into planktonic mode of life. Paleontological Research. 10.2517/2019PR020. Available online 17 Oct 2019. PDF
The three-dimensional morphology of the flat-shaped spumellarian radiolarian Dictyocoryne was analysed using a microfocus X-ray CT with a special focus on whether it was capable of a planktonic lifestyle. Two types of 3D models, the shell model, which represents realistic 3D shell, and the wrapped model, which mimics the whole body outline without axopodia, were reconstructed in order to estimate volume, surface area, and centre of gravity for the shell model and buoyancy for the wrapped model. The calculated values showed that the volume of shell with respect to the total volume was negatively allometric, regardless of the differences between threshold settings. Stepwise secretions of the patagium layer may result in a comparatively lightweight shell, thereby decreasing the total density during growth but not below the density of seawater. Estimated positions for the centres of gravity and buoyancy were too close to maintain an autonomous posture while floating. Instead, the ratio between surface area and volume was greater than that in an ideal sphere. Such a broad surface area could obtain the viscous resistance necessary for sinking retardation. Spumellarian radiolarians, including Dictyocoryne, have photosynthetic symbionts located primarily in the ectoplasmic layer, which is a habitable space that can be maximised within the larger surface area. Given that radiolarians float when extending their pseudopodia, it can be hypothesised that pseudopodia may play a role in the adjustment of life posture in a hydraulically unstable shell, which can be integrated into sinking retardation, enhancement of photosynthetic activity and manoeuvrability of life posture within a unique flat-shaped morphogenesis.
Matsuzaki, K. M., Itaki, T.a and Sugisaki, S., in press: Polycystine radiolarians vertical distribution in the subtropical Northwest Pacific during Spring 2015 (KS15-4). Paleontological Research. 10.2517/2019PR019. Available online 08 Oct 2019. PDF
Polycystine radiolarian remains were collected during Expedition KS15-4 in plankton tow sample from 0 to 3000 m of water depth at station (Sta.) 1 (30°28.8537N; 132°24.9532E). Obtained data from this site provide precious information about radiolarian assemblages living at deep-water depth. Based on R-mode cluster analysis, the surface water is characterized by well-known subtropical species such as Tetrapyle circularis/fruticosa group and Didymocyrtis tetrathalamus. The subsurface water depths (200–500 m) are characterized by species such as Siphonosphaera abyssi. The intermediate-water depths (500–1000 m) are characterized by relatively high abundances of Larcopyle weddellium and Actinomma boreale, while species such as Cycladophora davisiana inhabiths mainly water depths below 2000 m and Carpocanarium papillosum group inhabits the deep-water depths between 1000 and 2000 m.
Li, X. and Matsuoka, A., in press: Paleobiogeographic distribution of the Early Cretaceous radiolarian Turbocapsula costata and its correlation potential. Paleontological Research. 10.2517/2019PR015. Available online 10 Sep 2019. PDF
Turbocapsula, three (or four) segments with characteristic oval shape and a hemi-closed segmental end, is a radiolarian genus with a high stratigraphic value due to its phyletic evolution in the mid-Cretaceous period. An overview of lithology, depositional setting, associated fossil records, and paleomagnetic data from each locality of Turbocapsula costata (Wu) is summarized. All these data lead to the conclusion that locations of known T. costata-bearing strata are restricted to the low- to mid-latitude Mediterranean and eastern Tethys of the Tethyan realm. The results emphasize that the phyletic evolution of the genus Turbocapsula is significant in the correlations within the Tethys. The zonation established by the phyletic evolution of the genus Turbocapsula cannot be utilized for Early Cretaceous zonal correlations beyond the T. costata territory.
Suzuki, H., Ja, L., Maung, M., Thin, A. K. and Kuwahara, K., in press: The first report on Early Cretaceous Radiolaria from Myanmar. Paleontological Research. 10.2517/2019PR017. Available online 10 Sep 2019. PDF
An Early Cretaceous radiolarian fauna is found from the Tagaung Taung area, central Myanmar. The fauna consists of the following species: Acanthocircus dicranacanthos (Squinabol), Stylosphaera squinaboli Tan Sin Hok, Hiscocapsa cf. subcrassitestata (Aita), Svinitzium cf. depressum (Baumgartner), Thanarla brouweri (Tan Sin Hok), Archaeodictyomitra mitra Dumitrica, Archaeodictyomitra apiarium (Rüst), Archaeodictyomitra vulgaris Pessagno, Hemicryptocapsa cf. pseudopilula Tan Sin Hok, Hemicryptocapsa capita Tan Sin Hok, Holocryptocapsa hindei Tan Sin Hok and Cryptamphorella challengeri Schaaf. These species indicate clearly an Early Cretaceous age (Hauterivian), and therefore this is the first report on Cretaceous radiolarians from Myanmar.
Parent, H., Bejas, M., Greco, A., in press: Shell area-to-volume ratio in ammonoids. Paleontological Research. doi:10.2517/2019PR013. Available online 18 Jun 2019. PDF
The external area-to-volume ratio of the ammonite shell has been held to be related to morphology but never evaluated quantitatively. A dimensionless ratio, the Vogel number, was computed for large samples of Devonian to Cretaceous ammonites with a new method based on the ADA-model. The estimated ratios range from 2.4 to 3.4. The highest values are exhibited by uncoiled serpenticone ammonites, lowering in the sequence serpenticone-oxycone-spherocone. It is shown that the area-to-volume relationships are controlled by the involution (degree of overlapping) and the relative width of whorl section. The typical evolutionary trends serpenticone–spherocone and/or serpenticone–oxycone, broadly documented through the history of the Ammonoidea could have been driven, at least in part, by the lowering of the area-to-volume ratio.
Yabumoto, Y. and Nazarkin, M. V., in press: Clupea hanishinaensis nomen novum, a replacement name for the Miocene clupeid fish Clupea macrocephala Yabumoto and Nazarkin, 2018 from Nagano, Japan. Paleontological Research. doi:10.2517/2019PR011. Available online 10 May 2019. PDF
Isaji, S. and Okura, M., in press: Microgastropods from the late Carboniferous limestone in Fukuji, Gifu Prefecture, central Japan. Paleontological Research. 10.2517/2019PR010. Available online 10 Apr 2019. PDF
This paper describes microgastropod fossils from the Kasimovian (late Carboniferous) limestone floats collected from the Mizuboradani Valley, Fukuji, Okuhida-onsengou, Takayama City, Gifu Prefecture, central Japan. The microgastropod assemblage consists of a diversity of larval and/or early juvenile shells and represents late Palaeozoic cosmopolitan taxa, including Euomphalidae, Pleurotomarioidea, Anomphalidae, Naticopsidae, Trachyspiridae, Goniasmatidae, Orthonematidae, Pseudozygopleuridae, Subulitidae, Meekospiridae and Streptacididae. The microgastropod assemblage bears some resemblances to those from the early Carboniferous of New South Wales, Australia, and those from the latest Permian of Guangxi Province, China.
Handa, N., in press: Reappraisal of a rhinocerotid (Mammalia, Perissodactyla) from the lower Miocene Yotsuyaku Formation, Northeast Japan, with an overview of the early Miocene Japanese rhinocerotids. Paleontological Research. 10.2517/2019PR009. Available online 10 Apr 2019. PDF
A fragmentary femur of the Rhinocerotidae (Perissodactyla, Mammalia) from the lower Miocene Yotsuyaku Formation of the Shiratorigawa Group, Ichinohe, Iwate Prefecture, Northeast Japan is redescribed, and the fossil record of Japanese early Miocene rhinocerotids, including footprints, is briefly reviewed. The femur is identified as belonging to an indeterminate species of rhinocerotid, cf. Aceritherini, in having the distal portion of the base of the lesser trochanter situated near the apex of the third trochanter and a less projected third trochanter than in most rhinocerotids. Since ca. 20 Ma, rhinocerotids have inhabited and been widely distributed in Japan, which formed an eastern margin of continental East Asia at that time.
Misaki, A., Okazaki, Y., Mizuno Y. and Matsunaga, T., in press: Early Cenomanian (Late Cretaceous) ammonoids from the Miyanohara Formation in the Sakawa area, Shikoku, southwestern Japan. Paleontological Research. doi:10.2517/2019PR008. Available online 06 Apr 2019. PDF
Mid-Cretaceous ammonoids, Euhystrichoceras nicaisei, Mantelliceras japonicum, and Hypostlingoceras japonicum were collected from float rocks probably derived from the middle part of the Miyanohara Formation in the Sakawa area, Shikoku, southwestern Japan. Although it has been suggested that the stratigraphic correlation of this formation based mainly on bivalves such as trigoniids needs to be reexamined, the occurrence of these ammonoids confirms that the middle part of the Miyanohara Formation is correlated to the lower Cenomanian. The results of this study support the suggestion that the shallow marine deposits of the mid-Cretaceous that contain similar molluscan fauna are widely distributed throughout the northwestern Pacific region.
Danelian, T. and MacLeod, N., in press: Morphometric analysis of two Eocene related radiolarian species of the Podocyrtis (Lampterium) lineage. Paleontological Research. doi:10.2517/2019PR007. Available online 06 Apr 2019. PDF
A metric analysis of the morphology of two related Eocene species (Podocyrtis sinuosa and P. mitra) that are part of the Lampterium evolutionary lineage was undertaken in order to evaluate hypotheses related to their mutual taxonomic distinction statistically. All analyses (landmark, outline semi-landmark and landmark-constrained outlines) support an interpretation of statistically significant species-specific shape differences. Moreover, landmark and semilandmark-based morphometric characterizations can be used to identify which regions of the test are best suited for making reliable taxonomic distinctions. These results suggest that both abdomen and thorax shapes represent species-specific characters. While this agrees, in part, with previous, qualitative diagnoses, our results shed light on precisely how abdomen and thorax shape differ between these species. In addition, our investigation demonstrates the taxonomic value of a morphometric approach to character analysis as thorax shape differences had gone unnoticed by previous investigators.
Li, G., Matsuoka, A., Yang, Q., Sha, J., in press: Middle and Late Jurassic radiolarians from Nadanhada terrane of eastern Heilongjiang Province, northeastern China. Paleontological Research. doi:10.2517/2019PR006. Available online 25 Mar 2019. PDF
The Nadanhada terrane of northeastern Heilongjiang, northeastern China is composed of Jurassic accretionary complexes (i.e., the Yuejinshan Complex in the west, the Raohe Complex in the east) and succedent cover beds. In this paper we report two radiolarian assemblages recognized from four black claystone samples collected from the Dalingqiao Formation in the Dajiashan area of the Raohe Complex. The Striatojaponocapsa synconexa-“Tricolocapsa” tetragona assemblage of one sample consists of eleven species in eight genera, which indicates a middle Bathonian (Middle Jurassic) age, i.e., correlative with the uppermost Striatojaponocapsa plicarum Zone to the lower Striatojaponocapsa conexa Zone. The other three samples contain 34 species in 25 genera, which indicate a late Oxfordian–early Tithonian (Late Jurassic) age. The discovery of the Late Jurassic radiolarian assemblage indicates that the Raohe Accretionary Complex was formed during the Late Jurassic. The succedent late Tithonian–early Valanginian Buchia fauna-bearing Dong’anzhen Formation may be the earliest cover beds overlying the Raohe Accretionary Complex. Then, during the Early Cretaceous, the Nadanhada terrane received the deposition of the Aucellina bivalve fauna-bearing Dajiashan Group. This indicates paleogeographic differentiation between the Nadanhada terrane and the Tamba-Mino-Ashio terrane.
Ito, T., Takahashi, K. U., Matsuoka, A. and Feng, Q., in press: The Guadalupian (Permian) Gufeng Formation on the north margin of the South China block: A review of the lithostratigraphy, radiolarian biostratigraphy, and geochemical characteristics. Paleontological Research. 10.2517/2018PR025. Available online 19 Dec 2018. PDF
The Guadalupian (Permian) Gufeng Formation, distributed over the north margin of the South China block, is characterized by siliceous rock with muddy rock and yields several fossils representative by radiolarian fossils. The lithostratigraphy, radiolarian occurrences with biostratigraphy, and geochemical characteristics of the Gufeng Formation are reviewed and summarized in this paper. The distributional area of the Gufeng Formation is subdivided into the Lower, Middle, and Upper Yangtze regions. The Gufeng Formation in the Lower Yangtze region is lithostratigraphically characterized by muddy rock, chert, and muddy rock with nodules in descending order. The Gufeng Formation in the Middle Yangtze region is lithostratigraphically characterized by chert, muddy rock with carbonate, and muddy rock in ascending order. The lithostratigraphic characteristics of the “Gufeng Formation” in the south margin greatly differ from the typical Gufeng Formation in containing fewer siliceous rocks and much clastic rocks and in having a significantly thicker total thickness; therefore, this article considers that the “Gufeng Formation” in the south margin should be isolated from the Gufeng Formation in the north margin. The radiolarian biostratigraphy of the Lower and Middle Yangtze regions is composed of the Pseudoalbaillella globosa, Ps. monacantha, and Follicucullus scholasticus-Ruzhencevispongus uralicus assemblage zones in ascending order. The lower Gufeng Formation has abundantly yielded the Albaillellaria; however, its occurrence ratio decreases stratigraphically upward. The geochemical characteristics indicate the following facts: (1) the siliceous rocks are generally biogenic; (2) even though a previous study concluded that the organic matter within the cherts was originated from terrestrial or reworked organic matter, the origin remains debatable; and (3) the Gufeng Formation was formed on a continental shelf. The geochemical characteristics, in addition to the lithological characteristics, indicate that the redox conditions of the Gufeng Formation in the Lower Yangtze region changed from aerobic to suboxic to anoxic, while that in the Middle Yangtze region changed from oxic to anoxic to suboxic.
Kamikuri, S., Moore, T. C., Matsui, H. and Nishi, H., in press: Radiolarian biostratigraphy and faunal turnover across the early/middle Miocene boundary in the equatorial Pacific. Paleontological Research. doi:10.2517/2018PR024. Available online 30 Nov 2018. PDF
Sedimentary sequences obtained from drillings during Integrated Ocean Drilling Program (IODP) Expedition 320/321, “Pacific Equatorial Age Transect (PEAT)” at eight sites (Sites U1331–1338) in the equatorial Pacific offer an ideal record for reconstructing the evolution of the ocean/climate system throughout the Cenozoic. The sediments drilled at Site U1335 record short-term events of paleoceanographic significance, including the early Miocene climatic optimum (MCO) and the middle Miocene climatic transition (MMCT). Abundant well-preserved radiolarians were recovered from the lower Miocene radiolarian Zone RN2 through middle Miocene Zone RN5 at IODP Site U1335. A total of 46 radiolarian datum levels consisting of 20 first occurrences (FOs), 25 last occurrences (LOs), and one evolutionary transition (ET) was recognized within the studied interval at Site U1335. Of these datum levels, 36 radiolarian datum levels were directly tied to the geomagnetic polarity time scale (GPTS) across the early/middle Miocene boundary. The general magnitude of evolutionary change was estimated based on the total turnover rate (the sum of FOs and LOs per 0.5 m.y.) of tropical radiolarians, and two minor faunal turnovers of radiolarian species were recognized between 16.5 and 14.7 Ma and between 13.9 and 13.4 Ma. These faunal turnovers were associated with regional environmental changes such as the increased biological productivity in the equatorial Pacific during the MCO and the MMCT.
Tanaka, Y., Ohara, M. and Kimura, T., in press: A large fossil baleen whale from the Shikiya Formation (early Middle Miocene) of 4 Wakayama, Japan. Paleontological Research. doi: 10.2517/2018PR020. Available online 31 Oct 2018. PDF
A new large Chaeomysticeti indet., WMNH-Ge-1140240005 from the Shikiya Formation of Kumano Group (early Middle Miocene; about 16 to 15 Ma) of Wakayama, Japan is described here. It preserves a large rostrum (about 50 cm width at the base of the rostrum), which has gently tapered lateral margins of the rostrum, narrow mesorostral groove at the level of the narial fossa, wide premaxillae and maxillae. There are no diagnostic features on the specimen in family level, but it is comparable to two “cetotheres” sensu lato such 26 as Pelocetus calvertensis and Diorocetus hiatus by having wide premaxillae, which occupy 1/3 width of the rostrum at anterior to the narial fossa in dorsal view, which implies that WMNH-Ge-1140240005 is a possible member of “cetotheres” sensu lato. Its size is possibly between the two large species Pelocetus calvertensis and Diorocetus hiatus of the early Middle Miocene, and larger than the reported Middle Miocene mysticete specimens from Japan (“Diorocetus” chichibuensis, “Diorocetus” shobarensis, Parietobalaena sp. (SMNH-VeF-62)). The rostral width of WMNH-Ge-1140240005 suggests that this animal was middle size compare to extant species, but the largest class baleen whale as its age.
Ichinohe, R., Shiino, Y., Kurihara, T. and Kushimoto, N., in press: Active floating with buoyancy of pseudopodia vs passive floating by hydrodynamic drag force: A case study of the flat-shaped spumellarian radiolarian Dictyocoryne. Paleontological Research. doi:10.2517/2018PR023. Available online 23 Oct 2018. PDF
It has been suggested that the pseudopodia of radiolarians play a role in controlling buoyancy for floating behaviour. To understand the function of pseudopodia in terms of planktonic capability, we performed culture experiments on the flat-shaped radiolarian Dictyocoryne. A glass cell, a stereomicroscope and an X-Y-Z stage were used to observe the behaviour of Dictyocoryne from a lateral view. Under static conditions, Dictyocoryne grounded on the bottom of the glass cell extended pseudopodia from both sides of the flat disc surface. Subsequently, these individuals rose slightly by a length equal to that of the extended pseudopodia but remained attached to the bottom. These results suggest that Dictyocoryne lacks the ability to surface by obtaining buoyancy through the emergence of pseudopodia. Under conditions of convection flow, Dictyocoryne moved in the downstream direction only when pseudopodia were completely extended. When the convection flow moved upward, the individuals moved with the flow to just under the surface. In addition, convection flow changed the direction of a long, thick pseudopodium, called the axoflagellum, to be parallel with the flow. Consequently, the axoflagellum was always oriented towards the downstream side, being the disc face in the axoflagellum side on the upturn. Given that the flat-shaped spumellarians have symbiotic algae inside their cells, their unique planktonic capability leads to the stable efficiency of the algal photosynthesis.
Nishino, M. and Yamada, T., in press: Limnobiophyllum expansum (Araceae) from the early Miocene Hiramaki Formation in the Kani Basin, Gifu Prefecture, Central Japan . Paleontological Research. doi:10.2517/2018PR018. Available online 22 Aug 2018. PDF
Limnobiophyllum expansum (Lemnoideae, Araceae) was newly found in the lower Miocene (18.4–17.0 Ma) Hiramaki Formation of the Mizunami Group in the Kani Basin, Gifu Prefecture, central Japan. This genus was distributed widely in the Northern Hemisphere from the Late Cretaceous to the Paleocene, but a Miocene descendant (L. expansum) was reported only from Europe; the Bohemian Basin, the Czech Republic (early Miocene), Paldau, Austria (late Miocene), Schrotzburg, Switzerland (late Miocene) and Sośnica, Poland (late Miocene). Our finding is the first record of a Miocene Limnobiophyllum from an area other than Europe. The aquatic flora of Japan during the early Miocene is poorly documented, but this finding suggests a link between the aquatic floras of Europe and Asia.
Okanishi, A., Ishida, Y. and Mitsui, S., in press: Fossil gorgonocephalid basket stars (Echinodermata: Ophiuroidea: Euryalida) from the Middle Pleistocene of Japan; the first record from the Indo Pacific region. Paleontological Research. doi:10.2517/2018PR017. Available online 15 Aug 2018. PDF
Disarticulated fossil euryalid ophiuroid vertebrae from the Middle Pleistocene Miyata Formation, Miura, Kanagawa Prefecture, eastern Japan, are described. The vertebrae are assigned to the family Gorgonocephalidae on the basis of arm branching and the presence of an open oral groove along the entire arm. This is the first record of fossil euryalids from Indo-Pacific region.
Tanaka, Y. and Taruno, H., in press: The first cetacean record from the Osaka Group (Middle Pleistocene, Quaternary) in Osaka, Japan. Paleontological Research. doi:10.2517/2018PR016. Available online 13 Jul 2018. PDF
A new partial skeleton consisting of a left mandible and five caudal vertebrae, OMNH-QV 282 from the Osaka Group (Middle Pleistocene, about 0.3 million years ago) of Osaka City is reported as the first cetacean record from the Group. The skeleton is identified as Balaenopteridae gen. et sp. indet. based on the combination of mandibular characters, such as having a small mandibular foramen, reflected neck in dorsal view and lack of satellite process of the mandible. OMNH-QV 282 expands diversity for the local fauna, and also adds an evidence of existence for large sized balaenopterids from the poorly known epoch, the Middle Pleistocene.
Tanabe, K., Misaki, A., Ikeda, T., Izukura, M. and Moriya, K., in press: Taxonomic relationships and paleoecological significance of two exceptionally large lower jaws of Late Cretaceous ammonoids from Japan . Paleontological Research. doi:10.2517/2018PR015. Available online 09 Jul 2018. PDF
Two exceptionally large cephalopod jaws collected from the Upper Cretaceous marine deposits of the Hidaka area, Hokkaido (Yezo Group), and Awaji Island, Southwest Japan (Izumi Group), respectively, are described. Further, their taxonomic relationships and functional morphologic aspect for feeding are discussed. Based on a comparison to counterparts of modern and extinct cephalopods, they were identified as the lower jaws of ammonoids. Owing to the development of a thick calcareous tip in the large outer chitinous lamella, the lower jaw from the Yezo Group is classified as a rhychaptychus-type known from the Cretaceous Lytoceratina and Phylloceratina. The lower jaw from the Izumi Group lacks a sharply pointed calcareous tip and is characterized by a posteriorly elongated outer chitinous lamella, whose outer surface is sculptured by a median furrow in the anterior portion. These features categorize it as an intermediate-type lower jaw shared by the Cretaceous Desmoceratoidea. As determined from the co-occurring ammonoids and the relationship between the dimensions of in situ lower jaws and conchs for ammonoids previously described, the two lower jaws from the Yezo and Izumi groups were, respectively, thought to belong to large gaudryceratid and pachydiscid specimens, both of which have shell diameters greater than 40 cm. The overall shape and structure of the two lower jaws suggest a scavenging-predatory feeding habit for the gaudryceratid and a passive microphagous habitat for the pachydiscid.
Kitamura, N., in press: Features and paleoecological significance of the shark fauna from the Upper Cretaceous Hinoshima Formation, Himenoura Group, Southwest Japan . Paleontological Research. doi:10.2517/2018PR013. Available online 08 Jul 2018. PDF
The shark fauna of the Upper Cretaceous Hinoshima Formation (Santonian: 86.3–83.6 Ma) of the Himenoura Group (Kamiamakusa, Kumamoto Prefecture, Kyushu, Japan) was investigated based on fossil shark teeth found at five localities: Himedo Park, Kugushima, Wadanohana, Higashiura, and Kotorigoe. A detailed geological survey and taxonomic analysis was undertaken, and the habitat, depositional environment, and associated mollusks of each locality were considered in the context of previous studies. Twenty-one species, 15 genera, 11 families, and 6 orders of fossil sharks are recognized from the localities. This assemblage is more diverse than has previously been reported for Japan, and Lamniformes and Hexanchiformes were abundant. Three categories of shark fauna are recognized: a coastal region (Himedo Park; probably a breeding site), the coast to the open sea (Kugushima and Wadanohana), and bottom-dwelling or near-seafloor fauna (Kugushima, Wadanohana, Higashiura, and Kotorigoe). The shark fauna of the Hinoshima Formation is similar to that of the Yezo and Futaba groups, and also fauna from Angola, Australia, and Antarctica. However, based on the composition of taxa, the fauna differs from that of the Upper Cretaceous shark fauna of the Western Interior Seaway and Europe. The Upper Cretaceous shark fauna of Japan, including the Hinoshima Formation, contained active pelagic (Squalicorax and Cretoxyrhina mantelli) and bentho-pelagic (e.g. Notidanodon, Chlamydoselachus, Sphenodus) predators. These taxa probably coexisted, as they occupied different ecological niches. The characteristics of the Late Cretaceous shark fauna in Japan are similar to those of the contemporaneous Southern Hemisphere fauna (e.g. Angola, Australia, and Antarctica). This shows that the characteristic shark fauna (e.g. Notidanodon, Chlamydoselachus, Sphenodus) of the Southern Hemisphere had spread to the middle latitudes of the Northern Hemisphere by the Late Cretaceous.
Aiba, D., in press: A possible phylogenetic relationship of two species of Hyphantoceras (Ammonoidea: Nostoceratidae) in the Cretaceous Yezo Group, northern Japan. Paleontological Research. doi:10.2517/2018PR010. Available online 05 Jun 2018. PDF
A possible phylogenetic relationship of two species of Hyphantoceras (Ammonoidea: Nostoceratidae) was proposed, based on the newly found specimens with precise stratigraphic occurrences in the Kotanbetsu and Obira areas, northwestern Hokkaido. Two closely related species, Hyphantoceras transitorium and Hyphantoceras orientale, were recognized in the examined specimens from the Kotanbetsu and Obira areas. Specimens of H. transitorium show the wide intraspecific variation in the whorl shape. The stratigraphic occurrences of two species indicate that they occur successively in the Santonian–lowermost Campanian, without stratigraphic overlapping. The similarity of their shell surface ornamentations and the stratigraphic relationships possibly suggest that Hyphantoceras orientale was derived from Hyphantoceras transitorium. The presumed lineage is likely indigenous to the northwestern Pacific realm in Santonian–earliest Campanian. Hyphantoceras venustum and H. heteromorphum might be out of a lineage of Hyphantoceras transitorium¬–Hyphantoceras orientale, judging from differences of their shell surface ornamentations.
Handa, N., in press: Reassessment of a Pleistocene rhinocerotid (Mammalia, Perissodactyla) from Aira, Kagoshima, southwestern Japan. Paleontological Research. doi:10.2517/2018PR009. Available online 21 May 2018. PDF
This study describes right upper postcanaine teeth of a single individual of Pleistocene rhinocerotid (Mammalia, Perissodactyla) from the lower to lower middle Pleistocene Kamo Formation of the Kokubu Group in Aira City, Kagoshima Prefecture, southwestern Japan. These teeth are heavily worn and are identified as P2–M2 with missing M1. They are identified as an indeterminate genus and species of the Rhinocerotidae, although they were previously named as Rhinoceros aff. sinensis. These dental fossil specimens and the rhinocerotid footprints from the lower to lower middle Pleistocene of Japan indicate that rhinocerotid certainly existed in Japan during the early to early middle Pleistocene.
Ishida, Y., Fujita, T., Kohtsuka, H., Manabe, M. and Ohara, M., in press: A new example of the trace fossil Asteriacites quinquefolius from Japan and the process of production as revealed by observations of an extant sea star. Paleontological Research. doi:10.2517/2018PR003. Available online 19 Apr 2018. PDF
A star-shaped trace fossil here assigned to Asteriacites quinquefolius (Quenstedt) was found in the Miocene Shirahama Formation, Wakayama Prefecture. This is the first report in Japan and stratigraphically youngest record for the ichnospecies. The fossil has five distinct arms and wide striations on both lateral sides of each arm, and the shape is bilaterally symmetrical. To clarify the fossil producing process, we conducted burial experiments of extant asteroids in aquarium and in situ for the first time. Asteroids buried themselves in the substratum using the tube-feet, and when asteroids were covered with thin sand, they escaped slantingly upward onto the sand tilting their body in a bilaterally symmetrical posture. As a result, the remaining trace was very similar to the fossil of A. quinquefolius from the Shirahama Formation. Accordingly, the results suggest the present fossil was formed by the behavior of escaping from thin sand cover by asteroids.