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.