Paleontological Research

Ptilophyllum sp. and Zamites brevipennis are newly described from the middle Oxfordian Arimine Formation in the Arimine area, Toyama Prefecture, Central Japan. These two species characterize the vegetation of the Eurosinian paleophytogeographic province where a climate with dry season(s) prevailed. This finding, as well as the presence of a Kaizara Flora, suggests that Eurosinian-type vegetations continuously flourished during the late Bathonian to Oxfordian on the land of the Tetori Group. We also infer that Tetori-type floras first appeared during the Tithonian in the Tetori Group.

A fossil palatine of an alepisauroid fish collected from the middle Miocene Yokoo Formation in Nagano Prefecture, central Japan is described as Omosudis sp. Although the palatine is only a fragment, the palatine teeth arranged in a single row are well preserved. In fact, the palatine tooth characteristics are adequate for generic-level diagnosis. The fossil appears to be assignable to the genus Omosudis belonging to the family Alepisauridae by having the following characteristics: enormously large, posteriorly inclined teeth each with a sharply pointed apex, apico-basal striations, a nearly straight to arcuate anterior cutting edge, a wide pulp cavity surrounded by a thin dentine layer and a fang-like outline due to a basally elongated postapical barb. The Yokoo specimen represents the first reliable fossil record of the genus from the middle Miocene in Japan and appears to mark the earliest occurrence of this Recent genus in the Northwest Pacific region.

A new cyprinid fish, Nipponocypris takayamai sp. nov. is described from the Middle Pleistocene Nogami Formation in Oita Prefecture, Northern Kyushu, Japan. Nipponocypris takayamai differs from its congeners in the following combination of characters: the sensory canal of the parietal not reach to the medial edge of the parietal, and its length longer than half the width of the parietal; the notch of the orbital margin of the frontal weak; the dorsal margin of opercle concave; the posterodorsal margin of the fourth infraorbital L-shaped; the sensory canal of the dentary running slightly ventrally; 42–44 vertebrae; the flanges of the neural spines of the second and third preuralcentra elongated to the dorsal ends of the neural spines; eight supraneurals between the dorsal fin and the supraneural 3 bone; the dorsal fin base located slightly more posterior than the pelvic fin base; first three dorsal fin rays unbranched and unsegmented; eight dorsal fin rays, ten dorsal fin pterygiophores; and eleven anal fin pterygiophores with thirteen anal fin rays. A cladistic analysis suggests that this new species is related to N. sieboldii, N. koreanus and N. temminckii, and is probably the sister taxon to N. temminckii. This new species shows that an extinct species closely related to Recent Nipponocypris existed until the Middle Pleistocene in East Asia.

Cheilostome bryozoans (Phylum Bryozoa; Order Cheilostomata) originated in the latest Jurassic but remained at low diversity until the late Albian–early Cenomanian, when they began an explosive radiation that has continued to the present day. Most knowledge of Late Cretaceous cheilostomes comes from Europe and the USA from deposits associated with the western Tethys Ocean. Only a few previous records of Cretaceous bryozoans exist from the margin of eastern Asia in the NW Pacific. We examined material from four localities of early– middle Campanian age in the Himenoura Group, Shimokoshikijima Island, Kagoshima Prefecture, southern Kyushu, Japan, and detected six cheilostome species but no cyclostomes. Two species were relatively common. For one of these, we erect the new genus Kenocharixa and describe a new species, Kenocharixa kashimaensis; we also transfer Charixa goshouraensis Dick, Komatsu, Takashima, and Ostrovsky and Conopeum stamenocelloides Gordon and Taylor to Kenocharixa. We describe the other common species as Marginaria prolixa sp. nov. We detected four species as a single specimen each and identified them only to genus (Charixa sp. A) or listed them as Incertae sedis (A, B and C). Among the six cheilostomes, five are primitive anascan-grade (malacostegan) species and one is an anascan-grade neocheilostome. Compared to Campanian–Maastrichtian bryozoan faunas in Europe and the USA, the Himenoura fauna is low in diversity and morphological disparity, with no cribrimorph or ascophoran species detected. Previous researchers have suggested that the NW Pacific biota became isolated from that of the Tethys in the latest Aptian–middle Albian interval. We advance the hypothesis that the Late Cretaceous bryozoan fauna in the NW Pacific is a low diversity relict of the fauna present when this isolation occurred. While Tethyan cheilostome lineages underwent a major radiation associated with the origins of cribrimorph and ascophoran frontal shields, the NE Pacific lineages failed to diversify at the same rate and remained at low diversity and disparity. Testing this hypothesis will require much further sampling in Cretaceous deposits along the margin of East Asia.

The Polycotylidae are short-necked plesiosaurs known from the Cretaceous in various parts of the world, but only a few occurrences have been documented in Japan, where elasmosaurid remains are much more common. An indeterminate polycotylid specimen from the Upper Cretaceous in Obira Town, Hokkaido, is described. Characteristics of the vertebrae and clavicular arch support its taxonomic affinity. The Turonian occurrence of the specimen indicates the continuous presence of the Polycotylidae across the Cenomanian-Turonian boundary in the northwestern Pacific. Macroscopic osteological features of the vertebrae and clavicular arch indicate an advanced stage of ossification, and there are histological characteristics suggesting slowed growth. The osteoporotic-like condition implies a high degree of aquatic adaption.

Two Late Pennsylvanian conodont species, Gondolella sublanceolata Gunnell and Idiognathodus sulciferus Gunnell, were extracted from limestone boulders in the Mizuboradani Valley, Fukuji district, central Japan. These provide the first evidence of Missourian (Kasimovian) cosmopolitan conodonts in the Akiyoshi and Hida Gaien belts, Inner Zone of Japan. The limestone boulders might be derived from the Ichinotani Formation and/or from limestone clasts in conglomerates of the Permian Sorayama Formation that crop out in the Mizuboradani Valley.

We report the fourth record of a fossil whale-fall community in Japan. The new material consists of a single whale bone in association mainly with small bathymodiolin mussels, Adipicola sp., found in the Karumai Formation (late middle Miocene—early late Miocene) in the Hobetsu area of Hokkaido, Japan. This association of whale bone and Adipicola sp. and its mode of occurrence resembles the description of some other ancient whale-fall communities dominated by small mussels from the Olympic Peninsula in Washington State (early Oligocene), Shosanbetsu in Hokkaido (early middle Miocene) and Carpineti in northern Italy (middle Miocene) and constitutes an example of a chemosynthesis-based community sustained by whale-fall decay in the Miocene deep sea. The new example extends the Miocene distribution of bathymodiolin-dominated whale-fall communities to the northwestern Pacific Ocean.

Plant fossils are reported on from the Lower Cretaceous Upper Formation of the Sasayama Group, including the new species Otozamites toshioensoi sp. nov. The Sasayama Flora proposed here is characterized by an abundance of microphyllous conifers such as Brachyphyllum spp. and Pseudofrenelopsis sp., as well as by the rare occurrence of pteridophytes. These floristic components suggest that an arid climate prevailed in the land of the Sasayama Group and that the Ryoseki-type flora, which generally represents the Outer Zone of Japan during the Late Jurassic to Early Cretaceous, flourished in the Inner Zone of Japan during the late Early Cretaceous. Albian or Cenomanian ages have previously been proposed for the Upper Formation of the Sasayama Group by radiometric datings, but an Albian age is preferred here in regards to the extremely rare occurrence of possible angiosperms.

Late Devonian brachiopods from Choanji, South Kitakami Belt, northeastern Japan, are re-examined to determine their age. Two brachiopod assemblages are recognized in the Choanji area. The assemblage from locality CHJ2 in the middle part of the Choanji Formation consists of one species, Desquamatia (Seratrypa) sp., which indicates a Frasnian age. The assemblage from locality CHJ1 in the upper part of the Choanji Formation consists of 13 species in 10 genera, including a new species, Parallelora minatoi sp. nov., and is of late Famennian age. Thus, the Choanji Formation is correlated with the Upper Devonian (Frasnian–upper Famennian), although until quite recently it was considered to be the lowest Carboniferous (Tournaisian) in Japan.

A recently discovered, incomplete dentary with preserved incisor, premolar and first molar of a minute castorid from the Dota locality, Kani Basin, early Miocene (ca. 18.5 Ma) Nakamura Formation of the Mizunami Group in central Japan, is described as a new species of the trogontheriine beaver Euroxenomys. It represents the first record of Euroxenomys in Asia and one of the oldest records of this genus. Euroxenomys nanus sp. nov. is smaller than the type species, E. minutus from the Miocene of Europe and more hypsodont, but similar in size to E. inconnexus from the Barstovian of Montana. Dota is the first Asian locality with three cooccurring castorids, Youngofiber sinensis, Minocastor godai, and E. nanus.