Paleontological Research

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 the Indo-Pacific region.

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, in the Bohemian Basin, 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.

Two early Carboniferous brachiopod species, Levitusia humerosa (Sowerby) and Grandispirifer mylkensis Yang, are described from Japan for the first time. These species, from the middle part of the Arisu Formation in the Okuhinotsuchi area, South Kitakami Belt, indicate an early Visean age. Therefore, the middle part of the Arisu Formation is correlated with the lower Visean. In terms of palaeobiogeography, occurrence of the two species suggests that South Kitakami was probably the eastern extension of the North China Province, and located near and to the east of North China (Sino-Korea) during the early Visean.

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, and wide premaxillae and maxillae. There are no diagnostic features pertaining to the specimen at the family level, but it is comparable to “cetotheres” sensu lato such as Pelocetus calvertensis and Diorocetus hiatus by having wide premaxillae that occupy 1/3 width of the rostrum 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 medium-sized compare to extant species, but the largest in the class of baleen whales of its time.

Many molluscan fossils have been found from the turbidite deposits of the lower Miocene Kurosedani Formation at Kakuma in Toyama City. They include six chemosymbiotic bivalves, namely Solemyidae gen. et sp. indet., Nucinella sp., Luinoma cf. acutilineatum (Conrad), Conchocele yatsuoensis sp. nov., Pliocardia kawadai (Aoki) and Adulomya chitanii Kanehara. This is the oldest record of chemosymbiotic species in the Japan Sea region. Nucinella has been unknown from the Cenozoic deposits in the Japan Sea borderland and from the Recent Japan Sea. Moreover, the vesicomyids P. kawadai and A. chitanii are shared with those from the lower Miocene Honya Formation in Fukushima Prefecture and from the middle Miocene Nupinai Formation in eastern Hokkaido. This supports our hypothesis that the invasion of chemosymbiotic species from the Pacific side to the Japan Sea took place soon after the formation of a deep-sea basin in the Japan Sea.

An incomplete chimaeroid (Holocephali, Chimaeroidei) mandibular dental plate from the early Bajocian (Humphriesianum Zone) of Ottange-Rumelange, on the Luxembourg-French border, is described as a new genus and species of callorhinchid fish Ottangodus lotharingiae gen. et sp. nov. Comparison of the described plate with other Jurassic chimaeriform fish (both myricanthid and chimaeroid) mandibular plates shows relationships with callorhinchids and in turn close relationships with the genus Pachymylus Woodward, known from the Callovian of England. The plate of the new genus shows a set of primitive characters for chimaeroids (very high descending lamina, low oral part of the plate, large centrally placed median tritor occupying most of the oral part of the plate and an absence of mesio-labial system of the tritors) and could thus belong to a basal Chimaeroidei taxon. This is the first record of the Chimaeroidei from the Jurassic of Lorraine and the fourth chimaeriform genus known from the Mesozoic of the region.

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-three 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 to faunas 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.

Twenty species of fusulines and ten species of non-fusuline foraminifers have been identified from 20 limestone samples along Gongendani, a valley south of Mt. Ryozen, Shiga Prefecture, Japan. They are dominated by Pseudofusulina norikurensis, Ps. isomie, and Acervoschwagerina gongendaniensis, and considered to be late Cisuralian (late early Permian) based on their coexistence with five subordinate species reported from the Yakhtashian (Artinskian) and the Bolorian (lower Kungurian) in the southeast Pamir and other Tethyan regions. Taxonomic composition of the schwagerinids of Gongendani and other faunas in the Mino Terrane is considerably different from that of coeval faunas in the Permian Akiyoshi Terrane. It is highly probable that Acervoschwagerina is Yakhtashian and possibly ranging up to Bolorian. Thirteen species of fusulines including two new species, Acervoschwagerina gongendaniensis Kobayashi and Cuniculinella omiensis Kobayashi are described systematically.

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 habit for the pachydiscid.

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 a satellite process of the mandible. OMNH-QV 282 expands diversity for the local fauna, and also adds evidence for the existence of large-sized balaenopterids from a poorly known epoch, the Middle Pleistocene.