Paleontological Research

Two mandibles of fossil mysticetes from the early Pliocene, upper part of the Horokaoshirarika Formation at Numata Town, Hokkaido, Japan, belong to the archaic, extinct cetotheriid baleen whale, Herpetocetinae gen. et sp. indet. by having an elongated angular process projecting posteriorly beyond the mandibular condyle. The new materials of the Herpetocetinae represent their northernmost occurrence in the North Pacific.

The Upper Cretaceous Nakaminato Group, which contains the Chikko, Hiraiso, and Isoai formations in ascending order, crops out along the Pacific coast of Ibaraki Prefecture, central Japan. This group is composed mainly of sandstone, siltstone, and sandstone-siltstone alternations, with intercalated conglomerate layers at several levels. The siltstone of the Hiraiso and Isoai formations has yielded ammonites and inoceramid bivalves indicating a middle Campanian to Maastrichtian age. Some conglomerate layers in the Isoai Formation reach 1 m in thickness and mostly consist of pebbles and cobbles of rhyolite, dacite, chert, siliceous siltstone, siltstone, sandstone, and hornfels. We obtained late Paleozoic to Late Jurassic radiolarians from pebbles of argillaceous rock and chert from four levels of the conglomerate layers within the Isoai Formation. We describe the radiolarians systematically herein. The probable provenance of the radiolarian-bearing pebbles is interpreted as the Ashio and Yamizo terranes, which consist of Late Jurassic to Early Cretaceous accretionary complexes. We propose that there were two denudation stages of the accretionary complexes in the Kanto District, stages α (Barremian–) and β (Campanian–).

We studied the deep-sea benthic foraminiferal fauna from core NGC108 (36°36.85′N, 158°20.90′E; 645 cm long, recovered from 3,390 m water depth) collected from the Shatsky Rise in the central northwestern Pacific Ocean, to determine the relationship between benthic foraminifera and paleoproductivity in the transitional zone between the subtropical Kuroshio Extension and the subarctic current over the last 187 kyr. Cassidulina reniforme, Eilohedra levicula, and Epistominella exigua were the most abundant species. The relative abundance of C. reniforme was positively correlated with biogenic opal and organic carbon contents, which are proxies for paleoproductivity. The relative abundance of E. exigua was negatively correlated with biogenic opal and organic carbon contents, and the relative abundance of E. levicula had no correlation with biogenic opal and organic carbon contents. We conclude that C. reniforme and E. exigua are indicative of high and low productivity, respectively. The high relative abundances of C. reniforme reflect high paleoproductivity during glacial periods [oxygen isotope stages (OISs) 6, 3, and 2], indicating a southward advance of the Subarctic Front during global cooling. The high relative abundances of E. exigua reflect low paleoproductivity during the interglacial and postglacial periods (OISs 5 and 1). Faunal changes in the deep-sea benthic foraminifera on the Shatsky Rise suggest a direct response to orbital-scale paleoproductivity changes.

A fish fossil found from the Miocene Bessho Formation near Amikake, Sakaki Town, Hanishina County in Nagano Prefecture, central Japan is described as a new species of the genus Clupea, C. macrocephala of the family Clupeidae of Clupeiformes. This new species differs from other species of the genus in having a large head about 3.4 times in the standard length versus more than 4 times in other species. Clupea macrocephala sp. nov. is closer to Clupea pallasii than Clupea harengus in the number of vertebrae. Presence of the fossil herrings of the genus Clupea in the deposits of Pacific basin implies that the origin of this genus, probably, was in the Pacific Ocean in the Miocene.

A nearly complete right mandibular tooth plate of Ischyodus bifurcatus Case (Holocephali: Chimaeroidei) is reported from the Point Loma Formation (upper Campanian) of the Upper Cretaceous Rosario Group in southern California, USA. The individual is estimated to have measured nearly 1 m in total body length. Remains of I. bifurcatus have been reported from marine rocks deposited in epicontinental seas and continental shelf paleoenvironments of temperate latitudes in the Northern Hemisphere. Previous records of the species consist of specimens from Santonian to Maastrichtian strata of the US (Delaware, New Jersey, North Carolina, Georgia, Alabama, Arkansas, Montana, and Wyoming), Sweden, and European Russia. The tooth plate described herein is the first verifiable record of I. bifurcatus from California, and, more significantly, represents the only known definite Mesozoic record of Ischyodus from the entire North Pacific region.

Faunal composition and correlation of the late Carboniferous and Permian fusulines contained in exotic limestone blocks of the Sakamototoge area, Gifu Prefecture, Japan were reconsidered. Main alterations of the previous results are summarized as: (1) Moscovian fusulines are confined to the Myachkovian; (2) many species collectively included in Triticites are attributed to Protriticites, Montiparus, Rauserites, Triticites, or Schwagerina; (3) “Triticites” faunas designated as the Carboniferous–Permian are reorganized into three faunas: Protriticites subschwagerinoides (early Kasimovian), Montiparus mesopachus-Protriticites kiyomiensis (middle Kasimovian), and Rauserites arcticus-Carbonoschwagerina nakazawai-Quasifusulina longissimi (late Kasimovian to possibly early Gzhelian); and (4) the species assigned to Pseudoschwagerina is now attributed to late Gzhelian Carbonoschwagerina, implying the probable absence of the Asselian in the area. In addition, the Capitanian Yabeina fauna heterogeneous to the coeval neoschwagerinid faunas in the Permian terranes of Japan is ascertained.

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.