Futaba Group Research Articles

Inoceramid bivalves from the Ashizawa Formation of the Futaba Group (Upper Cretaceous) in the Iwaki area, Fukushima Prefecture, Northeast Japan and their geological implications: especially the Turonian-Coniacian stage boundary

Inoceramid bivalves from the Ashizawa Formation of the Futaba Group (Upper Cretaceous) in the Iwaki area, Fukushima Prefecture, Northeast Japan and their geological implications: especially the Turonian-Coniacian stage boundary

Large ammonoid shellbed and huge calcareous concretion bed in shallow-marine fine sandstone of the lower Coniacian Ashizawa Formation, Futaba Group, Northeast Japan

いわき市アンモナイトセンターの双葉層群足沢層大久川部層（コニアシアン下部）のアンモナイト（主に径40-60 cmのMesopuzosia yubarensis）密集層と直下にある炭酸塩コンクリーション濃集層の，堆積相・産状観察と地球化学分析から形成過程を考察した．M. yubarensisは，沖合いの生息場から死後浮遊で沿岸に達した軟体部の失われた殻が，ストーム波浪で住房部が破壊され，分級・集積・運搬され，下部外浜沖合い側で癒着HCS極細粒砂層のハンモックマウンドに沿って急速に埋積した．炭酸塩コンクリーションは，多様なサイズ（径15-194 cm）の長-扁球形で，密集しながらも比較的一様に分布する．コンクリーションの形成は，ストーム波浪で運搬された有機物と底質中のベントス遺骸の分解に伴って堆積物の浅所で始まり，その後，埋没に伴い堆積物深所におけるメタン生成帯での有機物分解が生じるまで継続した．

Two early eudicot fossil flowers from the Kamikitaba assemblage (Coniacian, Late Cretaceous) in northeastern Japan.

Two new fossil taxa referable to the basal eudicot grade are described from the Kamikitaba locality (ca. 89 MYBP, early Coniacian: Late Cretaceous) of the Futaba Group in Japan. These charcoalified mesofossils exhibit well-preserved three-dimensional structure and were analyzed using synchrotron-radiation X-ray microtomography to document their composition and internal structure. Cathiaria japonica sp. nov. is represented by infructescence segments that consist of an axis bearing three to four fruits. The capsular fruits are sessile and dehiscent and consist of a gynoecium subtended by a bract. No perianth parts are present. The gynoecium is monocarpellate containing two pendulous seeds. The carpel is ascidiate in the lower half and conduplicate in the upper part, and the style is deflected abaxially with a large, obliquely decurrent stigma. Pollen grains are tricolpate with a reticulate exine. The morphological features of Cathiaria are consistent with an assignment to the Buxaceae s. l. (including Didymelaceae). Archaestella verticillatus gen. et sp. nov. is represented by flowers that are small, actinomorphic, pedicellate, bisexual, semi-inferior, and multicarpellate. The floral receptacle is cup shaped with a perigynous perianth consisting of several tepals inserted around the rim. The gynoecium consists of a whorl of ten conduplicate, laterally connate but distally distinct carpels with a conspicuous dorsal bulge, including a central cavity. The styles are short, becoming recurved with a ventrally decurrent stigma. Seeds are ca. 10 per carpel, marginal, pendulous from the broad, oblique summit of the locule. Pollen grains are tricolpate with a reticulate exine pattern, suggesting a relationship to eudicots. The morphological features of Archaestella indicate a possible relationship to Trochodendraceae in the basal grade of eudicots. The fossil currently provides the earliest record of the family and documents the presence of Trochodendraceae in eastern Eurasia during the middle part of the Late Cretaceous.

Chemotaxonomy of plants by resistant macromolecular analysis in charred mesofossils from the Cretaceous Futaba Group

Chemotaxonomy of plants by resistant macromolecular analysis in charred mesofossils from the Cretaceous Futaba Group

A Late Cretaceous mammalian dentary from the Ashizawa Formation (Futaba Group), Fukushima, northeastern Japan

A mammalian dentary discovered in the Coniacian Ashizawa Formation (Fukushima, northeastern Japan) is described. The specimen is a fragment of the horizontal ramus of a left edentulous dentary with five alveoli, the distal four of which are plugged with broken roots. Based on the morphologies of the dentary and the roots, it is considered to be of a therian mammal. This constitutes the first discovery of a Mesozoic mammal in northeastern Japan and highlights the potential for future mammal discoveries in the Cretaceous System in northeastern Japan, which will be significant for disclosure of the mammalian faunal evolution in East Asia during the Late Cretaceous. 論文表題:福島県の上部白亜系足沢層 (双葉層群) から産出した哺乳類の歯骨 抄録 福島県のコニアシアン階足沢層 (双葉層群) より哺乳類の歯骨が発見された.標本は左下顎体の一部で,歯冠は保存されていないものの5つの歯槽があり,そのうち遠心の4つには壊れた歯根が入っている.歯骨や歯根の形態からみて,標本は獣類のものであると考えられる.この標本は東北日本から見つかった初めての中生代哺乳類化石であり,将来的に東北日本の白亜系からさらに哺乳類化石が見つかる可能性を示している.東アジアからは後期白亜紀の哺乳類化石があまり報告されていないので,今後追加標本が得られれば,東アジアにおける後期白亜紀の哺乳類進化を考える上で重要な情報を提供することができるだろう.

Analysis of alkyl lipids bound in sediment and plant mesofossils from the Cretaceous Futaba Group: Possible formation mechanism of organic matter-concentrated layer

Analysis of alkyl lipids bound in sediment and plant mesofossils from the Cretaceous Futaba Group: Possible formation mechanism of organic matter-concentrated layer

Lauraceous Fossil Flowers from the Kamikitaba Assemblage (Coniacian, Late Cretaceous) of Northeastern Japan (Lauraceae)

A new genus and species of Lauraceae, Microlaurus perigynus gen. et sp. nov. is described based on fossil charcoalified flower buds recovered from the Kamikitaba assemblage (early Coniacian, Late Cretaceous; ca. 89 million years before present (myr BP)) in the Ashizawa Formation (Asamigawa Member) of the Futaba Group in northeastern Japan. Analysis of the internal structure of the fossil buds using synchrotron-radiation X-ray microtomography (SRXTM) at the 2-BM-B beamline of the Advanced Photon Source (APS), Argonne National Laboratory, shows that the flowers are small, pedicellate, bisexual, trimerous, and actinomorphic, with small outer tepals, larger inner tepals, three whorls of stamens, an innermost androecial whorl composed of staminodia, and a unicarpellate gynoecium containing a single ovule. Microlaurus perigynus is assigned to the Lauraceae based on the regular trimerous floral organization and other details of floral structure, but it is distinguished from most previously described lauraceous fossil flowers by the poorly differentiated filament and anther in the stamens of the third whorl and the marked size difference between the small outer tepals and the large inner tepals. Also unusual are the paired glandular appendages that appear to be associated with the first (outermost) whorl of stamens, rather than the stamens of the third whorl, although the precise position is not fully clear. The same feature occurs in Hernandiaceae, the sister group of Lauraceae, as well as in Powhatania connata, an earlier but fragmentary lauralean fossil flower from the Early to Middle Albian of Virginia. Microlaurus perigynus adds to the floral diversity of Lauraceae known from the Late Cretaceous, and its presence in the Kamikitaba assemblage from Japan underlines the broad geographic distribution and floristic significance of lauraceous plants during the Late Cretaceous.

Floral Evidence of Annonaceae from the Late Cretaceous of Japan

Futabanthus asamigawaensis gen. et sp. nov. is based on a single fossil flower from the Kamikitaba locality in the Ashizawa Formation (Asamigawa Member), Futaba Group (early Coniacian: Late Cretaceous), of northeastern Honshu, Japan. The flower is small, pedicellate, bisexual, and actinomorphic. The floral receptacle is flattened and disk shaped, with a hypogynous perianth consisting of a small number of tepals, borne in at least two cycles, around the rim. The androecium comprises ∼90–100 stamens that are curved toward the center of the flower. Stamens are stout, with no clear differentiation into anther and filament. The thecae are lateral to abaxial in position, extend for most of the length of the stamen, and have latrorse‐extrorse dehiscence. The connective is expanded above the thecae into a prominent, flattened, sterile apical appendage. Together, the connective extensions of the stamens form a tessellated protective surface over the androecium. The gynoecium is composed of ∼100–120 free carpels borne on a small, conical projection in the center of the receptacle. The multipartite construction of the fossil flower, combined with the form of the stamens, indicates a relationship to Annonaceae in the order Magnoliales. The fossil currently provides the earliest record of the family and documents the presence of Annonaceae in eastern Eurasia during the middle part of the Late Cretaceous.

Fossil Flowers of Fagales from the Kamikitaba Locality (Early Coniacian; Late Cretaceous) of Northeastern Japan

Fossil flowers and fruits of Archaefagacea futabensis gen. et sp. nov., from the Asamigawa Member of the Ashizawa Formation (early Coniacian) of the Futaba Group in northeastern Honshu, Japan, are described. Flowers are small and bisexual, with an inferior ovary and an epigynous perianth composed of six small triangular tepals. The androecium is composed of six stamens positioned opposite the tepals, and the gynoecium consists of a trilocular ovary with three persistent apical styles. A single seed may develop in one or more locules. Archaefagacea is linked to extant and fossil Fagales, including flowers of fossil Normapolles plants, by its floral structure. Archaefagacea provides the earliest evidence of floral structure among those Fagales (Fagaceae, Nothofagaceae, and extinct taxa) that fall outside the group of six extant families and related fossil Normapolles plants that constitute the core Fagales. It is also the first record of Fagales in the Late Cretaceous of eastern Eurasia.

Fossil Seeds of Nymphaeales from the Tamayama Formation (Futaba Group), Late Cretaceous (Early Santonian) of Northeastern Honshu, Japan

Well‐preserved fossil seeds of an ANITA‐grade angiosperm are described from the Gokurakuzawa locality exposed along the Kobisa River, Fukushima Prefecture, in northeastern Honshu, Japan. Sediments at this locality compose the Tamayama Formation, the uppermost sedimentary unit of the Futaba Group, which is of early Santonian age (ca. 85 million yr BP). On the basis of comparison with extant taxa, these seeds are assigned to Nymphaeales. The presence of unequivocal Nymphaeales in the middle part of the Late Cretaceous contributes to emerging evidence for continuity in the fossil record of this key group of flowering plants from the very earliest phases of angiosperm diversification in the Early Cretaceous through their very extensive representation during the early Cainozoic and up to the Recent. The fossil seeds are assigned here to a new genus of Nymphaeales, Symphaenale futabensis gen. et sp. nov., with a combination of characters seen in the seeds of different extant genera of Nymphaeales.

A NEW ELASMOSAURID PLESIOSAUR FROM THE UPPER CRETACEOUS OF FUKUSHIMA, JAPAN

Abstract: A new genus and species of an elasmosaurid plesiosaur, Futabasaurus suzukii, is described based on a partial skeleton from the Inoceramus amakusensis Zone (Lower Santonian, Upper Cretaceous) of the Irimazawa Member of the Tamayama Formation, Futaba Group, in Fukushima Prefecture, Japan. The new taxon is characterized by a number of characters such as the wide space between the orbit and external naris, posterior extension of the interclavicle, relatively long humerus and prominent femoral muscle scar. The holotype includes a partial skull and mandible, posterior cervicals to sacrals, ribs, clavicular arch, pelvic girdle and four limbs. The remains are mostly in articulation, and exhibit evidence of predation/scavenging by sharks. The distribution of elasmosaurid species in the circum‐Pacific region remains unclear due to the lack of diagnostic materials. The occurrence of F. suzukii is geographically and stratigraphically significant, because it allows species‐level comparison; as a diagnosable elasmosaurid specimen, F. suzukii is the first and the oldest from the northern Pacific.

白亜紀〜古第三紀の陸上植物の変遷過程―白亜紀に多様化した被子植物群と古第三紀の植物相の地理的分布―

A new assemblage of Late Cretaceous fossil plants (the Kamikitaba assemblage) was discovered from the Ashizawa Formation of Futaba Group (Early Coniacian; 89Ma). The flora includes well-preserved mesofossils of angiosperm flowers, fruits, seeds, leaf fragments and woods.Some aspects on the origin of angiosperms and the vegetational consequences of Cretaceous and Paleogene flora are reviewed in the paper.

Hironoia fusiformis gen. et sp. nov.; a cornalean fruit from the Kamikitaba locality (Upper Cretaceous, Lower Coniacian) in northeastern Japan.

The application of sieving techniques to bulk samples from the Ashizawa Formation, Futaba Group (Lower Coniacian) of northeastern Honshu, Japan, has yielded well-preserved mesofossil assemblages comparable with those recently described from eastern North America, Europe, and central Asia. Among the most abundant and distinctive components of these assemblages are fusiform fruits that are assigned here to a new genus and species, Hironoia fusiformis gen. et sp. nov. The fruits developed from an epigynous ovary with three to four locules. Each locule bears one seed and has a distinctive dorsal germination valve. These features of the fruit, along with the adnate calyx, indicate an affinity to extant Cornales and specifically the Cornaceae sensu lato. The recognition of an unequivocal cornalean fruit in the Early Coniacian-Early Santonian of Japan provides the earliest record of this group in the fossil record. It also establishes a minimum age for the early divergence of the asterid clade, a major group of living angiosperms comprising more than a third of all species of extant flowering plants.

Fossil Flowers and Associated Plant Fossils from the Kamikitaba Locality (Ashizawa Formation, Futaba Group, Lower Coniacian, Upper Cretaceous) of Northeast Japan

Fossil Flowers and Associated Plant Fossils from the Kamikitaba Locality (Ashizawa Formation, Futaba Group, Lower Coniacian, Upper Cretaceous) of Northeast Japan

上部白亜系双葉層群の河川成〜浅海成堆積システム 堆積相と堆積シーケンス

The Futaba Group is the Lower Coniacian to Lower Santonian, fluvial to shallow-marine sedimentary succession distributed in the southern Abukuma Belt in north Honshu. Fifteen depositional facies were discriminated. They include alluvial fan, meandering river, braided river, transgressive lag and upper shoreface to inner shelf sediments. It is divided into three formations, the Ashizawa (the Asamigawa and Obisagawa Members), Kasamatsu and Tamayama Formations in upward sequence. Each formation consists of a third-order depositional sequence (DS).The first depositional sequence (DS 1) has a lowstand systems tract (LST) as the lower main part of the Asamigawa Member distributed in the northern half of the area. It is lithologically characterized by upper alluvial fan conglomerate and mid to lower fan sandstone and siltstone nonconformably overlying lower Cretaceous granite (sequence boundary 1 : SB 1). In the southern periphery of the distributed area the member contains talus or upper fan angular conglomerate. The upper part of the member distributed over the area is interpreted to be transgressive conglomerate with a sharp base called a ravinement surface (RS 1) which was formed by transgressive shoreface erosion. It may represent transgressive systems tract (TST). The following highstand systems tract (HST) is represented by the upper shoreface to inner shelf, medium to fine sandstone of the Obisagawa Member.The Kasamatsu Formation (DS 2) unconformably (SB 2) overlying the preceding depositional sequence (DS 1), is substantially characterized by fluvial upward-fining successions composed of channel sandstone and flood plain siltstone. The third sequence (DS 3) is characterized by thick fluvial (braided river) sandstone of the lower to middle part of the Tamayama Formation interpreted as LST and the upper shoreface to inner shelf sandstone of the upper Tamayama interpreted as HST. The sudden lithofacies change between the Kasamatsu and Tamayama Formations implies some large-scale changes in fluvial depositional systems by sea-level fall (SB 3). The transgressive conglomerate just above RS 2 in the basal part of the upper Tamayama Formation represents a thin TST. The upper sequence boundary (SB 4) of DS 3 shows the conspicuous clinounconformity between the Futaba Group and the Paleogene Shiramizu Group.

Spores and Pollen Flora from the Upper Cretaceous Futaba Group in Northeastern Japan

Spores and Pollen Flora from the Upper Cretaceous Futaba Group in Northeastern Japan

Discovery of elasmosaur from the Upper Cretaceous Futaba Group

Discovery of elasmosaur from the Upper Cretaceous Futaba Group

Additional note on the upper limit of the Cretaceous Futaba Group

Additional note on the upper limit of the Cretaceous Futaba Group

Discovery of Nannoplanktons from the Upper Cretaceous Futaba Group

Discovery of Nannoplanktons from the Upper Cretaceous Futaba Group

Note on the Upper Limit of the Cretaceous Futaba Group

Note on the Upper Limit of the Cretaceous Futaba Group