2025 年 29 巻 p. 64-75
Five species of Late Cretaceous ammonoids, Yezoceras miotuberculatum, Yezoceras elegans, Eubostrychoceras indopacificum, Pseudoxybeloceras sp., and Yabeiceras orientale, have been identified from the Obisagawa Member of the Ashizawa Formation, Futaba Group, Fukushima Prefecture, northeastern Japan. Notably, Yezoc.miotuberculatum, Yezoc. elegans, and the genus Pseudoxybeloceras, previously known from the Yezo Group in Hokkaido, northern Japan, were identified for the first time from the Futaba Group. The presence of Yezoc.miotuberculatum and Yezoc. elegans indicates faunal similarities between the Ashizawa Formation and the Yezo Group in Hokkaido. Co-occurrence of age-diagnostic inoceramids suggesting latest Turonian to earliest Coniacian age allows us to provide a detailed biostratigraphic correlation between the Futaba and Yezo groups.
The Futaba Group comprises terrestrial to shallow marine strata of the Upper Cretaceous in the Iwaki area in southeastern Fukushima Prefecture, northeastern Japan (Figure 1). It is divided into the Ashizawa, Kasamatsu, and Tamayama formations in ascending order (Konno, 1938; Saito, 1960, 1961; Ando et al., 1995; Kubo et al., 2002). Among them, the Ashizawa Formation, representing latest Turonian (?) to Coniacian shallow marine strata, contains abundant and diverse ammonoid fossils, including taxa useful for international correlation (e.g. Tokunaga and Shimizu, 1926; Matsumoto et al., 1964, 1989, 1990; Matsumoto, 1967; Ando et al., 1995; Futakami et al., 2016; Inose, 2018; Futakami and Suzuki, 2019). Numerous ammonoid specimens have been collected by local fossil collectors and are stored, with precise locality information, at the Iwaki City Board of Education, Hirono Town Board of Education, and Fukushima Museum. However, detailed paleontological descriptions of these specimens are still in progress, and the ammonoid fauna of the Ashizawa Formation remains to be completed. This study describes five species of ammonoids from the Obisagawa Member of the Ashizawa Formation, including first reported species.
The Cretaceous Futaba Group is distributed narrowly from north of Iwaki City to south of Naraha Town, Fukushima Prefecture, northeastern Japan. This group was deposited on the western margin of the forearc basin along the eastern margin of the paleo-Asian continent (Ando, 1997, 2003; Ando and Takahashi, 2024). According to Ando et al. (1995), the Futaba Group nonconformably covers Lower Cretaceous granitic rocks on the western margin and unconformably covers the Permian Takakurayama Group at its southwestern margin. Along the eastern margin, it is unconformably overlain by the upper Eocene–lower Oligocene Shiramizu Group. On its southern margin, the group is cut by a fault. The strata of this group trends NNE–SSW and dips 10–20°E. The Ashizawa Formation discussed here is subdivided into the fluvial Asamigawa and shallow marine Obisagawa members and is conformably overlain by the Kasamatsu Formation, which shows fluvial or estuarine to shallow marine environments (Ando et al., 1995; Kubo et al., 2002).
Ammonoids described in this study were collected from the Obisagawa Member (Figure 2). This member, having a thickness of up to 170 m, consists mainly of bioturbated fine sandstone with intercalations of cross-stratified medium to coarse sandstone layers and lenticular beds of gravelly fine to medium sandstone (Ando et al., 1995; Kubo et al., 2002). The sedimentary facies of this member indicate that it was deposited within upper shoreface to inner shelf environments (Ando et al., 1995). This member contains at least two ammonoid-rich horizons: the basal and middle parts.
The basal part of the Obisagawa Member is well exposed in the Sakurazawa Valley, Hirono Town (Loc. 1 in Figure 1). This horizon comprises calcareous conglomeratic sandstone with well-rounded pebbles and contains abundant fossils, such as shallow marine bivalves (e.g. Glycymeris amakusensis Nagao, 1930 and Apiotrigonia minor (Yabe and Nagao, 1925)), inoceramids (e.g. Inoceramus uwajimensis Yehara, 1924 and “Cremnoceramus rotundatus” (sensu Matsumoto and Noda, 1985)), and ammonoids (e.g. Yabeiceras orientale Tokunaga and Shimizu, 1926 and Eubostrychoceras indopacificum Matsumoto, 1967) (Ando et al., 1995; Futakami et al., 2016; Inose and Watanabe, 2020; Inose et al., 2024a). There are no signs of shell dissolution before burial, although bivalves are disarticulated and ammonoids are fragmented. This mode of occurrence indicates that they are not derived fossils from older strata.
The second fossiliferous horizon is intercalated in the middle part of the Obisagawa Member, approximately 40 m above the base, which is correlated to the units 2 to 5 of Omori et al. (2023). This horizon is well exposed around the Iwaki City Ammonite Center (Locs. 2 and 3 in Figure 1). It mainly comprises bioturbated, hummocky cross-stratified, very fine-grained sandstone (Omori et al., 2023). It contains abundant fossils, including large ammonoids over 30 cm in diameter (e.g. Mesopuzosia yubarensis (Jimbo, 1894)), several species of small ammonoids (e.g. Gaudryceras denseplicatum (Jimbo, 1894), Anagaudryceras limatum (Yabe, 1903), Forresteria alluaudi (Boule et al., 1907)), and some bivalves, including Inoceramus uwajimensis (Matsumoto et al., 1990; Ando et al., 1995; Nakada, 2017; Futakami and Suzuki, 2019).
Three newly identified species from the Futaba Group and two previously known species from this group are described and briefly discussed below.
Institution abbreviations.—GK = Department of Earth and Planetary Sciences, Kyushu University, Fukuoka; HBE = Hirono Town Board of Education, Fukushima; IGPS = Institute of Geology and Paleontology, Tohoku University, Miyagi; MCM = Mikasa City Museum, Hokkaido; I = Iwaki City Board of Education, Fukushima; FM = Fukushima Museum, Fukushima.
Family Nostoceratidae Hyatt, 1894
Genus Yezoceras Matsumoto, 1977
Type species.—Yezoceras nodosum Matsumoto, 1977.
Yezoceras miotuberculatum Matsumoto, 1977
Yezoceras miotuberculatum Matsumoto, 1977, p. 320, pl. 46, figs. 1, 2, pl. 61, fig. 2, text-fig. 5.
Holotype.—GK.H1391 from the Inoceramus uwajimensis Zone in the upper course of the Shuparo River in Oyubari, Hokkaido, Japan (Matsumoto, 1942, 1977).
Material examined.—FM-N202300030, a slightly deformed partial body chamber, was recovered from the middle part of the Obisagawa Member (Loc. 3 in Figure 1).
Description.—Coiled sinistrally. One and a half tightly coiled whorls are in contact. The shell surface has very fine, oblique ribs numbering 90 per whorl. Two rows of tubercles appear on every three or four ribs at the bottom and in umbilical peripheries (Figure 3O). The tuberculated ribs become slightly prominent at the bottom shoulder.
Remarks.—The helical whorls of the holotype of Yezoceras miotuberculatum (Matsumoto, 1977, pl. 46, fig. 2 (= GK.H1391)) are separated from each other, whereas those of the paratypes (Matsumoto, 1977, pl. 46, fig. 1 (= GK.H1392) and pl. 61, fig. 2 (= GK.H3541)) nearly contact each other. This indicates the wide intraspecific variation in the mode of coiling. A retroversal hook appears in the matured Yezoc. miotuberculatum (Matsumoto, 1977, pl. 61, fig. 2 (= paratype GK.H3541)), whereas it is not observed in the Futaba specimen.
Occurrence.—The described specimen was discovered in the middle part of the Obisagawa Member of the Ashizawa Formation. Yezoceras miotuberculatum is described from Coniacian deposits in the Oyubari and Urakawa areas, Hokkaido, Japan (Matsumoto, 1977).
Yezoceras elegans Aiba in Aiba et al., 2021
Yezoceras elegans Aiba in Aiba et al., 2021, p. 3, figs. 3–8.
Holotype.—MCM-K0044 from float calcareous concretions presumably derived from the Inoceramus uwajimensis Zone of the Lower Haborogawa Formation in the Nakanofutamata-gawa River in Haboro, Hokkaido, Japan (Aiba et al., 2021).
Material examined.—Four specimens, I-423238, I-423356, FM-N202200105, and FM-N202200106, from the middle part of the Obisagawa Member of the Ashizawa Formation (Loc. 2 in Figure 1). Another specimen, HBE.P-111, originates from the basal part of the Obisagawa Member (Loc. 1 in Figure 1). I-423238 consists of one whorl of the body chamber and a cast of a half whorl. I-423356, FM-N202200105, and FM-N202200106 comprise a half whorl of the body chamber. HBE.P-111 consists of a half whorl.
Description.—Early stage (up to 30 mm in coiling diameter; I-423238 and FM-N202200106): The whorls are loosely coiled and entirely separated from each other (Figure 3J). The shell surface is covered with very fine, slightly prorsiradiate ribs, numbering about 100 ribs per whorl. Two rows of tubercles on the ribs appear sparsely at the lower shoulder and the umbilical seam (Figure 3M and N). The tuberculate ribs thicken slightly between the tubercles. The original shape of the whorl section is uncertain due to deformation.
Middle to later stages (over 30 mm in coiling diameter; I-423356, FM-N202200105, and HBE.P-111): The whorl is loosely coiled and slowly increases in diameter (Figure 3A, C, and G). The shell surface is ornamented with periodic collar ribs associated with front adjacent constrictions and two rows of tubercules (Figure 3B and E). The periodic ribs are interspaced by 2–5 prorsiradiate non-tuberculate fine ribs. The ribs and constrictions become weaker on the dorsal seam. Tubercules are located on the lower ventral shoulder and lower flank (Figure 3B, F, and H).
Remarks.—The specimens described herein fall within the size range of known Yezoceras elegans specimens, and they share similar diagnostic characteristics of Yezoc. elegans tubercles, which appear sparsely at the lower shoulder and the umbilical seam. Although two specimens, I-423356 and FM-N202200105, were previously identified as Hyphantoceras cf. flexuosum by Muramiya et al. (2022), they can be reassigned to Yezoc. elegans on the basis of their tubercle characteristics stated above.
Occurrence.—The described specimens were discovered in the basal and middle parts of the Obisagawa Member of the Ashizawa Formation. Yezoceras elegans occurs in the Coniacian deposits in the Haboro and Mikasa areas, Hokkaido, Japan (Aiba et al., 2021).
Genus Eubostrychoceras Matsumoto, 1967
Type species.—Eubostrychoceras indopacificum Matsumoto, 1967.
Eubostrychoceras indopacificum Matsumoto, 1967
Bostrychoceras indicum Stoliczka, 1865. Tokunaga and Shimizu, 1926, p. 193, pl. 22, fig. 4, pl. 26, figs. 1–10.
Eubostrychoceras indopacificum Matsumoto, 1967, p. 333, pl. 18, fig. 1 (with synonymy); Klinger et al., 2007, p. 97, fig. 10A–H (with synonymy).
Holotype.—IGPS35159, originally illustrated as Bostrychoceras indicum by Tokunaga and Shimizu (1926) and redescribed as Eubostrychoceras indopacificum by Matsumoto (1967). It was discovered in Sakurazawa, Hirono, Fukushima, Japan.
Material examined.—Four specimens, I-423239, I-423357, I-423358, and I-423359, were recovered from the middle part of the Obisagawa Member of the Ashizawa Formation (Loc. 2 in Figure 1). I-423239 and I-423357 consist of two and a half whorls. I-423358 consists of three and a half whorls. I-423359 consists of three whorls of the phragmocone.
Description.—The whorls are helically and tightly coiled and in contact. In the early growth stage, whorl diameter increases relatively rapidly, resulting in a large apical angle (Figure 4C). During the middle to late growth stage, whorl diameter increases slowly, forming a high turricone (Figure 4B and D). In the last growth stage, the body chamber is out of the coiling, and the aperture turns upward (Figure 4B). The shell surface is ornamented with sigmoidally curved prorsiradiate fine ribs, numbering 40–50 ribs per whorl. Flared ribs and constrictions are not recognized in preserved parts.
Remarks.—The upward-facing aperture of I-423358 indicates maturity. Conversely, I-423239 and I-423357, despite being larger than I-423358, maintain regular coiling and appear to remain immature. This observation indicates that the mature size of E. indopacificum varies among individuals, as pointed out by Matsumoto (1967).
Occurrence.—In the Futaba Group, E. indopacificum is described in the basal (Tokunaga and Shimizu, 1926; Matsumoto, 1967) and middle parts of the Obisagawa Member of the Ashizawa Formation. Eubostrychoceras indopacificum is also found in the Coniacian of Japan, India, Madagascar, and KwaZulu of South Africa (Klinger and Kennedy, 2003).
Family Diplomoceratidae Spath, 1926
Genus Pseudoxybeloceras Wright and Matsumoto, 1954
Type species.—Hamites quadrinodosus Jimbo, 1894.
Remarks.—Wright et al. (1996) placed several genera that have crioconic or trombone-like shaped shells with outer and sometimes inner ventrolateral tubercles (e.g. Schlueterella Wiedmann, 1962, Pseudoxybeloceras, Parasolenoceras Collignon, 1969, and Lewyites Matsumoto and Miyauchi, 1984) into subgenera of Pseudoxybeloceras. We herein follow this classification.
Pseudoxybeloceras sp.
Material examined.—Two specimens, I-412006 and I-423431-1, originate from the middle part of the Obisagawa Member of the Ashizawa Formation (Loc. 2 in Figure 1).
Description.—The shell curves in one plane (Figure 5A and C). The shell surface is covered with fine, simple, and prorsiradiate ribs adorned with ventral shoulder tubercles. Mid-flank tubercles appear, and ventral shoulder tubercules are elongate on every 4–5 ribs; these ribs are slightly wider and higher than those with only ventral shoulder tubercles (Figure 5B and D).
Remarks.—Pseudoxybeloceras (Lewyites) is characterized by ventrolateral tubercules on periodic strong ribs and non-tuberculated intercalated minor ribs looping at tubercles (Matsumoto and Miyauchi, 1984). This feature of the ornamentation differs quite markedly from the described specimens. They resemble P. (Pseudoxybeloceras) and P. (Parasolenoceras) by having ventral shoulder tubercules on every rib. However, P. (Pseudoxybeloceras) differs by having outer ventrolateral tubercules on every rib except for the early stage (e.g. Wright and Matsumoto, 1954; Matsumoto, 1977; Matsumoto and Miyauchi, 1984; Wright et al., 1996; Kennedy et al., 2007), and P. (Parasolenoceras) differs by lacking outer ventrolateral tubercules (e.g. Matsumoto and Miyauchi, 1984, 1986; Wright et al., 1996; Shigeta et al., 2019). They also resemble some species of P. (Schlueterella) (e.g. Pseudoxybeloceras (Schlueterella) compressum Klinger, 1976 and Pseudoxybeloceras (Schlueterella) stinnesbecki Ifrim, 2019) by their crioconic coiling forms, ventral shoulder tubercles, and mid-flank tubercles on every 4–5 ribs. These characteristics suggest that they are most likely assigned to subgenus Schlueterella. Pseudoxybeloceras (Schlueterella) kawadai (Matsumoto and Miyauchi, 1984) occurs from the same northwestern Pacific realm. However, it is discriminated from Pseudoxybeloceras sp. by its huge size and the much higher stratigraphic occurrence, i.e., upper Campanian (Matsumoto and Miyauchi, 1984; Kodama et al., 2002).
Occurrence.—The described specimens were discovered in the middle part of the Obisagawa Member of the Ashizawa Formation.
Family Collignoniceratidae Wright and Wright, 1951
Genus Yabeiceras Tokunaga and Shimizu, 1926
Type species.—Yabeiceras orientale Tokunaga and Shimizu, 1926.
Yabeiceras orientale Tokunaga and Shimizu, 1926
Yabeiceras orientale Tokunaga and Shimizu, 1926, p. 201, pl. 22, fig. 7; pl. 27, fig. 1; Matsumoto et al., 1964, p. 323, pl. 48, figs. 1, 2, text-figs. 1–3; Matsumoto, 1969, p.324, pl. 44, figs. 1, 2; pl. 45, fig. 1, text-figs. 12, 13; Kennedy et al., 1983, p. 295, text-figs. 41, 42G–I, 43–45; Futakami et al., 2016, p. 223, text-figs. 4, 5, 6A, 6C–E, 7A–F, 8A–D, 10, 11; Futakami and Suzuki, 2019, p. 3, text-fig. 2.
Yabeiceras himuroi Tokunaga and Shimizu, 1926, p. 203, pl. 22, fig.9; pl. 27, fig. 2.
Neotype.—GK.H5556, initially illustrated by Matsumoto et al. (1964), was recovered from the basal part of the Obisagawa Member of the Ashizawa Formation distributed in Sakurazawa, Hirono, Fukushima, Japan.
Material examined.—Two specimens, I-423355 and FM-N202200104, originate from the middle part of the Obisagawa Member of the Ashizawa Formation (Loc. 2 in Figure 1). I-423355 comprises partly missing three and a half whorls of the phragmocone and a portion of the subsequent body chamber. FM-N202200104 comprises approximately three partly missing whorls of the phragmocone.
Description.—The shell is fairly evolute and widely umbilicate. In the early growth stage (less than 7 cm in diameter), the shell is adorned with broad and low indistinct ribs, accompanied by a single row of small distinct mediolateral tubercules. The ribs and tubercules become indiscernible with growth, and the later whorls become almost smooth (more than 7 cm in diameter). The original shape of the whorl section is uncertain due to deformation.
The suture on the later whorl appears relatively simple (see Figure 6C). The lateral lobe (L) exhibits a broad base and finely incised, deep grooves. The umbilical lobes are notably less intricate than the lateral lobes. The saddle head between the second (U2) and third umbilical lobe (U3) is tripartite.
Remarks.—Yabeiceras himuroi was initially described by Tokunaga and Shimizu (1926) by monotypy and was collected from the same locality as the type locality of Yabeiceras orientale. Now, Y. himuroi is lumped with Y. orientale (Futakami et al., 2016). Futakami et al. (2016) demonstrated that the vanishing point of lateral tubercules of Y. orientale generally lies at 7 cm shell diameter, as observed in the described specimens.
Occurrence.—In the Futaba Group, Y. orientale is described in the basal (Tokunaga and Shimizu, 1926; Matsumoto et al., 1964) and middle parts of the Obisagawa Member of the Ashizawa Formation. Yabeiceras orientale is also described in the Coniacian deposits of Hokkaido (Matsumoto, 1969) and South Africa (Kennedy et al., 1983).
The Obisagawa Member of the Ashizawa Formation has been correlated to the lower to middle (?) Coniacian on the basis of ammonoids, such as Forresteria alluaudi, Yabeiceras orientale, and Baculites yokoyamai (Tokunaga and Shimizu, 1926), and the bivalves of Inoceramus uwajimensis and Didymotis akamatsui (Yehara, 1924). However, Inose et al. (2024a) described several upper Turonian inoceramids from the basal to middle part of this member, such as Mytiloides incertus Jimbo, 1894, and “Cremnoceramus rotundatus” sensu (Matsumoto and Noda, 1985). Among them, the occurrence of Cremnoceramus waltersdorfensis waltersdorfensis (Andert, 1911) is confined to a relatively narrow biostratigraphic interval spanning the uppermost Turonian to the lowermost Coniacian in Europe and North America (Walaszczyk and Cobban, 1998, 2000; Walaszczyk and Wood, 1998, 2018). On the other hand, associated ammonoids from the Obisagawa Member generally suggest the lower to middle (?) Coniacian (Matsumoto et al., 1982; Ando et al., 1995; Futakami et al., 2016). Yezoceras miotuberculatum and Yezoc. elegans described here also indicate a Coniacian age (Matsumoto, 1977; Aiba et al., 2021). Although the strict horizons of these index taxa have not been completely confirmed (Figure 2), there remains a biostratigraphic inconsistency between some ammonoids and inoceramids. From current knowledge, the basal to middle part of the Obisagawa Member of the Ashizawa Formation can be provisionally correlated to the uppermost Turonian to the lowermost Coniacian interval. Further research on the Futaba Group is required for a more detailed biostratigraphic correlation in the northwestern Pacific region.
Faunal characteristics of the ammonoid assemblage of the Obisagawa Member of the Ashizawa FormationThe ammonoid assemblage of the Obisagawa Member of the Ashizawa Formation exhibits a mixture of endemic taxa in the northwestern Pacific region (e.g. Mesopuzosia yubarensis, Gaudryceras denseplicatum, Anagaudryceras limatum, Yezoceras miotuberculatum, and Yezoc. elegans) and so-called “Tethyan” species (e.g. Forresteria alluaudi, Yabeiceras orientale, Ya. manasoaense Collignon, 1965, Eubostrychoceras indopacificum). Such mixed ammonoid assemblages occur commonly in the northwestern Pacific region during the Late Cretaceous (e.g. early to middle Cenomanian: Hayakawa and Nishino, 1999; Misaki et al., 2020, late Turonian: Futakami, 1986, middle to late Campanian: Shigeta et al., 2016, 2019, late Maastrichtian: Shigeta et al., 2015; Shigeta and Maeda, 2023).
As highlighted by Ando et al. (2014) and Inose et al. (2024b), the ammonoid assemblage of the Ashizawa Formation resembles the Coniacian fauna of the Yezo Group in Hokkaido, northern Japan (e.g. Mesopuzosia yubarensis, Anagaudryceras limatum, Gaudryceras denseplicatum, Forresteria alluaudi, Tongoboryceras kawashitai (Matsumoto, 1979), Yezoites pseudoaequalis (Yabe, 1910)). The discovery of Yezoceras miotuberculatum and Yezoc. elegans from the Ashizawa Formation substantiates this similarity.
This study newly described Yezoceras miotuberculatum, Yezoceras elegans, Eubostrychoceras indopacificum, Pseudoxybeloceras sp., and Yabeiceras orientale, discovered from the Turonian/Coniacian boundary horizon in the Obisagawa Member of the Ashizawa Formation, Futaba Group. Notably, Yezoc. miotuberculatum, Yezoc. elegans, and the genus Pseudoxybeloceras are reported for the first time from the Futaba Group. The assemblage closely resembles the Coniacian fauna of the Yezo Group in northern Japan. Because the Futaba Group is key to correlating the Coniacian strata between northeastern and southwestern Japan, further detailed research on the Futaba Group is required.
We are very grateful to the reviewers Haruyoshi Maeda (Kyushu University) and Tomohiro Nishimura (Hobetsu Museum) and associate editor Yuta Shiino (Niigata University) for their valuable comments on the manuscript, substantially improving the manuscript. We thank Akihiro Ishigaki for donating specimens to the Fukushima Museum and the persons who donated specimens to the Iwaki City Ammonite Center and Iwaki City Coal and Fossil Museum. Thanks are extended to Yasunari Shigeta (National Museum of Nature and Science) for his help in photographing and providing the literature. We are indebted to the Iwaki City Coal and Fossil Museum for allowing us to examine ammonoid specimens.
YM, HI, and DA were responsible for the description and taxonomic aspects. FU, HA, and HO were responsible for the geological aspects. YM, HI, FU, DA, CS, and INHA carried out the field work and collected the materials. All authors contributed to the writing of the manuscript.
