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–).
Introduction
Microfossil-bearing clasts within conglomerates provide information on the denudation history of the hinterland and conglomerate provenance. The upper Mesozoic strata in East Asia interbed microfossil-bearing clasts that were presumably derived from mid-Mesozoic accretionary complexes (ACs) in East Asia, such as the Chichibu composite and Tamba–Mino terranes. Ishida et al. (2003) reviewed these microfossil-bearing clasts as tracers for erosional events in southwest Japan and Korea, and showed that the denudation of the mid-Mesozoic ACs started during the Late Jurassic, and intensified during the Early Cretaceous. Ito et al. (2017) also analyzed microfossil-bearing clasts within the upper Mesozoic strata in East Asia and recognized three stages of denudation history (stages A to C) and a pre-stage of the mid-Mesozoic ACs. Although some of the data used in these two studies were from neritic–terrestrial sedimentary rocks of southwest Japan, most data were obtained from the Hokuriku District (e.g. Saida, 1987; Takeuchi et al., 1991; Matsukawa and Takahashi, 1999; Tomita et al., 2007; Ito et al., 2012, 2014, 2015), in addition to that of the Korean Peninsula (e.g. Chang et al., 1990, 2003; Yao and Chang, 1990; Kamata et al., 2000; Mitsugi et al., 2001).
A succession of Cretaceous neritic–terrestrial strata is well exposed along the Pacific coast of the Kanto District in central Japan, in spite of the narrow exposure just in the coastline (Figure 1B). These strata yielded abundantly ammonoids and bivalves, so the ages of the strata have been determined in detail (Figure 2A). Furthermore, these strata interbed conglomerate layers includeing microfossil-bearing clasts. The investigation of the microfossil-bearing clasts within the age-controlled strata should provide the information of denudation history of the hinterland and conglomerate provenance in detail. In contrast to the abundant macrofossil studies on the Cretaceous neritic-terrestrial strata along the Pacific coast of the Kanto District, only one previous study reported microfossil-bearing clasts within these strata: Wordian–Capitanian (Guadalupian, Permian) and Bathonian–Callovian (Middle Jurassic) radiolarian assemblages were found from chert pebbles of the Barremian Ashikajima Formation of the Lower Cretaceous Choshi Group in this district (Kashiwagi and Isaji, 2015).
Figure 1.
A, distribution of mid-Mesozoic accretionary complexes in central Japan, with previous occurrences of microfossil-bearing clasts within upper Mesozoic neritic–terrestrial strata; 1, Saida (1987); 2, Matsukawa and Takahashi (1999); 3, Ito et al. (2015); 4, Kojima (1986); 5, Takeuchi et al. (1991); 6, Tomita et al. (2007); 7, Ito et al. (2012, 2014); 8, Kashiwagi and Isaji (2015); B, route map of the Hiraiso and Isoai formations of the Nakaminato Group in the Hiraiso–Isozaki area along the Pacific coast of the Kanto District, central Japan.
The Upper Cretaceous Nakaminato Group, which is younger than the Choshi Group, is exposed along the Pacific coast of Ibaraki Prefecture, and contains conglomerate interbeds at several horizons. We obtained late Paleozoic to Late Jurassic radiolarians from pebbles of argillaceous rock and chert in four levels of conglomerate layers in the Isoai Formation of the Nakaminato Group. In total, we discriminated 37 species belonging to 25 genera and four unidentified radiolarians, from the pebbles. We here systematically describe these radiolarians. The detailed systematic description, showing radiolarian assemblages in each radiolarian-bearing pebble, indicates the age of the pebbles. On that basis, we discuss the staged denudation of the mid-Mesozoic ACs in the provenance of the Kanto District by comparing the previously proposed staged denudation.
Geologic setting
Outline of the Cretaceous strata along the Pacific coast of the Kanto District
In the Kanto District, Neogene–Quaternary thick strata overlie widely pre-Neogene geologic units, so that the exposure of the pre-Neogene is restricted in distribution. They are, however, well exposed along the Pacific coast.
The Lower Cretaceous Choshi and Upper Cretaceous Nakaminato groups were described in the Pacific coast of the district. The former group comprises the Ashikajima, Kimigahama, Inubouzaki, Toriakeura, and Nagasakihana formations in ascending order (Obata et al., 1982). Age of the Choshi Group was determined in detail by ammonite occurrences (e.g. Obata and Matsukawa, 2007, 2009).
Lithology of the Nakaminato Group
The Nakaminato Group, which is regarded as the northeastern extension of the Izumi Group of southwest Japan (Ando, 2006), occurs along the Nakaminato Coast of the Pacific Ocean of Ibaraki Prefecture, central Japan (Figure 1). The Nakaminato Group consists of the Chikko, Hiraiso, and Isoai formations in ascending stratigraphic order (Tanaka, 1970), and mainly comprises sandstone, siltstone, and sandstone-siltstone alternations, with some intercalated conglomerate layers. The total thickness of the group is more than 1,500 m. The Nakaminato Group may be in faulted contact with Neogene rocks at the northern and southern ends of its outcrop, and extends into the ocean to the east (Figure 1). Concerning the depositional environment of this group, Tanaka (1970) regarded the lower part of the Hiraiso Formation as having been deposited on the offshore continental shelf, while the upper part of the Hiraiso Formation and the Isoai Formation correspond to a continental slope to trough slope setting. Masuda and Katsura (1978) and Katsura and Masuda (1978) inferred that the Nakaminato Group is composed of submarine fan sediments: the lower part of the Hiraiso Formation was deposited in the lower submarine fan: the upper part of the Hiraiso Formation and the Isoai Formation are in the middle to lower submarine fan.
The distribution of the Chikko Formation cannot now be recognized as it is covered by concrete resulting from harbor construction. According to Tanaka (1970), this formation is composed of pale green, massive, mediumgrained sandstone with a thickness inferred to be more than 30 m. There are no reports of fossils from this formation. The stratigraphic relationship between the overlying and underlying formations is uncertain because of the lack of outcrop.
The Hiraiso Formation consists mainly of siltstone, with the upper part predominantly containing sandstonesiltstone alternations. The total thickness of this formation is more than 580 m. The lower limit of this formation cannot be observed due to lack of outcrop. The siltstone in the lower part of the formation contains several thin (less than 2 cm thick) sandstone layers and thick (5 to 10 m thick) sandstone layers, and contains calcareous nodules at various levels. Slump structures and pebbly mud layers are also observed in the lower siltstone. The upper part of this formation is characterized by sedimentary structures such as grading, parallel- and cross-laminations, and ripup clasts.
The Isoai Formation is composed of turbiditic sandstone-siltstone alternations, thick sandstone beds, and siltstone layers, with some intercalated conglomerate layers. This formation conformably overlies the Hiraiso Formation. The total thickness of the Isoai Formation is 980 m. Overturned layers are present within the middle part of this formation, which may have been caused by large-scale submarine sliding.
Age of the Nakaminato Group
The age of the Nakaminato Group is estimated to be middle Campanian to early Maastrichitian on the basis of the ammonites and inoceramid bivalves in the Hiraiso and Isoai formations (Saito, 1961, 1962; Ando, 2006).
Saito (1961, 1962) reported the occurrence of ammonoid fossils such as, Didymoceras awajiense, D. nakaminatoense, and Pravitoceras? sp., and the bivalve Inoceramus cf. balticus in the Hiraiso Formation and Baculites inornatus, B. cf. rex, and Inoceramus cf. shikotanensis from the Isoai Formation. Tanaka (1970) reported Polyptycoceras sp. from the Isoai Formation. The presence of these fossils indicates that the age of the Hiraiso and Isoai formations is middle Campanian to early Maastrichtian and the Campanian–Maastrichtian boundary lies within the upper part of the Isoai Formation.
Recently, Kashiwagi et al. (2015) reported radiolarian fossils indicating a middle Campanian to middle Maastrichtian age from calcareous nodules in the Hiraiso Formation. The ages indicated by radiolarians are consistent with those suggested by ammonoid and bivalve fossils.
Lithostratigraphy of radiolarian-bearing conglomerate layers of the Isoai Formation in the studied sections
We made a route map in the Hiraiso–Isozaki area along the Pacific coast (Figure 1B) and established the lithostratigraphy of a sandstone and siltstone sequence with intercalated conglomerate layers, in an approximately 130 m thick section in the lower part of the Isoai Formation. We collected more than 20 radiolarian-bearing pebbles, of which seven pebbles from four levels, NK-1 to NK-4 (Figure 2B), yielded relatively well preserved radiolarians. The following is an outline of the lithostratigraphy of the measured section, which is divided into lower and upper sections.
The Lower section.—The thickness of this part of the section is about 65 m, and is divided into lower and upper subsections on the basis of lithological differences. The lower subsection is composed of sandstone-siltstone alternations and medium-grained sandstone with several conglomerate layers. The upper subsection consists of sandstone and conglomerate layers. Most of the sandstone and conglomerate layers exhibit a fining-upward sequence. Radiolarians were recovered from a pebble in a conglomerate layer (NK-1) of this lower subsection.
The Upper section.—The thickness of this section is about 65 m. The lithological sequence is characterized by frequent intercalations of conglomerate layers and medium-grained sandstone. Most of the conglomerate layers exhibit a fining-upward sequence. Radiolarians were identified from six pebbles from three conglomerate layers (NK-2, NK-3, NK-4).
Most of the studied conglomerate layers have almost the same lithological characteristics: the gravel size is generally smaller than cobble (<64 mm), the clasts are subangular to subcircular, sorting is poor, and the rock is clast-supported, sometimes with grading and no imbricated structures (Figures 3, 4). More than 75% of the pebbles are rhyolite and dacite. Other rock types are chert, siliceous siltstone, siltstone, sandstone, and hornfels. Granite is very rare.
Material and methods
We identified radiolarian-bearing pebbles using a hand lens at the outcrop and carefully collected these pebbles with a small chisel and hammer. The pebbles were isolated from matrices, and respective pebbles underwent chemical process for extraction of the radiolarian fossils.
Radiolarians were extracted from these pebbles using diluted hydrofluoric acid (HF), as described by Pessagno and Newport (1972). In general, the pebbles are gray chert, black to dark gray siliceous siltstone, and dark green siltstone. We recovered radiolarians from the following pebbles.
Pebble no. 1 from Sample NK-1; dark gray siliceous siltstone.
Pebble no. 2 from Sample NK-2; black siliceous siltstone.
Pebble no. 3 from Sample NK-3; gray chert.
Pebble no. 4 from Sample NK-3; gray chert.
Pebble no. 5 from Sample NK-3; gray chert.
Pebble no. 6 from Sample NK-3; dark green siltstone.
Pebble no. 7 from Sample NK-4; dark green siltstone.
Discussions
Age of pebbles based on radiolarian fossils
Seven pebbles from four levels in the conglomerate layers yielded 40 species belonging to 25 genera, including four unidentified radiolarians (Figures 4, 5), as stated in “Systematic paleontology”. We here infer the ages of these pebbles based on the previously established radiolarian biostratigraphy (Table 1).
Pebble no. 1 yielded Archaeodictyomitra sp. B, Archaeodictyomitra sp. C, Zhamoidellum sp., and Protunuma? sp. O'Dogherty showed the age range of Protunuma is middle Toarcian to late Tithonian and that of Zhamoidellum early Pliensbachian to late Tithonian. O'Dogherty et al. (2009) compiled the occurrence range of Jurassic radiolarian genera; therefore, generic components can provide reliable age determination. Based on the compilation of O'Dogherty et al. (2009), the generic components of Pebble no. 1 indicate an Early to Late Jurassic (middle Toarcian to late Tithonian) age.
Pebble no. 2 yielded the following radiolarians: Williriedellum sp. A group Matsuoka, 1983, Sethocapsa sp. B, Eucyrtidiellum sp., Spongocapsula? sp., Paronaella sp., and Spumellaria? gen. et sp. indet. B. Of these radiolarians, Williriedellum sp. A group Matsuoka indicates a Middle to Late Jurassic (late Bajocian to early Oxfordian) age (Baumgartner et al., 1995). Co-appearances of other radiolarians are consistent with the occurrence range of Williriedellum sp. A group.
Pebble no. 3 yielded only Pseudoalbaillella? sp. According to Holdsworth and Jones (1980), Pseudoalbaillella has a stratigraphic range of middle Carboniferous to middle Permian. Ishiga (1990) mentioned that the abundance of Pseudoalbaillella gradually diminished in the late Guadalupian. Therefore, we can say only that this pebble may have a late Paleozoic (middle Carboniferous? to Guadalupian) age.
Pebble no. 4 yielded Triassocampe sp. A, Triassocampe sp. B, Triassocampe sp. C, Triassocampe sp. D, multisegmented Nassellaria gen. et sp. indet., Betraccium? sp., Spumellaria? gen. et sp. indet. A, Spongostephanidium? sp., Cryptostephanidium? sp., and Capnuchosphaera sp. According to O'Dogherty et al. (2010) the occurrence range of Capnuchosphaera is early Carnian to late Campanian, Spongostephanidium is late Olenekian to late Carnian and Cryptostephanidium is early Anisian to late Carnian. Therefore, the age of Pebble no. 4 is Late Triassic (Carnian).
Pebble no. 5 yielded Katroma sp., Parashuum sp. A, Parashuum sp. B, Parashuum sp. C, and Parashuum? sp. Katroma has an age range of early Sinemurian to early Toarcian (O'Dogherty et al., 2009). The co-occurrence of the genera Katroma and Parashuum indicates an Early Jurassic (early Sinemurian to early Toarcian) age (O'Dogherty et al., 2009).
Pebble no. 6 yielded Podocapsa cf. amphitreptera, Cinguloturris cf. carpatica, Archaeodictyomitra minoensis, Archaeodictyomitra sp. A, Loopus sp. A, Loopus sp. B, Sethocapsa sp. A, Gongylothorax? sp., Eucyrtidiellum sp., Pseudoeucyrtis? sp., Poulpus? sp., Pantanellium sp., and Triactoma sp. Of these radiolarians, Archaeodictyomitra minoensis has an age range of middle late Oxfordian to early late Tithonian and Cinguloturris carpatica ranges from the late Bathonian to the early Tithonian (Baumgartner et al., 1995). Therefore, the age of this pebble is Late Jurassic (middle late Oxfordian to early Tithonian).
Pebble no. 7 yielded Sethocapsa sp. C, Loopus sp. B, and Ristola sp. Of these species, Ristola sp. is quite similar to R. altissima s.l. (Rüst), which has an age range of latest Bajocian to early late Tithonian (Baumgartner et al., 1995), and the genus Loopus is known to range from the early Bathonian to the early Aptian (O'Dogherty et al., 2009). Although data for the age of this pebble are quite scarce, it can be said to be Middle to Late Jurassic (early Bathonian to early late Tithonian) in age.
Figure 5.
SEM (Scanning Electron Microscope) photos of radiolarians from the pebbles of the Isoai Formation, Nakaminato Group. 1, Protunuma ? sp., EES-KS0101, from pebble no. 1; 2, Archaeodictyomitra sp. B, EES-KS0102, from pebble no. 1; 3, Archaeodictyomitra sp. C, EES-KS0103, from pebble no. 1; 4, Archaeodictyomitra sp. A, EES-KS0104 from Pebble no. 6; 5, Archaeodictyomitra minoensis (Mizutani), EES-KS0105 from pebble no. 6; 6, Parahsuum sp. B, EES-KS0106, from pebble no. 5; 7, Parahsuum sp. A, EES-KS0107, from pebble no. 5; 8, Parahsuum? sp., EES-KS0108 from pebble no. 5; 9, Multisegmented Nassellaria gen. et sp. indet., EES-KS0209, from pebble no. 4; 10, Triassocampe sp. D, EES-KS0110, from pebble no. 4; 11, Spongocapsula? sp., EES-KS0111, from pebble no. 2; 12, 16, Loopus sp. B; 12, EES-KS0112, from pebble no. 7; 16, EES-KS0116, pebble no. 2; 13, 14, Cinguloturris cf. carpatica Dumitrica; 13, EES-KS0113, from pebble no. 6; 14, EES-KS0214, from pebble no. 6; 15, Loopus sp. A, EES-KS0115, from pebble no. 6; 17, 23, Triassocampe sp. B; 17, EES-KS0117, from pebble no. 4; 23, EES-KS0113, from pebble no. 4; 18–20, Triassocampe sp. A; 18, EES-KS0118, from pebble no. 4; 19, EES-KS0119, from pebble no. 4; 20, EES-KS0110, from pebble no. 4; 21, Parahsuum sp. C, EES-KS0121, from pebble no. 5; 22, Ristola sp., EES-KS0122, pebble no. 6; 24, Pseudoalbaillella? sp., EES-KS0124, from pebble no. 3. Scale bars indicate 100 µm.
Table 1.
List of radiolarians from pebbles of the Isoai Formation. X shows the existence of radiolarian species.
Derivation of radiolarian-bearing clasts
Our examined radiolarian-bearing pebbles are late Paleozoic to Late Jurassic in age, as mentioned above. The lithologies and ages of these clasts are consistent with those of the mid-Mesozoic ACs in East Asia, such as the Chichibu composite and Tamba–Mino terranes. According to previous studies (e.g. Matsuoka et al., 1998; Nakae, 2000), cherts are known to occur in strata from the Pennsylvanian (upper Carboniferous) to the Upper Jurassic; argillaceous rocks range from the Upper Triassic to the lowest Cretaceous. Lithological similarities and age therefore suggest that the radiolarian-bearing pebbles were derived from the mid-Mesozoic ACs. We here explain the Ashio and Yamizo terranes, which are east extensions of the Tamba–Mino terrane, as neighbor examples of the Nakaminato Group.
Kamata (1996) extensively studied the lithostratigraphy, tectonics, and ages of the Ashio terrane in the Ashio Mountains. Hori and Sashida (1998) and Sashida and Hori (2000) clarified the lithology, ages, and tectonostratigraphy of the Yamizo terrane in the Yamizo Mountians. Permian radiolarian-bearing chert was reported from the Omama Complex of the Ashio terrane by Kamata (1996). Upper Triassic cherts are known to occur in the Kuzu and Kurohone-Kiryu complexes (Kamata, 1996) and the Takatoriyama and Kasama units of the Yamizo terrane (Sashida and Hori, 2000). Lower Jurassic cherts are also known from the Omama and Kurohone-Kiryu complexes of the Ashio terrane and the Takatoriyama and Kasama units of the Yamizo Terrane. Middle Jurassic siliceous siltstones have been reported from the Omama, Kuzu, and Kurohone-Kiryu complexes and the Takatoriyama Unit of the Yamizo terrane. Upper Jurassic siltstones occur in the Kuzu Complex and the Takatoriyama and Kasama units of the Yamizo terrane. On the basis of the lithological similarities between the geological units and the clasts in the conglomerate, we concluded that the clasts were derived from geologic units in the Ashio and Yamizo terranes and/or corresponding geologic units.
Provenance of the Nakaminato Group
Pebbles in the conglomerate layers of the Nakaminato Group are mostly volcanic rocks, such as rhyolite, dacite, and pyroclastic material with subordinate andesite (Tanaka and Kawada, 1971). Pebbles of chert, siliceous shale, sandstone, siltstone, and hornfels also occur. Volcanic rocks constitute more than 75% of pebbles, as mentioned above.
Concerning the provenance of the volcanic pebbles from the Nakaminato Group, Tanaka and Kawada (1971) inferred that volcanic pebbles were derived from the Late Cretaceous Okunikko Rhyolites, which are extensively distributed in and around the Ashio Mountains, based on the lithological and petrographic similarities between the clasts and the Okunikko Rhyolites. Furthermore, they showed the southeasterly to easterly current directions for the coarse-grained materials of the Nakaminato Group as the supporting evidence.
As mentioned in the previous section, we concluded that the radiolarian-bearing argillaceous rock and chert pebbles were derived from the geologic units in the Ashio terrane in the Ashio Mountians and the Yamizo terrane in the Yamizo Mountains and/or corresponding geologic units. Briefly, it is plausible that the Okunikko Rhyolites provided the greatest supply of clasts to the formation of the Nakaminato Group while the Ashio and Yamizo terranes contributed a moderate supply of clasts during the depositional time of the group. These sources are also consistent with the current direction of the Nakaminato Group shown by Tanaka and Kawada (1971).
Denudation stages of mid-Mesozoic ACs in the Kanto District
Ito et al. (2017) summarized microfossil-bearing clasts within upper Mesozoic strata in East Asia from the standpoint of the stages of denudation of the mid-Mesozoic ACs. They recognized three stages of denudation (Stages A, B, and C) of the mid-Mesozoic ACs based on previous studies. Stage A (Oxfordian–Hauterivian) is characterized by an initial and narrow denudation; Stage B (Barremian–early Albian) is characterized by wide denudation; Stage C (late Albian–) is characterized by denudation of almost all strata in the mid-Mesozoic ACs (Ito et al., 2017). The characteristics of Stage C are based mainly on the occurrences of Tithonian and Cretaceous radiolarians in the upper Albian Kisadong Formation of the Hayang Group in the Korean Peninsula reported by Kamata et al. (2000). However, those radiolarians were extracted from residues of whole conglomerates by use of HF solution, so the origins of these radiolarians are uncertain.
In this study, we discovered Late Jurassic radiolarians from argillaceous rock pebbles within the Campanian–Maastrichtian Isoai Formation of the Nakaminato Group. These pebbles are components of the youngest geologic units in the mid-Mesozoic ACs in East Asia. Consequently, the Late Jurassic ACs, which are younger and located in a structurally lower position, started to be denuded in the Campanian in the Kanto District at the latest. Kashiwagi and Isaji (2015) reported late Bathonian–early Callovian radiolarians from a chert pebble of the Barremian Ashikajima Formation of the Choshi Group in the Kanto District. Based on these studies, the denudation history of the mid-Mesozoic ACs in the Kanto District is summarized below. Early Late Jurassic ACs began to be denuded during the Barremian at the latest, after which Late Jurassic ACs were eroded in the Campanian at the latest (Figure 7). Here we propose that the denudation occurred in two stages: stages α (Barremian–) and β (Campanian–). The former and latter correspond partially to stages B and C proposed by Ito et al. (2017).
Systematic paleontology
Classification method and framework basically follow De Wever et al. (2001). Synonymies are given only for selected taxa. All specimens described herein are deposited at the Doctoral Program in Earth Evolution Sciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, with the prefix EES.
Class Actinopoda
Subclass Radiolaria Müller, 1858
Superorder Polycystina Ehrenberg, 1838, emend.
Riedel, 1967
Order Albaillellaria Deflandre, 1953
Family Follicucullidae Ormiston and Babcock, 1979
Genus
Pseudoalbaillella
Holdsworth and Jones, 1980
Pseudoalbaillella
? sp.
Figure 5.24
Remarks.—We recovered only one specimen under questionably assigned genus Pseudoalbaillella. This species has an apical horn and a pseudothorax with two bases of wings. Form and structure of pseudoabdomen cannot be determined well due to the poor preservation.
Occurrence.—Pebble no. 3 from Sample NK-1, Isoai Formation.
Order Nassellaria Ehrenberg, 1875
Family Ruesticyrtiidae Kozur and Mostler, 1979
Genus
Triassocampe
Dumitrica, Kozur and Mostler, 1980
Triassocampe
sp. A
Figures 5.18–5.20
Remarks.—Our examined specimens are poorly preserved. They have a cephalis with a small horn, trapezoidal thorax, and more than seven segments. Outer shell form resembles that of the spined morphotype of T. coronata Bragin. However, giving these specimens a specific name is put off until better preserved specimens are obtained.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Triassocampe
sp. B
Figures 5.17, 5.23
Remarks.—Examined specimens have a small domeshaped cephalis, trapezoidal thorax and 7 or more segments. Outer shell shape of this species is rather similar to that of Triassocampe deweveri (Nakaseko and Nishimura). However, a precise comparison cannot be made due to the poor preservation.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Triassocampe
sp. C
Figure 6.1
Remarks.—The examined specimen is characterized by a small cephalis with very long and needle-like apical horn whose length attains two-thirds of the shell length. Outer surface cannot be observed due to the poor preservation. In outer shell appearance this species rather resembles Triassocampe eruca Sugiyama, but is distinguished from the latter by having a longer apical horn.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Triassocampe
sp. D
Figure 5.10
Remarks.—This species characteristically has a long conical shell with eight segments. Cephalis is long conical, and thorax is barrel-shaped. Width of all but final one or two post-abdominal chambers increasing rapidly. Final one or two post-abdominal chambers decreasing in width. Outer surface of the shell cannot be observed well due to the poor preservation.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Family Syringocapsidae Foreman, 1973
Genus
Katroma
Pessagno and Poisson, 1979
Katroma
sp.
Figure 6.12
Remarks.—A poorly preserved specimen is examined. The present species possesses a cephalis, thorax, abdomen, and three post-abdominal chambers. The last postabdominal chamber is inflated and subspherical in outline, but distal tube is broken away. Cephalis may have a horn. Four or more spines are present in one plane above the equator of the last post-abdominal chamber. Outer surface of shell cannot be observed due to the poor preservation. General shell feature is similar to that of Katroma bicornus De Wever and K. clara Yeh, however, a precise comparison cannot be made.
Occurrence.—Pebble no. 5 from Sample NK-3, Isoai Formation.
Figure 6.
SEM photos of radiolarians from the pebbles of the Isoai Formation, Nakaminato Group. 1, Triassocampe sp. C, EESKS0201, from pebble no. 4; 2, Spumellaria? gen. et sp. indet. B, EES-KS0202, from pebble no. 2; 3, Pantanellium sp., EES-KS0203, from pebble no. 6; 4, Spongostephanidium? sp., EES-KS0204, from pebble no. 4; 5, Cryptostephanidium? sp., EES-KS0205, from pebble no. 4; 6, Poulpus? sp., EES-KS0206, from pebble 6; 7, Capnuchosphaera sp., EES-KS0207, from pebble 4; 8, Triactoma sp., EES-KS0208, pebble from 6; 9, Betraccium? sp., EES-KS0209, from pebble no.4; 10, Spumellaria? gen. et sp. indet. A, EES-KS0210, from pebble no. 4; 11, Pseudoeucyrtis? sp., EES-KS0211, from pebble no. 6; 12, Katroma sp., EES-KS0212, from pebble no. 5; 13, Paronaella sp., EES-KS0213, from pebble no. 2; 14, Podocapsa cf. amphitreptera Foreman, EES-KS0214, from pebble no. 6; 15, Sethoocapsa sp. C, EES-KS0215, from pebble no. 7; 16, Gongylothorax? sp., EES-KS0216, from pebble no. 6; 17, Zhamoidellum sp., EES-KS0217, from pebble no. 1; 18, 19, Sethocapsa sp. B; 18, EES-KS0218, from pebble no. 2; 19, EES-KS0219, from pebble no. 2; 20, Sethocapsa sp. A, EES-KS0220, from pebble no. 6; 21, Williriedellum sp. A group Matsuoka, 1983, EES-KS0221, from pebble no. 2; 22, Eucyrtidiellum sp., EES-KS0221, from pebble no. 2. Scale bars indicate 100 µm.
Figure 7.
Age relationships between neritic–terrestrial strata and microfossil-bearing clasts in the Kanto District, central Japan, with denudation stages proposed by this study and Ito et al. (2017). Guad.= Guadalupian; Lop.= Lopingian. Radiolarian zones are after Ishiga (1990), Matsuoka (1995), Sugiyama (1997) and Kuwahara et al. (1998); P. u-II = Pseudoalbaillella u-forma m. II Zone; P. lom = Pseudoalbaillella lomentaria Zone; P. rho = Pseudoalbaillella rhombothoracata Zone; A. sin = Albaillella sinuata Zone; P. lon = Pseudoalbaillella longtanensis Zone; P. glo = Pseudoalbaillella globosa Zone; P. mo = Pseudoalbaillella monacanthus Zone; F. sc = Follicucullus scholasticus Zone; F. ch = Follicucullus charveti Zone; N. or = Neoalbaillella ornithoformis Zone; N. op = Neoalbaillella optima Zone.
Genus
Podocapsa
Rüst, 1885, emend. Foreman, 1973
Podocapsa
cf.
amphitreptera
Foreman, 1973
Figure 6.14
Podocapsa amphitreptera Foreman, 1973, p. 267, pl. 13, fig. 11; Jud, 1994, p. 94, pl. 17, figs. 2, 3; Baumgartner et al., 1995, p. 428, pl. 3171, figs. 1–5; Hori, 1999, p. 101, fig. 10.9.
Remarks.—Cephalis and a terminal tube of our specimen are broken away. Abdomen is flattened. This species characteristically has large pores. Although our specimens are incomplete, general shell features are comparable to those of P. amphitreptera.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Family Hsuidae Pessagno and Whalen, 1982
Genus
Parahsuum
Yao, 1982
Parahsuum
sp. A
Figure 5.7
Remarks.—This species has a spindle-shaped shell without well developed strictures. Cephalis may be conical without an apical horn. Trapezoidal thorax with sparse irregularly displaced pores. Abdomen and post-abdominal segments have continuous edged costae. Single row of square pore frames has circular, primary pores between costae. Our specimen resembles Parahsuum simplum Yao but it differs from the latter by having greater numbers of costae and transverse rows of pores.
Occurrence.—Pebble no. 5 from Sample NK-3, Isoai Formation.
Parahsuum
sp. B
Figure 5.6
Remarks.—This species is characterized by a spindled shell with perhaps six to seven post-abdominal chambers. Cephalis is dome-shaped without a horn. Thorax and subsequent chambers are trapezoidal in outline. Abdomen and post-abdominal chambers comprised of 13 longitudinal costae superimposed on each row of pore frames. Chambers increasing gradually in width as added with final post-abdominal chamber decreasing slightly in width. The examined specimen is similar to Parahsuum mostleri (Yeh) but it differs from the latter by having a much greater number of costae.
Occurrence.—Pebble no. 5 from Sample NK-3, Isoai Formation.
Parahsuum
sp. C
Figure 5.21
Remarks.—This species resembles Parahsuum simplum Yao but differs from it by having a conical shell with a lesser number of costae. This species is also similar to P. sp. B of the present study but it differs from the latter by having fewer costae.
Occurrence.—Pebble no. 5 from Sample NK-3, Isoai Formation.
Parahsuum
? sp.
Figure 5.8
Remarks.—This species has a spindled shell with small cephalis, trapezoidal thorax and as many as five post-abdominal chambers. Fifteen to sixteen longitudinal costae are counted on the half shell surface. However, longitudinal costae are discontinuous at the surface on the abdomen. Therefore, we questionably included this species in the genus Parahsuum herein.
Occurrence.—Pebble no. 5 from Sample NK-3, Isoai Formation.
Family Canoptidae Pessagno and Poisson, 1979, emend. Yeh, 1987
Genus
Cinguloturris
Dumitrica, 1982, in Dumitrica and Mello, 1982
Cinguloturris
cf.
carpatica
Dumitrica, 1982, in Dumitrica and Mello, 1982
Figures 5.13, 5.14
Cinguloturris carpatica Dumitrica in Dumitrica and Mello, 1982, p. 23, pl. 4, figs. 7–11; Gorican, 1994, p. 64, pl. 23, figs. 1, 6–11; Baumgartner et al., 1995, p. 142, pl. 3139, figs. 1–6(H); Hori, 1999, p. 91, fig. 9.1.
Remarks.—Examined specimens have a conical shell. Cephalis is conical and thorax is trapezoidal in outline. Post-thoracic chambers are barrel-shaped. A spongy network is developed between segments. Second and third segments are convex with many costae. The fourth segment is not convex but there are deep strictures instead. These shell characters slightly differ from Cinguloturris carpatica.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Family Parvicingulidae Pessagno, 1977a
Genus
Ristola
Pessagno and Whalen, 1982
Ristola
sp.
Figure 5.22
Remarks.—Our examined specimen is poorly preserved, but it has a conical cephalis, trapezoidal thorax, abdomen and six or seven post-abdominal chambers. These characters fall within the genus Ristola. Outer surface of our examined specimen is not clear due to the silica mineral encrustation. Outer morphological features of this species are similar to those of Ristola altissima (Rüst).
Occurrence.—Pebble no. 7 from Sample NK-4, Isoai Formation.
Family Archaeodictyomitridae Pessagno, 1976
Genus
Archaeodictyomitra
Pessagno, 1976
Archaeodictyomitra minoensis
(Mizutani, 1981)
Figure 5.5
Pseudodictyomitra minoensisi Mizutani, 1981, p. 178, pl. 58, fig. 4, pl. 63, figs. 9–10.
Archaeodictymitra minoensis (Mizutani). Matsuoka and Yao, 1985, pl. 2, fig. 5; Gorican, 1994, p. 62, pl. 20, figs. 14, 15, 19, 20;
Baumgartner et al., 1995, p. 104, pl. 3305, figs. 1–5; Hori, 1999, p. 81, fig. 7–12.
Remarks.—Although a part of the distal portion of our examined specimen is broken away, our specimen is identical to Archaeodictymoitra minoensis.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Archaeodictyomitra
sp. A
Figure 5.4
Remarks.—This species resembles Archaeodictymoitra minoensis, but it differs from the latter by having a weak depression at the segments of the post-abdominal chambers. This species is also similar to A. apiarium (Rüst) but it differs from the latter by having a lesser number of costae.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Archaeodictyomitra
sp. B
Figure 5.2
Remarks.—Although our examined specimen lack the last abdomen, this species resembles Archaeodictymoitra lacrimula (Foreman) in general shell shape but has a short conical form of cephalis to abdomen.
Occurrence.—Pebble no. 1 from Sample NK-1, Isoai Formation.
Archaeodictyomitra
sp. C
Figure 5.3
Remarks.—This species characteristically has a small and conical shell. Cephalis is small and dome-shaped and thorax to first post-abdominal chambers is trapezoidal in outline. The last chambers suddenly become narrow.
Occurrence.—Pebble no. 1 from Sample NK-1, Isoai Formation.
Family Pseudodictyomitridae Pessagno, 1977b
Genus
Loopus
Yang, 1993
Loopus
sp. A
Figure 5.15
Remarks.—General shell features of this species fall within the genus Loopus Yang. Cephalic part of this species is broken but it may be small and dome-shaped. Our examined specimen has six post-abdominal chambers. This species is rather similar to Loopus primitivus (Matsuoka and Yao) but it differs from the latter by having fewer and weaker costae on the outer surface of the chambers.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Loopus
sp. B
Figures 5.12, 5.16
Remarks.—This species has a spindle-shaped shell with dome-shaped cephalis and trapezoidal thorax, abdomen, and first post-abdominal chamber, other post-abdominal chambers are barrel-shaped. The last chamber decreases in width compared with the previous one. Although it is difficult to determine well the shell surface structure of one of the illustrated specimens (Figure 5.12), we included these two specimens in Loopus sp. B. This species resembles Loopus sp. A of the present study, but it differs from the latter by having fewer chambers.
Occurrence.—Pebble no. 7 from Sample NK-4 and Pebble no. 2 from Sample NK-2, Isoai Formation.
Family Williriedellidae Dumitrica, 1970
Genus
Williriedellum
Dumitrica, 1970
Williriedellum
sp. A group Matsuoka, 1983
Figure 6.21
Williriedellum sp. A group, Matsuoka, 1983, p. 23, pl. 4, figs. 1–3, pl. 8, figs. 11–15; Baumgartner et al., 1995, p. 628, pl. 4060, figs. 1–2.
Remarks.—Our specimen has a dome-shaped cephalis, campanulate thorax which is half depressed in the abdominal cavity, and barrel-shaped abdomen with small basal appendage. Although the abdomen of our specimen is slightly depressed, general shell features are identical to those of Williriedellum sp. A group Matsuoka.
Occurrence.—Pebble no. 2 from Sample NK-2, Isoai Formation.
Genus
Zhamoidellum
Dumitrica, 1970
Zhamoidellum
sp.
Figure 6.17
Remarks.—This species has an oval shell with domeshaped cephalis, trapezoidal thorax and globular abdomen. Constriction is visible at the base of the thorax. Outer view of this species is similar to that of Zhamoidellum ovum Dumitrica.
Occurrence.—Pebble no. 1 from Sample NK-1, Isoai Formation.
Family Sethocapsidae Haeckel, 1881
Genus
Sethocapsa
Haeckel, 1881
Sethocapsa
sp. A
Figure 6.20
Remarks.—The examined specimen of the present species is rather well preserved and consists of four segments of which the last segment is large and spherical. The last segment has circular pores with pentagonal to hexagonal pore frames. This species is similar to Sethocapsa kitoi Jud but our specimen has larger circular pores. This species also resembles Crococapsa sp. A described by O'Dogherty et al. (2017) from the Eastern Alps. That species has more globular last segments.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Sethocapsa
sp. B
Figures 6.18, 6.19
Remarks.—The two examined specimens have four segments. Cephalis is dome-shaped, thorax and abdomen form trapezoidal outline. The fourth segment is large and globose, and bears many circular pores with tetragonal to hexagonal pore frames. This species is distinguished from Sethocapsa sp. A of the present study by having a smaller shell with high numbers of pores on the last segment.
Occurrence.—Pebble no. 2 from Sample NK-2, Isoai Formation.
Sethocapsa
sp. C
Figure 6.15
Remarks.—Our examined specimen is incomplete. This specimen differs from Sethocapsa sp. B of the present study by having a weak depression between the third and last chambers.
Occurrence.—Pebble no. 7 from Sample NK-4, Isoai Formation.
Genus
Gongylothorax
Foreman, 1968, emend.
Dumitrica, 1970
Gogylothorax
? sp.
Figure 6.16
Remarks.—One specimen was examined. We cannot determine the cephalic and thoracic parts, therefore, we questionably included this species in Gongylothorax.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Family EucyrtidiellidaeTakemura, 1986
Genus
Eucyrtidiellum
Baumgartner, 1984
Eucyrtidiellum
sp.
Figure 6.22
Remarks.—Our examined specimen has three segments. Cephalis is subspherical without an apical horn, thorax is subspherical, and third segment is inflated annular with longitudinal plicae. This species is similar to Eucyrtidiellum ptyctum (Riedel and Sanfilippo) which has irregularly arranged nodes and pores on the surface of the thorax.
Occurrence.—Pebble no. 2 from Sample NK-2, Isoai Formation.
Family Eucyrtidiidae Ehrenberg, 1847
Genus
Pseudoeucyrtis
Pessagno, 1977b
Pseudoeucyrtis
? sp.
Figure 6.11
Remarks.—This species has a spindle shell with a cephalis, trapezoidal thorax, abdomen and possibly six or more post-abdominal chambers. Pores are small and circular and regularly spaced in transverse rows. This species is questionably assigned to Pseudoeucyrtis because of the absence of the apical horn and terminal tube.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Family Unumidae Kozur, 1984
Genus
Protunuma
Ichikawa and Yao, 1976
Protunuma
? sp.
Figure 5.1
Remarks.—This species is characterized by having a spindle-shaped shell with longitudinal costae. Costae number about 12 on hemisphere. Cephalis may be broken away, and thorax is trapezoidal in outline. Last chamber is inversely subconical. Aperture is not identified at the base of the last chamber. Small, numerous pores are present at the surface of the next-to-last chamber. Single row of small pores is recognized between the costae. Outer shell form is similar to that of Protunuma, however, this species has many more costae and many small pores at the surface of the next-to-last chamber. Therefore, we questionably included this species in Protunuma.
Occurrence.—Pebble no. 1 from Sample NK-1, Isoai Formation.
Family Poulpidae De Wever, 1981
Genus
Poulpus
De Wever in De Wever et al., 1979
Poulpus
? sp.
Figure 6.6
Remarks.—Illustrated specimen is viewed from the direction of divergent feet. This species has a spherical shell with three three-bladed feet. Internal spicule system of this species may be attributed to the genus. As we cannot recognize the general outer shell features of this species, we questionably included this species in Poulpus. Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Family Spongocapsulidae Pessagno, 1977a
Genus
Spongocapsula
Pessagno, 1977a
Spongocapsula
? sp.
Figure 5.11
Remarks.—The examined specimen has a shell with small conical cephalis, trapezoidal thorax, abdomen and first post-abdominal chamber, and inflated second postabdominal chamber. Distinct stricture present between the first and second post-abdominal chambers. Outer surface of the second post-abdominal chamber may consist of a coarse spongy meshwork. We questionably included this species in Spongocapsula because it has a smaller number of chambers and the outer surface of all of the chambers cannot be observed well.
Occurrence.—Pebble no. 2 from Sample NK-2, Isoai Formation.
Multisegmented Nassellaria gen. et sp. indet.
Figure 5.9
Remarks.—Examined specimen has a conical shell with six chambers. Cephalis is dome-shaped without a horn, and the thorax to the last chambers is trapezoidal in outline. Thoracic surface has small pores and other chambers have irregularly shaped pores which may be arranged horizontally. Rather strong and narrow strictures present between the last two and three chambers. Our examined specimen cannot be placed generically because its shell surface cannot be compared to previously reported multisegmented nassellarians.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Order Spumellaria Ehrenberg, 1875, emend. De Wever et al., 2001
Family Angulobracchidae Baumgartner, 1980, emend. De Wever et al., 2001
Genus
Paronaella
Pessagno, 1971
Paronaella
sp.
Figure 6.13
Remarks.—Distal half of one of rays of our specimen is broken away. Rays may have linearly arranged tetragonal or pentagonal pore frames. Terminal point of one of rays seems to have bifurcate spines. These shell forms are rather similar to those of P. bandyi Pessagno.
Occurrence.—Pebble no. 2 from Sample NK-2, Isoai Formation.
Family Pantanellidae Pessagno, 1977b
Subfamily Pantanellinae Pessagno, 1977b
Genus
Pantanellium
Pessagno, 1977b
Pantanellium
sp.
Figure 6.3
Remarks.—One of the bipolar spines is broken away. This species is similar to Pantanellium squinaboli (Tan) by having a small number of large hexagonal or pentagonal pores. But we cannot compare this species with P. squinaboli because our specimen of this species is incomplete.
Occurrence.—Pebble no. 6 from Sample NMK-3, Isoai Formation.
Genus Betraccium Pessagno et al., 1979
Betraccium
? sp.
Figure 6.9
Remarks.—One of the main spines of our examined specimen is broken away. One of the main spines displays strong torsion of ridges and grooves. Due to the poor preservation, we cannot observe the surface of the spherical shell. Therefore, we questionably included this species in the genus Betraccium.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Family Xiphostylidae Haeckel, 1881, sensu Pessagno and Yang in Pessagno et al., 1989, emend.
De Wever et al., 2001
Genus
Triactoma
Rüst, 1885
Triactoma
sp.
Figure 6.8
Remarks.—The examined specimen has lost one of the main spines. Although the internal shell structure cannot be observed, this species is identical to Triactoma. This species is similar to T. tithonianum Rüst, but the latter species differs from the former by having more slender spines.
Occurrence.—Pebble no. 6 from Sample NK-3, Isoai Formation.
Spumellaria? gen. et sp. indet. A
Figure 6.10
Remarks.—Our examined specimen is incomplete but may have a shell with five or six spines among which four spines lie in a horizontal plane at almost 90 degrees to each other while the other one or two spines arise perpendicular to this plane. Two of the spines on the horizontal plane are very large and three-bladed and large pores are present at the base of each spine. The other two horizontal spines may be small and cone-shaped, but their length attains two-thirds of the diameter of the shell. We questionably included this species in Spumellaria because we cannot determine the internal shell structure.
Occurrence.—Pebble no. 4 from Sample Nk-3, Isoai Formation.
Spumellaria? gen. et sp. indet. B
Figure 6.2
Remarks.—The examined specimen is poorly preserved but appears to have a globular shell with three triradiate spines. Internal shell structure and outer shell features cannot be observed well, therefore we placed this species in Spumellaria? gen et sp. indet. B.
Occurrence.—Pebble no. 2 from Sample NK-2, Isoai Formation.
Order Entactiniaria Kozur and Mostler, 1982
Family Eptingiidae Dumitrica, 1978
Genus
Spongostephanidium
Dumitrica, 1978
Spongostephanidium
? sp.
Figure 6.4
Remarks.—This species has a spongy spherical shell with three rodlike main spines. General shell features are identical to those of Spongostephanidium. As we cannot identify the internal shell structure of the present species, we questionably included it in this genus.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Genus
Cryptostephanidium
Dumitrica, 1978
Cryptostephanidium
? sp.
Figure 6.5
Remarks.—One poorly preserved specimen was examined. This species has a spherical shell with three threebladed main spines. Shell surface consists of coarse spongy layers. Internal shell structure cannot be observed, therefore, we questionably included this species in Cryptostephanidium.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Family Capnuchosphaeridae De Wever in De Wever et al., 1979, emend. Pessagno et al., 1979
Genus
Capnuchosphaera
De Wever in De Wever et al., 1979
Capnuchosphaera
sp.
Figure 6.7
Remarks.—One of the main spines of our examined specimen is broken away. This species has a spherical shell with many small circular pores surrounded by polygonal frames and three main spines which twist at almost the middle of the spine. Distal part of the spine seems to be rodlike. These shell features may be identical to those of Capnuchosphaera deweveri De Wever. However, our examined specimen is incomplete and we refrain from assigning a species name to it.
Occurrence.—Pebble no. 4 from Sample NK-3, Isoai Formation.
Concluding remarks
We recovered 40 radiolarian species belonging to 25 genera, including four unidentified radiolarians, from seven pebbles in four horizons of the Isoai Formation of the Nakaminato Group exposed along the Pacific coast of the Kanto District, central Japan. The pebbles are argillaceous rock of Early–Late Jurassic (middle Toarcian–late Tithonian), Middle–Late Jurassic (late Bajocian–early Oxfordian), Late Jurassic (middle late Oxfordian–early Tithonian), and Middle–Late Jurassic (early Bathonian–early late Tithonian) age and chert of late Paleozoic (middle Carboniferous?–Guadalupian), Late Triassic (Carnian), and Early Jurassic (early Sinemurian–early Toarcian) age. Based on the lithological similarities and ages, the radiolarian-bearing pebbles were presumably derived from the mid-Mesozoic ACs, such as the Ashio terrane in the Ashio Mountains and the Yamizo terrane in the Yamizo Mountains.
The present result from the Nakaminato Group, in addition to the previous study on the Choshi Group, suggests that early Late Jurassic ACs began to be denuded in the Barremian and then Late Jurassic ACs started to be eroded in the Campanian at the latest. Meanwhile the Nakaminato and Choshi groups form a mostly complete succession with interbedded conglomerate layers at several horizons. The ages of these strata are constrained, mainly by ammonoid dating. Further detailed work, therefore, can provide information on the stages of denudation of the mid-Mesozoic ACs in the Kanto District.
Author contributions
Inose, H. made geological field works, geologic map, columnar section and collecting samples. Furuuchi, K. contributed to make a geologic map and columnar section and collecting samples. He investigated the washing samples to extract radiolarian fossils. Ito, T. summarized the radiolarian-bearing pebbles from East Asia and prepared figures for this manuscript. Sashida, K. made field investigations and collecting samples. He took radiolarian SEM photos and was primarily responsible for the taxonomic aspects. Agematsu, S. made field investigation and preparation for the radiolarian specimens. All authors contributed to the writing of the paper.
Acknowledgements
We express our sincere thanks to Atsushi Takemura (Hyogo University of Education) for providing us important literature for this study. We further express our gratitude to two anonymous reviewers for many helpful suggestions and comments that have improved the manuscript.
