Microfossils abundantly occur in the upper part of the Pliocene Kuwae Formation in the Sakai stratigraphic section, Tainai City, northern Niigata Prefecture. We clarified the biostratigraphy and reconstructed the vertical paleoenvironmental changes based on micro fossils such as ostracodes, diatoms and planktonic foraminifers. As a result, the sequence in the Sakai section was divided into three parts based on lithology and correlated with the Neodenticula koizumii-N. kamtschatica diatom zone (NPD 8, 3.5-2.7 Ma). At least the lower and middle parts of the study sequence were assigned to the horizon below the datum of the rapid increase of N. koizumii (D85, 3.1-3.0 Ma). An index fossil planktonic foraminiferal species, Globorotalia inflata (s.l.), was abundantly found only in the middle to upper part of the study interval. This interval belonged to the No. 3 G. inflata bed, which has been used as a biozone in the Sea of Japan side during the Pliocene. Four ostracode bio associations were identified using R-mode cluster analysis. Based on ostracode analysis, the Kuwae Formation in the present study section was shown to have benn deposited in the sublittoral to upper bathyal zone and at least one cycle of water depth change was found. In addition, one new ostracode species, Hemicythere sakaiensis sp. nov., was described in this study.
Introduction
The Sea of Japan is an enclosed marginal sea between the Eurasian continent and the Japanese Islands (Figure 1). It is now connected to the East China Sea through the Tsushima Strait to the southwest, the Okhotsk Sea through the Soya Strait to the north, and the Pacific Ocean through the Tsugaru Strait to the northeast. The Tsushima Current is a branch of the warm Kuroshio Current. It flows into the Sea of Japan through the Tsushima Strait. From the late Pliocene to early Pleistocene, 41-kyr glacial-interglacial cycles became clear and paleobathymetry changed cyclically. The inflow of a warm-water current to the Sea of Japan through the southern entrance was limited because a land bridge was intermittently formed during the periods of lower sea level (Tada, 1994; Kitamura and Kim oto, 2004).
Plio-Pleistocene deposits were widely distributed on the Sea of Japan side of central and northeastern Japan. They contain the cool-temperate Omma-Manganji molluscan fauna, which has been studied to reconstruct paleoenvironments (e.g. Otuka, 1939; Ogasawara, 1986, 1994; Chinzei, 1991; Amano et al., 2000). The upper Pliocene Kuwae Formation, which is the objective of the present study, is distributed in Shibata and its neighboring Tainai cities, northern Niigata Prefecture, central Japan (Figure 1). Several paleontological studies have been so far conducted on this formation. Amano et al (2000) inferred that a warm-water current had flowed into the Sea of Japan during the deposition of the middle to upper parts of the Kuwae Formation in Shibata City based on fossil molluscan assemblages. Watanabe et al. (2003) and Miwa et al. (2004) established diatom and planktonic foraminiferal biostratigraphies, respectively, in the Tainai section, Tainai City, where the continuous sequence aged at 3.55-2.5 Ma is well exposed (Watanabe et al., 2003; Inoue et al., 2003). Miwa et al. (2004) showed nine zones with abundant occurrence of Globorotalia inflata (s.l.) in the No. 3 G. inflata bed, which was used as a biozone in the Sea of Japan side during the Pliocene (Maiya et al., 1976).
Figure 1.
Maps showing locations of the study area in Niigata Prefecture, Northeast Japan. TWC: Tsushima Warm current.
Yamada et al. (2005), Irizuki et al. (2007), and Irizuki and Ishida (2007) studied fossil ostracodes from the Tainai section, which is the same locality as that in the study of Miwa et al. (2004). They showed cyclic changes in fossil ostracode assemblages based on several quantitative analyses, suggesting global eustatic changes. Irizuki et al. (2007) and Irizuki and Ishida (2007) also suggested that temperate intermediate water was present at the deposition of the Kuwae Formation and that a cold-water mass such as Japan Sea Intermediate Water and Japan Sea Proper Water in the present day had not been developed at that time.
The aims of this paper are to report the microfossil assemblages and to discuss the paleoenvironment from the Pliocene Kuwae Formation distributed in the Sakai section near the Tainai section, central Japan. This study will contribute to delineate spatiotemporal changes in paleoenvironments in the Sea of Japan side, central Japan, in the future. We also describe one new ostracode species, Hemicythere sakaiensis sp. nov.
Geology of the study area
The Kuwae Formation in the Sakai section is exposed along a valley formed sporadically in low hills (Figure 2). This section is approximately 17 m in thickness, and mainly consists of fine- to medium-grained sandstone containing many shell fragments. Based on the lithofacies, it is divided into three parts (Figures 2, 3). In the lower part, an approximately 20 cm-thick lenticular tuff bed is intercalated in the fine- to medium-grained sandstone approximately 1 m above the base, and trace fossils such as Teichichnus are abundantly found in the lower part. In the middle part, hummocky cross stratification is developed 8–9 m above the base. Trace fossils such as Rosselia are found in the entire section; however, they are concentrated only in the middle part. In the upper part, sandy siltstone is developed in the lower portion, and bioturbation is abundantly found in the uppermost part. Strikes and dips show nearly N-S and 3–4°E, respectively.
Materials and methods
We examined three types of microfossils: diatoms, planktonic foraminifers, and ostracodes. In total, 36 sediment samples for microfossil analyses. Of these, five Rosselia samples (2R, 8R, 18R, 23R, and 28R, in descending order), which are infilled with finer muddy sediments, were collected only for diatom analysis from the study section. The sediment sample interval was approximately 50 cm (Figure 3). Fossil diatoms and planktonic foraminifers were used for biostratigraphy. Fossil ostracodes were analyzed to infer the depositional environment.
Diatoms
We used the method described by Akiba (1986) to prepare diatom slides. Approximately 1 g of sediment was soaked in distilled water to prepare a solution. A 0.5-cc aliquot of the suspended solution was placed on a cover glass. An 18 × 18 mm cover glass and Pleurax mounting medium were used to prepare the slides. Fifty diatom valves were counted for each slide at ×600 magnification using a binocular microscope. One hundred diatom valves were counted for samples 1, 2 and Rosselia samples 18R, 23R, and 28R. Only 30 diatom valves were counted for Rosselia sample 8R. The diatom valves found by additional scans in an 18×5 mm area of the cover glass were marked as present (+). We used the diatom biostratigraphy for the northwestern Pacific of Akiba (1986) and Yanagisawa and Akiba (1998). The ages of diatom biohorizons were corrected using a method described by Watanabe and Yanagisawa (2005) and were calibrated based on a geomagnetic polarity time scale reported by Gradstein et al. (2004). Furthermore, biohorizon numbers tentatively proposed by Yanagisawa and Kudo (2011) are used in this paper.
Planktonic foraminifers and ostracodes
80 g of dried sediment samples was weighed and disaggregated by the sodium sulfate and naphtha method (Maiya and Inoue, 1973). These processes were repeated until all fragment rocks were disaggregated. After wet sieving using a 200-mesh sieve screen (opening size: 0.075 mm), the residues were dried. Then, they were divided using a sample splitter, and approximately 200 specimens were picked up from residues coarser than 0.125 mm under a stereoscopic microscope. The number of ostracode specimens refers to both valves and carapaces. One carapace was counted as two valves.
Results
Diatoms
Fossil diatoms were contained in all 36 samples; however, they were poorly preserved. They were composed of Pliocene pelagic, reworked Miocene, nonmarine, and other marine diatoms (Appendix 1). In particular, many nonmarine diatoms were present although the study section consists exclusively of marine deposits (Figure 4). Thalassionema nitzschioides was the most dominant taxon. Coscinodiscus marginatus, Actinocyclus ingens, Paralia sulcata, Stephanopyxis spp., and Aulacoseira spp. were also abundant. Of the nonmarine diatoms, Aulacoseira was the most dominant taxon. Pliocaenicus, Cyclotella, and Stephanodiscus were also abundant. The percentage of nonmarine taxa was >30% in samples 28, 7, 6, 5, and 2. Twenty-five reworked Miocene diatom taxa were contained in the samples. Among them A. ingens and Denticulopsis spp. were abundant (20–30% in samples 27, 22, 4, and 2).
Figure 4.
Stratigraphic distribution of the planktonic foraminifer Globorotalia inflata (s.l.) and diatoms
Index species such as Neodenticula kamtschatica, N. koizumii, Shionodiscus oestrupii, Thalassiosira antiqua, and T. convexa var. aspinosa were found in the study interval. N. kamtschatica and N. koizumii intermittently occurred in the interval from samples 31 to 2 and from samples 31 to 6, respectively (Appendix 1). In particular, N. kamtschatica was abundant in samples 23,20,18, and 17.
The planktonic foraminifer G. inflata (s.l.)
The planktonic foraminiferal species group Globorotalia inflata (s.l.) in the present study is composed of G. orientalis Maiya, Saito et Sato and G. inflata praeinflata Maiya, Saito et Sato sensu Miwa et al. (2004). This species grouop was found in the interval from samples 14 to 6. This interval was divided into two parts at the boundary of sample 11. The lower part from samples 14 to 12 was characterized by a high relative abundance of >50%, with the maximum abundance in sample 12. Sample 11 had a relatively low abundance. The upper part from samples 10 to 6 was characterized by a high relative abundance of >25% in most samples. G. inflata (s.l.) was absent or rarely present in the other horizons (Figure 4).
Ostracodes
More than 170 ostracode species were found from 31 sediment samples (Appendix 2, Figure 5). Cythere sp., Schizocythere kishinouyei, Cytheropteron sawanense, and Cytheropteron miurense were very abundant. They inhabit shallow water areas in a temperate zone (e.g. Ikeya and Itoh, 1991; Irizuki and Ishida, 2007; Iwatani and Irizuki, 2008). Several deep-water taxa such as Acanthocythereis dunelmensis, C. carolae, and Robertsonites tabukii were concentrated in the interval from samples 10 to 5. To reconstruct the vertical changes in paleoenvironments, we conducted quantitative analyses using 31 samples containing >150 specimens.
Figure 5.
Fossil ostracodes from the Kuwae Formation in the Sakai section. RV: right valve, LV: left valve. 1, Acanthocythereis dunelmensis (Norman), RV, adult, sample 5; 2, Aurila tsukawakii Ozawa and Kamiya, RV, adult, sample 4; 3, Cornucoquimba moniwensis (Ishizaki), RV, adult, sample 5; 4, Cornucoquimba cf. tosaensis (Ishizaki), RV, adult, sample 12; 5, Cornucoquimba sp., LV, juvenile, sample 9; 6, Cythere sp. 1, RV, adult, sample 11; 7, Cythere sp. 2, RV, juvenile, sample 06; 8, Cytheropteron carolae Brouwers, LV, adult, sample 9; 9, Cytheropteron miurense Hanai, RV, adult, sample 17; 10, Cytheropteron sawanense Hanai, RV, adult, sample 14; 11, Cytherura? sp. 1, RV, juvenile, sample 22; 12, Cytherura? sp. 2, RV, juvenile, sample 19; 13, Finmarchinella hanaii Okada, RV, adult, sample 4; 14, Finmarchinella cf. japonica. (Ishizaki), RV, adult, sample 28; 15, Hemicythere kitanipponica (Tabuki), RV, adult, sample 4; 16, Loxoconcha subkotoraforma Ishizaki, LV, juvenile, sample 25; 17, Neonesidea sp., LV, juvenile, sample 22; 18, Robertsonites tabukii Yamada, RV, adult, sample 23; 19, Schizocythere kishinouyei (Kajiyama), RV, adult, sample 4; 20, Semicytherura kazahana Yamada, RV, adult, sample 26; 21, Semicytherura subslipperi Ozawa and Kamiya, RV, adult, sample 12; 22, Semicytherura sp. 1, RV, adult, sample 27. Scale bars are 0.2 mm.
Diversity, equitability, and total number of ostracodes
We used H(S), Eq., and the number of ostracodes as ecological indices (Figure 6). H(S) is a diversity index and was calculated on the basis of the Shannon-Wiener function 
, where S is the total number of species and pi is the proportional abundance of the ith species. Eq. is a measure of species equitability and was calculated using the equation provided by Buzas and Gibson (1969) (Eq. = eH(S)/S). These indices are generally used to clarify the structure of fossil assemblages. As a result, the H(S) index indicated entirely large values. However, it showed slightly smaller values in the upper part (samples 7 and 1). Small Eq. values were observed in the upper part (samples from 9 to 5 except 6 and 1). Two peaks of the total number of ostracodes were found at around sample 26 and sample 12.
R-mode cluster analysis
The relative abundance of 22 dominant ostracode taxa containing more than 80 individuals as a total number were selected for R-mode cluster analysis (Figure 5). The Paleontological Statistics (PAST) program (Hammer, 2013) was used for R-mode cluster analysis using Horn's modification of Morishita's overlap index as a similarity (Morishita, 1959; Horn, 1966) and the unweighted pair group method with arithmetic average (UPGMA) as a clustering method. As a result, these dominant taxa were classified into the four bioassociations, namely I, II, III, and IV (Figures 7, 8).
Bioassociation I was composed of 17 taxa (Aurila tsukawakii, Cornucoquimba moniwensis, Cornucoquimba cf. tosaensis, Cornucoquimba sp., Cythere sp. 1, Cythere sp. 2, C. miurense, C. sawanense, Cytherura? sp. 2, Finmarchinella hanaii, Finmarchinella cf. japonica, Hemicythere kitanipponica, Loxoconcha subkotoraforma, S. kishinouyei, Semicytherura kazahana, Semicytherura subslipperi, and Semicytherura sp. 1). C. moniwensis, Cythere sp. 1, C. miurense, C. sawanense, Finmarchinella cf. japonica and S. kishinouyei were the dominant taxa in this bioassociation. C. moniwensis was reported from Sendai Bay in the Tohoku area in mild to cool temperate zones (Ikeya and Itoh, 1991; Irizuki and Ishida, 2007). The genus Cythere is a representative phytal taxon living in the tidal zone (e.g. Tsukagoshi and Ikeya, 1987). C. miurense lives in shallow seas in warm to mild temperate waters (Irizuki and Ishida, 2007). C. sawanense lives in mild to cool temperate waters (Ikeya and Itoh, 1991; Irizuki and Ishida, 2007). S. kishinouyei lives in coastal sandy bottoms in warm to mild temperate waters (Iwatani and Irizuki, 2008) and is mainly distributed at <80 m depths in the Sea of Japan (off Oki Islands, Noto Peninsula, and Tsugaru Peninsula; Ozawa, 2003). Thus, they are components of the upper to middle sublittoral zone in several temperate ranges around Japan.
Figure 7.
Dendrogram of R-mode cluster analysis based on the index of overlap (Horn, 1966). I, II, III, and IV are ostracode bioassociations.
Bioassociation II was composed of three taxa (A dunelmensis, C. carolae, and R. tabukii). A. dunelmensis is the second most dominant species and is a circumpolar species living in water temperatures <6–8°C in high latitude areas of the Northern Hemisphere (North Atlantic, Arctic, and Northern Pacific seas) (Cronin and Ikeya, 1987). Although it was thought that A. dunelmensis could only live in shallow waters in the relatively cold zones of northern areas, it has been reported that this species lives in the upper to middle bathyal zones in the Japan Sea Intermediate-Proper Water of the Recent Sea of Japan off Honshu, where the water temperature is approximately 0–5°C (Ishizaki and Irizuki, 1990; Ozawa, 2003; Ozawa and Kamiya, 2005; Irizuki et al., 2007). It also lives at >20 m depths and is most abundant at a water depth of 100 m in Alaska Bay or Arctic Siberia (Brouwers, 1993; Stepanova et al., 2003). Cytheropteron carolae lives in the mid-outer sublittoral and upper bathyal zones in Alaska Bay (Brouwers, 1994). Several species in the genus Robertsonites (R. hanaii, R. tabukii, and R. tsugaruana) occur at >150 m depths in the Sea of Japan, and the water mass at this depth is characterized by temperatures <5°C (Yamada, 2003; Ozawa, 2003; Ozawa and Kamiya, 2005; Irizuki et al., 2007). Thus, this bioassociation evidences deep and cold waters.
Bioassociation III was composed of only one species, Cytherura? sp. 1. This taxon has not been identified at the species level until now. Therefore, the paleoenvironment for this bioassociation remains unknown.
Bioassociation IV was composed of only one species, Neonesidea sp. Species in the genus Neonesidea are common on algae in the intertidal and littoral zones (e.g. Hanai et al., 1977). Thus, this bioassociation indicates the shallowest waters of all the bioassociations.
Biostratigraphy
Almost all samples of this section except for the uppermost two samples can be assigned to the N. koizumii-N. kamtschatica Zone (NPD 8, 3.5-2.7 Ma) based on the cooccurrence of both species. Furthermore, the lower half of this section from samples 31 to 17 could be placed below the rapid increase of N. koizumii (D85, 3.1-3.0 Ma) because N. kamtschatica is more dominant than N. koizumii in this interval. However, the paucity of age-diagnostic Neodenticula species hinders determining whether the upper interval of this section from sample 18 to 2 is placed below the biohorizon D85. Although the uppermost two samples lack index species, they can be probably assigned to NPD 8 because no erosional surfaces were observed in the study section and the thickness above D85 was only 9 m.
Depositional environment and correlation
Vertical changes in the relative abundance of the first 22 dominant ostracode taxa were arranged on the geological column (Figure 8).
Figure 8.
Vertical changes of percentages of ostracode species in each bioassociation (I, II, III, and IV) in the Sakai section.
The lower part was mainly characterized by species of bioassociation I, and species of bioassociation II were rarely found from samples 30 to 28. Therefore, the lower part was suggested to have been deposited in mild to cool temperate waters in the upper to middle sub littoral zone.
The middle to upper part was characterized by an increase in the relative abundance of species of bioassociation II. Species of bioassociation I were abundant in the middle part; however, they were relatively few in samples 9 to 7 in the upper part, which indicated a combination of higher mud content and smaller H(S), Eq., and total number of ostracodes. Species of bioassociation II had large peaks in this interval. Several peaks of R. tabukii were found in samples 20, 16 to 14, 9 to 7, and 5. A. dunelmensis was abundantly found in samples 10 to 4 but was rare in other horizons. Thus, the middle to upper part was estimated to have been deposited in mild to cool temperate waters in the sublittoral to upper bathyal zone. In particular, sample horizons from 9 to 7 in the upper part were assigned to the maximum water depth (upper bathyal zone). Sample 5 in the upper part showed relatively high abundance of C. miurense and S. kishinouyei of bioassociation I, and A. dunelmensis and R. tabukii of bioassociation II. Therefore, this horizon was characterized by a mixture of several taxa inhabiting different water depths, suggesting that the depositional environment was nearshore but deep.
The uppermost part (samples 4 to 1) was characterized by a decrease in the species of bioassociation II and an increase in the species of bioassociation I, suggesting a decrease in water depth upward. Neonesidea sp., a species of bioassociation IV, shows its maximum abundance. Thus, the depositional environment again became the shallowest sea.
The result of R-mode cluster analysis of ostracode assemblages therefore indicated that the Kuwae Formation in the Sakai section was deposited in the upper sublittoral to upper bathyal zones with at least one cycle of water depth change.
Figure 9 shows the correlation between relative abundance of G. inflata (s.l.) and diatoms in the Tainai and Sakai sections. In the Tainai section, Watanabe et al. (2003) proposed that nonmarine and reworked Miocene diatoms were transported by rivers from hinterland and that the relative frequency of these diatoms was related to the amount of terrigenous clastic supply. In addition, they suggested that their cyclic fluctuations probably correlate with orbital obliquity cycles of approximately 41 kyr because one cycle includes approximately several tens of thousands years. Nonmarine and reworked Miocene diatoms in the Sakai and Tainai sections seem to indicate cyclic fluctuation. Thus, this cyclic fluctuation may correlate with orbital obliquity cycles of approximately 41 kyr (Figure 4).
Figure 9.
Diagram showing comparison between the Sakai and the Tainai sections on the basis of planktonic foraminifers and diatoms. D80 and D85 show diatom datums (Yanagisawa and Akiba, 1998).
Based on the diatom biostratigraphy in the present study, we inferred that the interval yielding G. inflata (s.l.) between samples 15 and 5 can be correlated with any combination of zones 1 and 2 or zone 3 of the No. 3 G. inflata bed shown by Miwa et al. (2004) in the Tainai section (Figure 9). If the diatom biohorizon D85 would be situated around sample 17, the latter case is more reasonable than the former one. According to Irizuki et al. (2007), zone 3 of the No. 3 G. inflata bed was inferred to have been assigned to MIS G19 (approximately 3.0 Ma).
Conclusions
In total, 36 sediment samples for microfossil analyses from the study section. Of these, five Rosselia samples were collected only for diatom analysis.
The study interval is correlated to the N. koizumiiN. kamtschatica zone (NPD 8, 3.5-2.7 Ma), and at least the lower to middle part is placed below the rapid increase of N. koizumii (D85, 3.1-3.0 Ma).
The result of R-mode cluster analysis of fossil ostracode assemblages indicates that the Kuwae Formation in the Sakai section was deposited in the sublittoral to upper bathyal zone with at least one cycle of water depth change.
Nonmarine and reworked Miocene diatoms in the Sakai section indicated a cyclic fluctuation, perhaps the signal of orbital obliquity cycles of approximately 41 kyr.
The horizon of occurrences of G. inflata (s.l.) can be correlated with any combination of zones 1 and 2 or zone 3 of the No. 3 G. inflata bed in the Tainai section.
Figure 10.
Scanning electron micrographs of Hemicythere sakaiensis sp. nov. la-d, holotype, female left valve, sample 31, DGSU no. CO0291; 1a, lateral view; 1b, internal view; 1c, closeup view of hingement; 1d, closeup view of muscle scars; 2a, b, paratype, female right valve, sample 31, DGSU no. CO0292; 2a, lateral view; 2b, internal view; 3a, b, paratype, male left valve, sample 26, DGSU no. CO0293; 3a, lateral view; 3b, internal view; 4, paratype, (A-1 instar) left valve, sample 26, DGSU no. CO0294; 5, paratype, (A-1 instar) right valve, sample 26, DGSU no. CO0295.
Systematic description
by T. Goto and T. Irizuki
One species belonging to the genus Hemicythere is newly described. All specimens used in this study are deposited in the Department of Geoscience, Interdisciplinary Graduate School of Science and Engineering, Shimane University (DGSU).
Order Podocopida Sars, 1866
Suborder Cytherocopina Baird, 1850
Superfamily Cytheroidea Baird, 1850
Family Hemicytheridae Puri, 1953
Genus
Hemicythere
Sars, 1925 in Sars (1922–1928)
Hemicythere sakaiensis
sp. nov.
Figures 10, 1–5
Etymology.—After Sakai, which is the type locality of this species.
Types.—Holotype: female LV, DGSU no. CO0291 (Figures 10, 1a–d). Paratypes: female RV, DGSU no. CO0292 (Figures 10, 2a–b); male LV, DGSU no. CO0293 (Figures 10, 3a–b); juvenile (A-1 instar) LV, DGSU no. CO0294 (Figure 10, 4); juvenile (A-1 instar) RV, DGSU no. CO0295 (Figure 10, 5).
Type locality and horizon.—The upper Pliocene Kuwae Formation in the Sakai section, Tainai City, northern Niigata Prefecture, central Japan, sample 31. (Lat. 38°02′39″N, Long. 139°47′28″E)
Diagnosis.—Carapace large size, subrectangular and heavily calcified, nearly rounded anterior margin. Robust marginal rim.
Measurements.—Length = 0.74 mm, height = 0.41 mm (holotype; female LV, DGSU no. CO0291): length = 0.74 mm, height = 0.39 mm (paratype; female RV, DGSU no. CO0292): length = 0.70 mm, height = 0.37 mm (paratype; male LV, DGSU no. CO0293): length = 0.59 mm, height = 0.33 mm (paratype; juvenile LV, DGSU no. CO0294): length = 0.62 mm, height = 0.33 mm (paratype; juvenile RV , DGSU no. CO0295).
Description.—Valve moderate to large in size, subrectangular in lateral view, and highest at anterior cardinal angle. Dorsal margin nearly straight in left valve. Ventral margin straight. Anterior margin broadly rounded, slightly extended below. Posterior margin concave in its upper half, and lower half protruding into a caudal process. Surface ornamented with reticulation except for posterior area, eye tubercle and subcentral tubercle. Robust marginal rim present in anterior. Ventral ridge distinct, running along ventral margin. One robust ridge prominent in the posterior area, starting from posterodorsal comer, running obliquely downward, and connecting to ventral ridge at posterior one-third of valve length. Weak oblique ridge present in the anterocentral area, starting from mid-anterior margin and connecting to subcentral tubercle. Marginal infold broadly anteriorly and posteriorly. Hingement holamphidont with anterior stepped subround socket, subround tooth, weak and finely crenulated bar and curved elongate socket. Hinge - ment in right valve complementary. Frontal muscle scars consisting of two subround scars. Adductor muscle scars consisting of vertical row of four scars: upper three distinctly subdivided and lower one elongate. Sexual dimorphism not distinct but present. Males more slender than females.
Occurrence.—This species was recorded in 24 samples (2, 3, 5–8, 10, 12–16, 18, 20–24, 26–31) from the Kuwae Formation in the Sakai section.
Remarks.—This species closely resembles and is probably related to Caudites japonicus Ishizaki, 1971 from Aomori Bay, northeastern Japan, in the general outline of the valves, but differs from the latter in having a distinct oblique ridge in the posterior area. Hanai et al. (1977) thought that C. japonicus belonged to the genus Hermanites but with a question mark. Okubo (1979, 1980) suggested that C. japonicus be placed in the genus Ambostracon. Caudites has three frontal scars and an inner lamella with a peculiar secondary fusion (van Morkhoven, 1963). Hermanites has probably V-shaped or crescent frontal scars (van Morkhoven, 1963). Hazel (1962) established the genus Ambostracon but he could not describe frontal scars. Valicenti (1977) mentioned that Ambostracon has three frontal scars. As C. japonicus of Ishizaki (1971) and the present new species have two frontal scars, they are not placed in the genera mentioned above. On the basis of the number of frontal scars and other features of the carapace, the new species and C. japonicus are possibly assignable to the genus Hemicythere.
Acknowledgments
We are grateful to Katsura Yamada (Shinshu University) for her advice and discussions. Our thanks are also extended to Yuki Najima for his helping our planktonic foraminiferal analysis. We thank Den-ichi Nakamura for permission to collect samples even though private property. We also thank Thomas M. Cronin and an anonymous reviewer who provided constructive comments to improve the manuscript. This study was partly supported by a Grant-in Aid for Scientific Research (C) from the Ministry of Education (No. 22540476 to T. Irizuki).
