This study investigated early to middle Miocene ostracods from the Yatsuo Group (ca. 17–15 Ma), located on the Japan Sea coast of central Japan. Ostracods from this area consist of 100 species in 60 genera, with two assemblages defined by Q-mode cluster analysis. The five most abundant genera of each assemblage are Krithe, Propontocypris?, Cytherella, Argilloecia, and Falsobuntonia in assemblage A, and Schizocythere, Cornucoquimba, Callistocythere, Neomonoceratina, and Paracytheridea in assemblage B. The depositional environment of each assemblage can be defined as (1) outer-shelf to bathyal areas (assemblage A) and (2) inner-shelf areas (assemblage B). Stratigraphic changes in the faunal composition, with changes in lithofacies, suggest a shallowing-upward phase in this sedimentary basin. These assemblages contain the same species or genera as corresponding Pacific faunas in the early to middle Miocene. The assemblage containing bathyal genera such as Cytherella and Krithe from Yatsuo at 17–16 Ma is the oldest record from the entire Japan Sea coast, and their fossil records are common to Cenozoic bathyal genera among the Pacific fauna. This fact indicates that these genera could migrate from the Pacific Ocean through the deep-sea passageway to the Japan Sea since at least around 17 Ma during the early Miocene. The shallow-water assemblage of the inner-shelf area around 15.7 Ma has only a small percentage of cold-water genera. The paleo-water temperature in the shallow-sea area around 15.7 Ma in Yatsuo would have been intermediate between warm (subtropical) and cold marine climates around 16 Ma in the Japanese Islands. The ostracod fauna from Yatsuo is significant for the paleogeography and faunal migration of benthos in the Northwest Pacific Ocean during the formation period of the Japan Sea caused by back-arc spreading.
Introduction
The paleogeography and aquatic environments around the Japanese Islands in the Northwest Pacific region changed greatly during the early to middle Miocene (e.g. Chinzei, 1991; Ogasawara et al., 2008). In the period pre-20 Ma, the Japanese Islands were part of the eastern margin of the Asian Continent, and the Japan Sea still did not exist (Iijima and Tada, 1990; Uemura, 2006; Kano et al., 2007; Tsutsumi, 2014). The Japan Sea began to open as a marginal sea around 20 Ma, caused by back-arc spreading due to pull-apart opening, until 15 Ma (Otofuji et al., 1994; Hoshi and Takahashi, 1999; Hoshi et al., 2006; Baba et al., 2007; Yanai et al., 2010; Sawada et al., 2013). Major geological and tectonic events in this region from the early to middle Miocene led to drastic changes of the aquatic environment from freshwater lakes to shallow seas and deep-sea basins with corresponding changes of benthic faunas (Chinzei, 1991; Tada, 1994; Ogasawara, 1994; Irizuki and Matsubara, 1994; Uemura, 2006).
Benthic ostracods provide good clues to understand the relationship between aquatic environmental fluctuations and faunal changes, because they are sensitive to environmental changes, with high endemism due to no planktic growth stages (Boomer et al., 2003; Ozawa, 2010, 2013). Several studies have documented the ostracod fauna in shallow-water areas of the inner-shelf region in the Japan Sea for the early to middle Miocene during 20–15 Ma (e.g. Ishizaki, 1963; Yajima, 1988; Tanaka et al., 2004). However, to date, few studies have presented accounts of early to middle Miocene (20–15 Ma) deep-water ostracods from outer-shelf to bathyal areas of the Japan Sea. A preliminary report of Irizuki et al. (2006) only touched on this topic. Some studies of shallow- and deep-water ostracods in this sea have been conducted on slightly younger records from the middle to late Miocene (15–7 Ma; Yajima, 1988; Irizuki, 1994; Tanaka et al., 2002; Tanaka, 2003; Tanaka and Nomura, 2009; Matsuura et al., 2013) or shallow-sea assemblages of the early to middle Miocene (17–16 Ma; Ishizaki, 1963; Yajima, 1988; Huh and Paik, 1992; Huh and Whatley, 1997; Tanaka et al., 2004).
The Yatsuo Group comprises the Miocene bathyal stratigraphic unit during 17–16 Ma outcropping in the Japan Sea region. These deposits contain many bathyal calcareous fossils and much chronological data around 16 Ma (Yanagisawa, 1999a; Amano et al., 2004). Kamiya et al. (2006) briefly mentioned fossil occurrences of several genera of bathyal ostracods from here, but they did not provide basic details for their specimens such as their geological age, fossil localities or a faunal list with their SEM images. Therefore there are no faunal reports for bathyal ostracods from Yatsuo in detail.
This study investigated ostracods from two formations (Kurosedani and Higashibessho formations) in the Yatsuo Group in order to better understand this important area in the faunal history of the Japan Sea and the Northwest Pacific. Specifically, this study revealed the taxonomic composition and diversity in ostracod faunas, and inferred the nature of the paleo-environment, with representative ostracod faunal lists from this area and their SEM images. The present paper discusses the paleobiogeographical relevance of selected ostracod taxa in bathyal areas, comparing their occurrence with that in other coastal regions of the Japanese Islands during the late Cenozoic. These results allow for inferences regarding the paleogeography of the Japan Sea, an era in which deep-water basins appeared just after the back-arc spreading.
Geological setting and age
The Kurosedani and Higashibessho formations in the Yatsuo Group, along the Toyama Bay coast of the Japan Sea, are well exposed in the Yatsuo area, southwest of Toyama City, central Japan (Figure 1). Sakamoto and Nozawa (1960) and Hayakawa and Takemura (1987) described their geology in detail. The Kurosedani Formation conformably overlies the Iozen Formation, and is overlain conformably by the Higashibessho Formation (Figure 2). The Higashibessho Formation is unconformably overlain by the Tonami Group.
Figure 1.
Map showing sampling localities for fossil ostracods and distribution of the Kurosedani and Higashibessho formations in the Yatsuo Group, central Japan, showing two studied areas (1 and 2), simplified from Yamada et al. (1974) and Kaseno et al. (1992). Letters 1 and 2 correspond to use in Figure 4.
The Kurosedani Formation is composed mainly of mudstone, accompanied by alternations of mudstone-sandstone. This formation consists of five members including the Fushikidani Mudstone Member with the Yamadanaka Tuff (Figure 2). The Higashibessho Formation is composed mainly of mudstone, accompanied by sandstone and alternations of mudstone and sandstone, and consists of four members (Tochiage Mudstone, Shiotani Sandstone, Asatani Mudstone, and Mitani Mudstone members; Figure 2).
The Kurosedani Formation contains early to middle Miocene planktic microfossils (Tanimura, 1979). Previous studies (e.g. Hasegawa and Takahashi, 1992) had placed microfossils from this formation at the lower part of the planktic foraminifer zone N.8 of Blow (1969; Figure 3) and the lower part of the nannofossil CN3 zone of Okada and Bukry (1980). The andesitic lava in the uppermost part of the Iwaine Formation, which is in the lower part of the Yatsuo Group (Figure 2), has a K-Ar age of 16.4 ± 0.9 Ma (Shibata, 1973).
The Higashibessho Formation also contains early to middle Miocene planktic microfossils (Tanimura, 1979; Figure 3). According to Yanagisawa's (1999a) biochronological summary of the Yatsuo Group (Figure 3), microfossils from here correspond to the diatom NPD 3A, 3B, and 4A zones of Akiba (1986), the planktic foraminifer zone N.8 of Blow (1969) and the calcareous nannofossil CN3 zone of Okada and Bukry (1980) with the radiolarian Calocyletta costata zone of Motoyama and Maruyama (1998). Yanagisawa (1999a) recognized six diatom biohorizons from D35 (FO Denticulopsis praelauta; 16.4 Ma) to D43 (LO Denticulopsis okunoi; 15.4 Ma) in the Higashibessho Formation (Figure 3; age from Berggren et al., 1995; Yanagisawa and Akiba, 1998; Watanabe and Yanagisawa, 2005).
Figure 2.
Stratigraphy of the Yatsuo Group in central Japan, after Hayakawa and Takemura (1987), Hasegawa and Takahashi (1992) and Yanagisawa (1999a). Bold-italic letters show the three members examined for ostracods in this study. M: Member.
Therefore, the Kurosedani and Higashibessho formations likely date to ca. 17–15 Ma, corresponding to the early to middle Miocene, based on planktic microfossil records (Figure 3). These formations were deposited during the mid-Neogene climatic optimum ca. 17–15 Ma (e.g. Chinzei, 1991) around the isotope event Mi 2 (16.0 Ma; age from Takahashi, 2004) of Miller et al. (1998).
Materials and methods
Twenty-two rock samples were collected for microfossil analysis from the three members of the Kurosedani and Higashibessho formations (Figures 4, 5). Nine samples (F1–F9) were obtained mainly from the mudstone of the Fushikidani Mudstone Member in the Kurosedani Formation along the Yachi River. Seven samples (T1–T7) were collected mainly from the mudstone of the Tochiage Mudstone Member in the Higashibessho For mation along the Wada and Akae rivers (Figure 4). Six samples (S1–S6) were obtained from sandy mudstone layers within alternations of sandy mudstone and fine sandstone of the Shiotani Sandstone Member in the Higashibessho Formation along the Wada River (Figure 5). These samples were collected in the 1970's by Y. Tanimura for his own research (Tanimura, 1979). We cannot observe outcrops for most of these samples at present, because of conservancy work or residential development since the 1980's.
Figure 3.
Chronology of the Kurosedani and Higashibessho formations in the Yatsuo Group of central Japan related to biochronology and the geomagnetic polarity time scale, modified from Yanagisawa (1999a). Bold-italic letters show the three members studied for ostracods. Ages from Berggren et al. (1995), Lourens et al. (2004) and Watanabe and Yanagisawa (2005); P: planktic foraminifer zone (Blow, 1969); D: diatom zone (Akiba, 1986; Yanagisawa and Akiba, 1998), CN: nannofossil zone (Okada and Bukry, 1980), Chron and Mp: geomagnetic polarity (Cande and Kent, 1995); Mi2: Miocene oxygen isotope event 2 (Takahashi, 2004). M, Member; Md, Mudstone; S, Sandstone Member; Mi2, event according to Miller et al. (1998) and Takahashi (2004).
Dried sediment samples in amounts of 80 g were disaggregated using a saturated sodium sulfate solution and naphtha, washed through a 63-µm (250-mesh) sieve, and dried at 80°C in an oven. This procedure was repeated until the entire sediment sample had disintegrated. Approximately 200 ostracod individuals were picked from six samples (S1–S6) from fractions between 0.125 and 1.0 mm for each sample split. Other samples yielded less than 90 specimens in total when all specimens that were presented were picked from fractions between 0.125 and 1.0 mm for each sample split. The number of ostracod specimens refers to both valves and carapaces. All the identified adult and juvenile valves (left and right) were counted, and each carapace was counted as one individual. The species diversity was calculated by the Shannon-Wiener function for samples yielding more than 30 specimens.
Figure 4.
Map showing sampling localities for fossil ostracods with distribution of three members in the Yatsuo Group, central Japan, simplified from Sakamoto and Nozawa (1960), Hayakawa and Takemura (1987) and Yanagisawa (1999a). Letters 1 and 2 correspond to use in Figure 1. M: Member.
The author determined fossil ostracod assemblages using Q-mode cluster analysis. In the analysis, Horn's modification of Morishita's overlap index (Morishita, 1959; Horn, 1966) and the unweighed pair-group average (UPGMA) were used as a similarity index and a linkage method, respectively. This study used the free software package PAleontological STatistics (PAST) provided by Hammer (2014) for the cluster analysis. Samples containing at least 30 specimens were subjected to the cluster analysis. Species represented by less than three individuals in any sample examined were discarded from the present analysis.
All the specimens examined were deposited at the Earth Sciences Laboratory, College of Bioresource Sciences, Nihon University, Fujisawa, Japan. All the sediment samples studied here are stored at the National Museum of Nature and Science, Tsukuba, Japan.
Results
Occurrence of ostracod
In total, 100 ostracod species in 60 genera were identified from the 22 samples. The Fushikidani and Tochiage mudstone members yielded just under 90 individuals and less than 10 species from each sample (F1– F9 and T1–T7; Table 1), even though ostracod specimens were picked from all fractions, except for the sample F5 yielding 14 species and sample T3 yielding 15 species. The total number of species collected from the Fushikidai Mudstone and Tochiage Mudstone members is 25 and 36, respectively. The species diversity in these samples, calculated by the Shannon-Wiener function, was lower than 1.8, except for sample T3 (2.5; Table 1).
The Shiotani Sandstone Member yielded higher numbers of ostracod taxa (80 species; Table 2) than did the Fushikidani and Tochiage mudstone members. Approximately 20–50 species accounted for approximately 170– 300 specimens from each sample of the Shiotani Sandstone Member (S1–S6). The species diversity in these samples, calculated by the Shannon-Wiener function, was approximately 2.4–3.2 (Table 2), which is higher than that for most samples from the Fushikidani and Tochiage mudstone members.
The six samples from the Shiotani Sandstone Member showed an absolute abundance of ostracod fossils (=number of all specimens per 100 g sediment) of approximately 220–740, which is considerably higher than the absolute abundance of approximately 3–110 for the 16 samples from the Fushikidani and Tochiage mudstone members (Tables 1, 2).
The two most abundant species are Schizocythere sp. 1 and Cornucoquimba saitoi accounting for ca. 30% of the total number of specimens (Table 3). Other species in the genera Cornucoquimba, Neomonoceratina, Paracytheridea, Callistocythere, and Neonesidea are commonly found from many horizons (Figure 5; Table 2). Species in the above six genera accounted for more than 50% of the total number of individuals (Table 3). These genera currently inhabit shallow-water areas with sandy and muddy sediments around the Japanese Islands (Frydl, 1982; Ikeya and Itoh, 1991; Kamiya et al., 2001; Ozawa, 2003; Tanaka, 2008; Irizuki et al., 2009).
Figure 5.
Columnar sections, frequencies of ostracod species and taxa, ostracod assemblages of the studied area, simplified from Tanimura (1979). YMT, Yamadanaka Tuff; SSM, Shiotani Sandstone Member; MM, Mudstone Member; Fm., Formation. Bold-italic letters show samples used for quantitative analyses. Assemblage names correspond to their use in Figure 6 and Table 4.
Table 1.
List of fossil ostracods from the Fushikidani and Tochiage mudstone members in the Yatsuo Group. “Irizuki & Yamada, 2004” in this table means Irizuki and Yamada in Irizuki et al., 2004.
Table 2.
List of fossil ostracods from the Shiotani Sandstone Member in the Yatsuo Group. “Irizuki & Yamada, 2004” in this table means Irizuki and Yamada in Irizuki et al., 2004.
Continued.
Table 3.
Twenty-ranked ostracod species, their numbers of individuals (No.), and their percentages against all individuals from the Yatsuo Group.
Species in the genera Krithe, Propontocypris?, Cytherella, Falsobuntonia, Acanthocythereis, Robertsonites, and Palmenella are commonly or rarely found from several horizons (Figure 5; Tables 1, 2). These seven genera are found from outer-shelf and bathyal areas with muddy sediments around Japan (Ishizaki and Irizuki, 1990; Ikeya and Suzuki, 1992; Zhou and Ikeya, 1992; Zhou, 1995; Ozawa and Kamiya, 2005; Ozawa and Tsukawaki, 2008; Tanaka et al., 2012a).
There are common or rare occurrences of the genera and Xestoleberis along with Loxoconcha pulchra from many horizons (Figure 5; Tables 1, 2). These four taxa are all phytal dwellers, living on sea plants, e.g. the seagrass Zostera in sandy substrates, and calcareous algae on rocky shores, along the present-day Japanese coasts (Kamiya, 1988; Tsukagoshi, 1990; Kamiya et al., 2001; Ishii et al., 2005; Sato and Kamiya, 2007; Ozawa and Ishii, 2008, 2014; Ozawa, 2013). The two species of Pseudoaurila (P. ishizakii and P. okumurai) occur also from many horizons (Figure 5; Tables 1, 2). Iriziuki et al. (2004) stated that these two species probably went extinct during the Miocene, and presumed that they were phytal dwellers on the basis of their carapace shape, which resembles a rugby ball, as a result of adaptation to life on leaves of seagrass or algae (e.g. Kamiya, 1988).
Ostracod assemblage
Two fossil assemblages A and B were defined by the Q-mode cluster analysis (Figure 6) using 52 species and 11 samples. Assemblage A (samples F3–F5 and T1), from the Fushikidani and Tochiage mudstone members mainly in mudstone samples, was mainly composed of five genera, i.e., Krithe, Propontocypris?, Cytherella, Argilloecia, and Falsobuntonia (Figures 5, 6; Table 4). It is enriched by characteristic occurrences of Bradleya, Hirsutocythere?, Henryhowella, Legitimocythere, and Palmoconcha (Figures 5, 7, 8, Table 4). This study allocated the 11 samples from the Fushikidani and Tochiage mudstone members (samples F1, F2, F6–F9, T2, and T4–T7) that yielded less than 15 specimens and were not examined by cluster analysis to assemblage A on the basis of the species composition, because these samples commonly yielded the above genera (Figure 5).
Most of these characteristics and the genus composition are typical of ostracod assemblages in muddy sediments from outer-shelf to bathyal areas of the present Northwest Pacific and Japan Sea (e.g. Ikeya and Suzuki, 1992; Zhou and Ikeya, 1992; Ikeya and Cronin, 1993; Ozawa, 2003; Ozawa and Kamiya, 2005; Tanaka et al., 2012a). Individuals of these ostracod taxa accounted for approximately 80% of ostracod specimens in the 15 samples of assemblage A, which was characterized by a low number of species and low absolute abundance in muddy sediments, and a species diversity index of approximately 1–2 (Tables 1, 4).
In regard to assemblage A, Krithe and Cytherella were more dominant in the Fushikidani Mudstone Member than in the Tochiage Mudstone Member, and were accompanied by genera such as Argilloecia, Falsobuntonia, Henryhowella, and Hirsutocythere? (Figures 5, 6; Tables 1, 4). In contrast, in the Tochiage Mudstone Member, Propontocypris? dominated and was accompanied by Acanthocythereis and Robertsonites (Figures 5, 6; Tables 1, 4). At present along the Pacific and Japan Sea coasts of Japan, the former genera such as Krithe are dominant in relatively bathyal areas at approximately 300–1,000 m water depths, while the latter genera such as Propontocypris and Robertsonites dominant or common from outer-shelf to bathyal areas at approximately 150–300 m water depths (e.g. Zhou and Ikeya, 1992; Zhou, 1995; Ozawa and Kamiya, 2001, 2005; Ozawa, 2003).
Table 4.
Summary for two ostracod assemblages (Ass.) from the Yatsuo Group. Diversity index (Diversity) in samples used in cluster analysis, number of species (No. spp.) in all samples, paleoenvironments, and five-ranked ostracod species of each assemblages, with their mean percentages in samples used in cluster analysis of each assemblage (RA) and their maximum percentage in a single sample (RM). Assemblage names correspond to use in Figures 5 and 6.
Figure 6.
Dendrogram showing the result of Q-mode cluster analysis of 11 samples and the relative abundances of the dominant ostracod species in each assemblage. Assemblage names correspond to their use in Figure 5 and Table 4.
Figure 7.
SEM images of fossil ostracods from the Yatsuo Group. RV: right valve, LV: left valve, CA: carapace. 1, Acanthocythereis koreana Huh and Whatley, 1997, RV; 2, 3 Acanthocythereis quadrata Irizuki and Yamada in Irizuki et al.,2004; 2, LV; 3, LV; 4, Acanthocythereis cf. tsurugasakensis Tabuki, 1986, CA right side; 5,Argilloecia cf. toyamaensis Ishizaki and Irizuki, 1990, CA left side; 6, Bradleya cf. sendaiensis Ishizaki, 1966, CA right side; 7, Falsobuntonia sp., CA left side; 8, Hirsutocythere? nozokiensis (Ishizaki, 1963), CA left side; 9, Hirsutocythere? sp., CA right side; 10, 11, Henryhowella sp.; 10, CA right side; 11, CA right side; 12, Krithe cf. sawanensis Hanai, 1959, CA left side; 13, Krithe cf. antisawanensis Ishizaki, 1966, CA right side; 14, Legitimocythere sp., RV; 15, Palmenella limicola (Norman, 1865), LV; 16, Propontocypris? sp., RV; 17, 18, Robertsonites reticulatusIrizuki and Yamada in Irizuki et. al., 2004; 17, LV; 18, RV; 19–21, Cytherella sp.; 19, CA left side; 20, CA left side; 21, CA left side. All specimens are adults, except for 6, 11, 17 and 20 which are juveniles. Scale bars 0.1 mm: A for 9; B for 1, 8, 10, 11, 16; C for 4; D for 2, 3, 6, 7, 12–14, 17–21; E for 5; F for 15. Specimens of 1 from sample T7, 2–4 from sample T2, 5 from sample F8, 6 from sample F4, 7–9 from sample T4, 10–12, 19–21 from sample F5, 13, 15, 17 from sample S1, 14, 16 from sample T1, and 18 from sample T6.
Figure 8.
SEM images of fossil ostracods from the Yatsuo Group. CA: carapace. 1–3, Palmoconcha sp. 1; 1, male, CA right side; 2, female, CA right side; 3, female, CA dorsal side; 4–6, Palmoconcha sp. 2; 4, male, CA right side; 5, female, CA right side; 6, female, CA dorsal side. All specimens are adults. Specimens of 1 from sample F8, 2, 3 from sample T4, 4, 6 from sample S1, and 5 from sample S2.
A small number of shallow-sea genera on inner-shelf areas, such as Schizocythere and Cornucoquimba present Japan Sea and Northwest Pacific (e.g. Ikeya and Cronin, 1993; Ozawa, 2003), occurred from the Tochiage Mudstone Member (samples T1 and T5–T7; Table 1). These genera occurred very rarely in the Fushikidani Mudstone Member.
Assemblage B (samples T3 and S1–S6) was dominantly composed of the shallow-sea genera Schizocythere and Cornucoquimba, accompanied by Callistocythere, Neomonoceratina, Pseudoaurila, Cythere, Paracytheridea, Loxocorniculum, Neonesidea, Pontocythere, and Xestoleberis (Figures 5, 6, 9, 10; Table 4). This assemblage mainly occurred in sandy mudstone layers within alternations of mudstone and sandstone of the Shiotani Sandstone Member (samples S1–S6) and in part of the Tochiage Mudstone Member (sample T3; Figures 5, 6). Most of these dominant genera inhabit inner-shelf areas of the modern Japan Sea and Northwest Pacific (e.g. Ikeya and Cronin, 1993; Zhou, 1995; Kamiya et al., 2001; Ozawa and Kamiya, 2001; Ozawa, 2003, 2010; Tanaka, 2008; Irizuki et al., 2009). Individuals of species in these genera accounted for approximately 80% of ostracod specimens in this assemblage, which was characterized by a high number of species and absolute abundance in sandy sediments, and a species diversity index of approximately 3 (Tables 1, 2, 4). Assemblage B had a much higher number of species and higher ostracod abundance than did assemblage A from the Fushikidani and Tochiage Mudstone members, except for sample T3 (Tables 1, 2, 4).
Most species found in assemblage B, e.g. Callistocythere kyongjuensis, Cornucoquimba saitoi, Loxoconcha nozokiensis, Neonesidea sp., Pseudoaurila ishizakii, P. okumurai, Schizocythere sp. 1, and Trachyleberis mizunamiensis belong to the main species in the ostracod TS (Trachyleberis mizunamiensis-Schizocythere sp.) association, defined by Irizuki et al. (2004) from the Pacific inner shelf (shallower than 30 m) of the Japanese early Miocene fauna.
Discussion
Water depth of depositional environments
The presence of assemblage A indicates a paleoenvironment in outer-shelf to bathyal areas, while assemblage B suggests an inner-shelf paleo-environment. Further detailed examinations of differences of the genus composition in assemblage A between the two mudstone members suggest a shallowing upwards-phase.
Common or rare occurrences of phytal-dwelling taxa such as Cythere and Xestoleberis from assemblage B (samples S1–S6) suggest the existence of beds of calcareous algae on the rocky shore and eelgrass Zostera on a sandy substrate around the intertidal zone near the site of these samples. The fact that the depositional environment has relatively strong water currents at the site of these samples can explain the high abundance of ostracod fossils and the high occurrence of phytal species which were probably transported from near the intertidal zone to the inner-shelf area, also considering that samples S1–S6 were composed of sandstone, which is evidence for some sorting influence.
Figure 9.
SEM images of fossil ostracods from the Yatsuo Group. RV: right valve, LV: left valve, CA: carapace. 1, 2, Callistocythere kyongjuensis Huh and Whatley, 1997; 1, CA left side; 2. CA left side; 3, Cornucoquimba cf. kagitoriensis Ishizaki et al., 1996, CA left side; 4, Cornucoquimba moniwensis (Ishizaki, 1966), RV; 5, 6, Cornucoquimba saitoi (Ishizaki, 1963); 5, LV; 6. RV; 7, Cythere omotenipponica Hanai, 1959, RV; 8, Cytheropteron cf. sendaiensis Ishizaki, 1966, CA right side; 9, Cytheropteron sp., LV; 10, Kotoracythere cf. abnorma Ishizaki, 1966, LV; 11, Laperousecythere ishizakii Irizuki and Matsubara, 1995, LV; 12, Loxoconcha nozokiensis Ishizaki, 1963, RV; 13, Neomonoceratina japonica (Ishizaki, 1966), LV; 14, Neonesidea sp., CA left side; 15, Paracytheridea neolongicaudata Ishizaki, 1966, RV; 16, 17, Schizocythere sp. 1; 16, LV; 17, LV; 18, Schizocythere sp. 2, LV; 19, Pseudoaurila ishizakii Irizuki and Yamada in Irizuki et al., 2004, CA left side; 20, Trachyleberis leei Huh and Whatley, 1997, RV; 21 ,Xestoleberis cf. hanaii Ishizaki, 1968, CA right side. Scale bars are 0.1 mm: A for 3, 4, 14, 19–21; B for 1, 2, 5–8, 10,11, 15–18; C for 9, 12, 13. All specimens are adults, except for 11 and 15 which are juveniles. Specimens of 1, 2 from sample S2, 3, 4, 9, 11, 12 from sample S6, 5, 6, 10, 13, 15–17 from sample S5, 7, 8, 14, 19 from sample S1, 18 from sample S3, 20 from sample T2, and 21 from sample S4.
Figure 10.
Relative frequency (%) of dominant ostracod genera from the Shiotani Sandstone Member (samples S1–S6; assemblage B).
Stratigraphic changes in these genera or species composition for the assemblages A and B, with lithofacies from mudstone to alternation of mudstone and sandstone in the study area (Figure 5), suggest a shallowing-upward phase of the paleo-setting for this sedimentary basin. The paleo-environment changed from bathyal to inner-shelf as a whole between ca. 17 and 15.7 Ma.
Temperature of bathyal water
Numerous paleo-environmental studies, including those based on molluscan analyses, have suggested that a warm tropical—subtropical aquatic environment appeared in shallow- and deep-sea areas of Japan during the mid-Neogene climatic optimum at ca. 17–15 Ma, except for the cooling event Mi 2 around 16 Ma (e.g. Chinzei, 1991; Ogasawara, 1994; Amano et al., 2004; Irizuki et al., 2004). The deep-water ostracod genera found in the assemblage A from the Fushikidani and Tochiage mudstone members (ca. 17–16 Ma; Figures 3, 5), e.g. Cytherella, Bradleya, and Hirsutocythere? (e.g. Figure 7), are distributed together in the present Kuroshio Warm Current area in the outer-shelf to bathyal regions off southern Japan on the Pacific and East China Sea coasts (Ishizaki, 1983; Zhou, 1995). Therefore, during the depositional period of the two mudstone members, a relatively warm-water environment of 5–10°C like that created by the present Kuroshio Current (Ikeya and Cronin, 1993) would have been found in the outershelf to bathyal areas at Yatsuo.
This condition was different from the relatively coldwater environment prevailing in the outer-shelf to bathyal areas in the present Japan Sea (Japan Sea Intermediate—Proper Water; Ozawa, 2003), less than 2°C (e.g. Ikeya and Cronin, 1993; Ozawa and Kamiya, 2001, 2005). The depositional period of the two mudstone members in Yatsuo, with bathyal ostracods, correlates to the appearance period of warm-water shallow-sea ostracods in the Mizunami, Kanazawa, Tomioka and Ota areas and lower horizons of the Ninohe area (lower part of the Kadonosawa Formation), with numerous subtropical warm-water ostracod and molluscan assemblages characteristic of Japan (Figure 11; e.g. Irizuki and Matsubara, 1994; Amano et al., 2004; Tanaka et al., 2013).
Previous studies have proposed a global cooling period, i.e., the Miocene oxygen isotope event Mi2 around 16.0 Ma (e.g. Miller et al., 1998; age from Takahashi, 2004; Figure 11). In Yatsuo, the Mi2 event correlates to the depositional period of the Tochiage Mudstone Member (Figure 11), but samples from this member contain no cold-water genera such as Finmarchinella (Table 1). According to Irizuki et al. (2004), the rapid and dominant appearance of shallow-dwelling cold-water ostracods around 16.4 Ma at Ninohe in northeastern Japan could reflect a cold marine climate influenced by this cooling event. Evidence of this event was not recorded in the ostracod fauna from the Tochiage Mudstone Member at Yatsuo, probably due to the deepwater environment.
Paleobiogeography of bathyal fauna between 17–16 Ma
The assemblage A from Yatsuo contains in conjunction with Cytherella species of Henryhowella and Legitimocythere (Figures 5, 6). All these are common bathyal genera that have lived since the Paleogene in many such regions of the Pacific and other oceans (e.g. Cronin and Compton-Goodin, 1987; Cronin et al., 2002; Yamaguchi, 2004; Bergue and Nicoladis, 2012). Modern and Eocene—Pleistocene sediments from the Pacific coasts of Japan and Southeast Asia include these genera as well as Krithe (Figures 7, 12–15) (e.g. Hanai et al., 1980; Ishizaki, 1983; Zhou, 1995; Yamaguchi, 2004). However, up to now, considering the entire coastline of the Japan Sea, from the early Miocene to the present there has been just one preliminary report of the genus Cytherella, which was found at the coastline's southwestern end in the middle Miocene Yasuda Formation of the Masuda Group (Irizuki et al., 2006; shown as G in Figure 12). Moreover, the geological age of this formation is still vague, because previous studies have presented several different ages, such as the foraminifer Zone N.9 of Blow (1969; Nomura et al., 1993; 14.7–14.2 Ma) and ca. 16– 15 Ma (e.g. Irizuki et al., 2014). So the bathyal ostracod assemblage containing the several genera Cytherella, Henryhowella, and Legitimocythere from Yatsuo, which is below D35 (FO D. praelauta; 16.4 Ma), is the oldest fossil record from the entire Japan Sea coast.
Figure 11.
Stratigraphic correlation of the early—middle Miocene (17–15 Ma) strata in Yatsuo and seven areas of Japan referred in the text. P. planktic foraminiferal zone of Blow (1969); D, diatom zone of Barron and Gladenkov (1995); Mi2 event according to Miller et al. (1998) and Takahashi (2004). Age data; a, Irizuki and Matsubara (1994, 1995); b, Takahashi and Yanagisawa (2003); c, Takahashi and Hayashi (2004); d, Yanagisawa (1999a); e, Yanagisawa (1999b); f, Irizuki et al. (2004); g, Irizuki (2003); h, Irizuki et al. (2006, 2014). S. M.: Sandstone Member.
This study also found two Palmoconcha species from Yatsuo (Figures 5, 8; Tables 1, 2), constituting the oldest fossil record of this genus from the entire Japan Sea coast. Yamaguchi and Kamiya (2007) divided Palmoconcha species from the North Pacific (latitude 20°–60°N) into two groups by the carapace morphology. One is the P. parapontica group, characterized by distinct posterodorsal muri with no prominent eye spots. The other is the P. laevimarginata group, which have no postero-dorsal muri and a distinct eye spots. The former group is distributed only in the Northwest Pacific around Japan from the Oligocene to the present, being endemic to this area, and the latter group is found on the Northwest and Northeast Pacific coasts (Yamaguchi and Kamiya, 2007). Zhou (1995) reported common occurrences of this genus from outer-shelf to bathyal areas around present-day Japan.
The author considers that Palmoconcha sp. 1 from Yatsuo belongs to the P. parapontica group, and Palmoconcha sp. 2 from the same locale belongs to the P. laevimarginata group, based on the existence or absence of the postero-dorsal muri and eye spot (Figure 8). The occurrence of Palmoconcha sp. 2 from Yatsuo is the oldest and sole record of the P. laevimarginata group from the entire Japan Sea, and that of Palmoconcha sp. 1 is the oldest record of this genus from the Japan Sea.
The genus Falsobuntonia also has been reported only from the Northwest Pacific (latitude 20°–60°N), and is known as one of the endemic genera in this region from the Oligocene to the present (Yamaguchi and Kamiya, 2007). Previous studies (Ikeya and Suzuki, 1992; Zhou, 1995; Ozawa et al., 2004) reported common occurrences of this genus from outer-shelf to bathyal areas around present-day Japan. Bradleya, Krithe, and Pacambocythere are also bathyal genera, and are commonly reported from outer-shelf to bathyal areas around present-day Japan (e.g. Ikeya and Suzuki, 1992; Zhou and Ikeya, 1992; Ozawa, 2003).
Figure 12.
Geographical and geological occurrences of the genus Cytherella in and around the Japanese Islands from the Eocene to Recent. A, This study; B, Nakao et al. (2001); C, Irizuki (2003); D, Yamaguchi et al. (2005); E, Zhou (1995); F, Ishizaki (1983); G, Irizuki et al. (2006); H, Yamaguchi et al. (2006); I, Yamaguchi (2004); J, Yamaguchi (2006); K, Iwatani and Irizuki (2008); L, Iwatani et al. (2011); M, Yamaguchi et al. (2012); N. Nohara (1987).
The occurrence of the last-named three genera from Yatsuo is their oldest record from the entire Japan Sea. These ostracod genera from Yatsuo are significant for the origin, evolution and extinction of the bathyal benthos during the Cenozoic in and around the Japan Sea.
Faunal interchange and deep-sea passageway
A Miocene fossil assemblage around 17 Ma containing Cytherella and Pacambocythere was reported also from the Pacific side of Japan in the Makino Formation of the Awa area, Mie Prefecture (Irizuki, 2003; Figures 11, 12, 14). Therefore, occurrences of outer-shelf and bathyal ostracods in the Yatsuo Group strongly indicate faunal interchange between Northwest Pacific and Japan Sea faunas and the existence of a deep-sea passageway, connected to both areas around 17–16 Ma.
Figure 13.
Geographical and geological occurrences of the genus Krithe in and around the Japanese Islands from the Eocene to middle Miocene. A, This study; B, Irizuki et al. (1998); C, Tanaka and Hasegawa (2013); D, Tanaka et al. (2002); E, Tanaka (2003), Tanaka and Nomura (2009); F, Yamaguchi and Kamiya (2007); G, Yamaguchi et al. (2006); H, Yamaguchi (2004); I, Yamaguchi (2006). Fossil occurrence of Krithe from the Miocene Yasuda Formation, southwestern Japan Sea coast (Irizuki et al., 2006; the same locality shown as G in Figure 12) is not presented here, because of its vague age.
Amano et al. (2004) have also suggested the existence of Miocene deep-sea passageways connecting to the Pacific, based on molluscan analysis from Yatsuo. They represented occurrences of deep-sea molluscan fossils containing taxa of outer shelf and bathyal areas, from a horizon that is ca. 75 m lower than the Yamadanaka Tuff, correlated to the uppermost part of the Kurosedani Formation. The depositional periods of the Yamadanaka Tuff and the sampling horizon of Amano et al. (2004) are approximately 16.6 Ma and 16.8 Ma, respectively, based on the diatom biohorizons of Yanagisawa (1999a) as dated by Watanabe and Yanagisawa (2005) (Figure 3), if the rate of sediment accumulation of the Kurosedani Formation is the same as that of the lower horizons of the overlying Higashibessho Formation, i.e., ca. 3.3 cm/1,000 years between two diatom biohorizons D35 and D40.
A precise age for the beginning of the formation of the deep-sea basin in the Japan Sea and the time during which deep-sea passageways existed to the Pacific are still vague, because the starting age of the deposition for the Kurosedani Formation is unclear. However, Cytherella is found from the sample F3 in the Kurosedani Formation, and this sample is from ca. 500 m lower than the Yamadanaka Tuff (Tanimura, 1979; Figure 5). If the sedimentation rate of the Kurosedani Formation (Fushikidani Mudstone Member) is equal to that of the lower horizons of the Tochiage Mudstone Member (ca. 3.3 cm/1,000 years; Yanagisawa, 1999a), the age of the horizon F4 is estimated to be much older than 17 Ma, i.e.. ca. 18.2 Ma. Considering the age of lava from the uppermost part of the underlying Iwaine Formation (Figure 2; 16.4 ± 0.9 Ma; Shibata, 1973), this estimated age (ca. 18.2 Ma) for the horizon F4 would be much older than this lava age. So the time when these tectonic and faunal-migration events would have started has to be at least around 17.0 Ma, which is slightly older than the age estimated from the molluscan study of Amano et al. (2004). Thus, the deep-water basin of the Japan Sea at central Japan had been formed by ca. 17 Ma at least, considering these facts. These bathyal ostracods would have passed through the “Paleo-Sanin-Hokuriku Trough” or “Paleo Fossa Magna Trough”, named by Iijima and Tada (1990), connected to the Pacific around 16 Ma (Figures 14, 15).
Figure 14.
Maps for the Japanese Islands of three Cenozoic periods showing with occurrences of the genus Cytherella. 1, early Miocene; 2, early to middle Miocene; 3, middle to late Miocene. Map 1 is simplified from Uemura (2006), and maps 2 and 3 are simplified from Chinzei (1991). Names of paleo-troughs are from Iijima and Tada (1990). Arrows with dotted lines in map 2 indicate the inferred invasion routes of bathyal ostracods from the Pacific to the Japan Sea. Area names in map 2; a, Ninohe; b, Ota; c, Tomioka; d, Yatsuo; e, Kanazawa; f, Mizunami; g, Awa. Area names in map 3; h, Matsue; i, Izumo; j, Tanegashima. Occurrence data of the genus Cytherella in maps 2 and 3; d, this study; g, Irizuki (2003); j, Yamaguchi et al. (2012). Occurrence data of Cytherella from the Miocene Yasuda Formation, southwestern Japan Sea coast (Irizuki et al., 2006; shown as G in Figure 12) is not presented here, because of its vague age.
Figure 15.
Maps for the Japanese Islands of three Cenozoic periods showing with occurrences of the genus Krithe. 1, early Miocene; 2, early to middle Miocene; 3, middle to late Miocene. Map 1 is simplified from Uemura (2006), and maps 2 and 3 are simplified from Chinzei (1991). Names of paleo-troughs are from Iijima and Tada (1990). Arrows with dotted lines in map 2 indicate the inferred invasion routes of bathyal ostracods from the Pacific to the Japan Sea. Area names a–;j in maps 2 and 3 correspond to their use in Figure 14, area names in map 3 are k, Tanagura; l, Karasuyama. Occurrence data of the genus Krithe in maps 2 and 3; a, Irizuki and Matsubara (1994, 1995), b, Tanaka et al. (2013), d, this study; h, Tanaka et al. (2002); i, Tanaka (2003), Tanaka and Nomura (2009); k, Irizuki et al. (1998); l, Tanaka and Hasegawa (2013). Occurrence data of Krithe from the Miocene Yasuda Formation, southwestern Japan Sea coast (Irizuki et al., 2006; shown as G in Figure 12) is not presented here, because of its vague age.
The modern shallow-depth limit of several bathyal genera, such as Krithe, Legitimocythere, and Henryhowella, was reported on the basis of researches for grab and coretop samples to be under 1,000 m in the Northwest and Southwest Pacific (e.g. Zhou, 1995; Ayress et al., 1997). Based on these data, the paleo-water-depth of this deep-water passageway might have been shallower than 1,000 m at least.
Water temperature of shallow water
According to previous studies for many regions shown in Figure 11, shallow-sea ostracod species dominated during 17.0–16.2 Ma, e.g. Pseudoaurila ishizakii, Cornucoquimba saitoi, and Schizocythere sp. 1 (e.g. Yajima, 1992; Irizuki and Matsubara, 1994; Irizuki, 2003; Tanaka et al., 2004, 2012b, 2013). These were warm-subtropical species during the Miocene (Irizuki, 2003; Goto et al., 2013). Among these shallow-sea ostracods, species of Schizocythere and Cornucoquimba are dominant also in the Shiotani Sandstone Member (15.7 Ma) of Yatsuo with small percentages of species of Trachyleberis and Loxoconcha (Figures 5, 6, 10, 11). Three shallow coldwater genera, Finmarchinella, Hemicythere and Kotoracythere, defined by Irizuki and Matsubara (1994, 1995), also occurred in this sandstone (Figure 10; Table 2). However, these three taxa were not found in assemblages from Mizunami—Kanazawa (17.0–16.7 Ma), Tomioka—Ota (16.7–16.2 Ma) or the lower horizons of Ninohe (17.0– 16.4 Ma) during 17.0–16.2 Ma (Yajima, 1992; Irizuki and Matsubara, 1994; Tanaka et al., 2004, 2012b, 2013). These genera were dominant in the upper horizons of Ninohe during 16.4–15.0 Ma (in the NPD3B and 4A zones) in the upper part of the Kadonosawa and Suenomatsuyama formations (Irizuki and Matsubara, 1994, 1995; Figure 11). Thus, dominant occurrences of these cold-water genera indicate a cooling event in the shallow-marine environment of the Ninohe area (since 16.4 Ma; Irizuki and Matsubara, 1994, 1995).
However, deposits of the Shiotani Sandstone Member in Yatsuo around 15.7 Ma have quite small ratios of the three genera Finmarchinella, Hemicythere, and Kotoracythere (ca. 5% of the total number of specimens of samples S1–6; ca. 2–7% in each sample; Figures 6, 10). These numbers were much lower than at Ninohe since 16.4 Ma (e.g. Clusters D and E of Irizuki and Matsubara, 1994). There the ratio in total specimens of their two clusters (consisting of 42 samples) at Ninohe around 16.0 Ma was more than 20%, and the maximum ratio of one sample was ca. 70% (Irizuki and Matsubara, 1994). For these horizons, it is inferred that the paleo-environment of Ninohe since 16.4 Ma was under cold-marine climate condition (Irizuki and Matsubara, 1994, 1995; Figure 11).
The shallow-sea water temperature during the period of deposition of the Shiotani Sandstone Member (15.7 Ma) in Yatsuo differed from both the warm subtropical condition during 17.0–16.2 Ma in Mizunami—Kanazawa, Tomioka—Ota and Ninohe (17.0–16.4 Ma) and the cold condition since 16.4 Ma at Ninohe. The shallow-sea water temperature of Yatsuo at 15.7 Ma suggest an intermediate environment between those of the warmsubtropical and cold conditions around 16 Ma for Japan. The cause of the difference in the faunal composition of shallow-water environments between Yatsuo and other areas in this cooling period, e.g. differences in latitudes or geographical settings, is not clear. We have to examine more examples of ostracod faunas around 16 Ma from many areas in Japan and its adjacent areas.
Conclusions
(1) Twenty-two samples from the lower to middle Miocene Yatsuo Group in central Japan on the Japan Sea coast contained 100 species of ostracods in 60 genera.
(2) Q-mode cluster analysis distinguished two assemblages. The depositional environment of each assemblage can be inferred as (a) the outer-shelf to bathyal area and (b) the inner-shelf area. Both assemblages contain the same species or genera as respective Pacific faunas in the early to middle Miocene during 17–16 Ma. Stratigraphic changes in the taxonomic composition and lithofacies suggest a shallowing upward phase in this sedimentary basin, from bathyal to inner-shelf depths.
(3) This study represents the oldest record of the bathyal benthonic ostracod fauna (17–16 Ma) from the entire Japan Sea coast. This ostracod fauna is significant for its origin, evolution, and extinction during the Cenozoic in the Japan Sea and its adjacent areas. The assemblage includes bathyal genera, e.g. Krithe, Propontocypris?, Cytherella, Argilloecia, and Falsobuntonia, that are common to Pacific faunas and those of the Japan Sea coast. This result indicates that these ostracods migrated from the Pacific coast through a deep-sea passageway to the Japan Sea since the early Miocene at least ca. 17 Ma.
(4) The shallow-water assemblage of the inner-shelf area around 15.7 Ma has just small percentages of cold-water genera. This result suggests that the paleo-water temperature at the shallow sea around 15.7 Ma in this region was in an intermediate condition between warm (subtropical) and cold marine climates found around 16 Ma in the Japanese Islands.
Taxonomic notes
The author briefly describes the morphology of six species below, using the following measurements in the text: L: carapace length (mm), H : carapace height (mm). This study follows the classification of ostracod orders in Horne et al. (2002).
Subclass Podocopa Sars, 1866
Order Platycopida Sars, 1866
Superfamily Cytherelloidea Sars, 1866
Family Cytherellidae Sars, 1866
Genus
Cytherella
Jones, 1849
Cytherella
sp.
Figures 7.19–7.21
Remarks.—Specimens from Yatsuo are similar to Cytherella japonica Ishizaki, 1983, described from Late Pliocene sediments of Shikoku, southwestern Japan in the Northwest Pacific realm, having faint subparallel costae along the anterior margin and granular elements in the posterior margin, with similar carapace length and height. However, they differ in having large and distinct granular elements in the posterior margin with a straight outline of the dorsal and ventral margins. This genus is one of the representative bathyal genera that have been found from the Paleogene to the present in many bathyal regions of the Pacific (Figures 12, 14) and along oceanic coasts elsewhere (e.g. Cronin et al., 2002; Bergue and Nicoladis, 2012). Specimens of Figure 7.19 and 7.21 are adult male and female, and a specimen of Figure 7.20 is juvenile.
Order Podocopida Sars, 1866
Superfamily Cytheroidea Baird, 1850
Family Cytheridae Baird, 1850
Subfamily Schizocytherinae Mandelstam, 1960
Genus
Schizocythere
Triebel, 1950
Schizocythere
sp. 1
Figures 9.16, 9.17
Remarks.—The present species is similar to Schizocythere okhotskensis Hanai, 1970 (L = 0.79–0.89, H = 0.47–0.53 in types; Hanai, 1970) from modern surface sediments obtained east of Soya Cape, Hokkaido, northern Japan on the southwest coast of the Okhotsk Sea. However, it is distinguished from S. okhotskensis in having weaker ridges in the anterior marginal, mid-central, postero-dorsal and postero-ventral areas, a different pattern of ridges in the posterior half, and a smaller carapace even in the adult stage (L = 0.60,H = 0.32; based on Figure 9.16). This species is different from Schizocythere kishinouyei (Kajiyama, 1913) from modern surface sediments of the Miura Peninsula, central Japan in the Northwest Pacific realm, in having weaker ridges in the mid-central, ventral marginal and dorsal marginal areas, and different distributional patterns of ridges and reticulations in the postero-ventral and postero-dorsal areas.
Schizocythere
sp. 2
Figure 9.18
Remarks.—This species is similar to Schizocythere ikeyai Tsukagoshi and Briggs, 1998 described from modern surface sediments of Akkeshi Bay, Pacific coast of Hokkaido, northern Japan. However, the present species differs from S. ikeyai in having much weaker ridges in the mid-central, antero-dorsal, and postero-dorsal areas, a different distributional patterns of ridges and reticulations in the mid-central and postero-ventral areas, and a different outline of the anterior and posterior ends of the carapace. This species is different from Schizocythere sp. 1 of this study in having less developed reticulations and ridges on the entire carapace, a different distributional patterns of ridges in the posterior half, and a different outline at the posterior end.
Family Trachyleberididae Sylvester-Bradley, 1948
Genus
Falsobuntonia
Malz, 1982
Falsobuntonia
sp.
Figure 7.7
Remarks.—Specimens from Yatsuo are similar to Falsobuntonia hayamii (Tabuki, 1986), from Pleistocene sediments of the Tsugaru Peninsula, on the Japan Sea coast of northeastern Japan, in general outline, but they differ in having well developed reticulation and ridges in the posterior area with a larger valve size (L = 0.76). This genus is endemic to the Northwest Pacific coastal regions from the Oligocene to the present (Yamaguchi and Kamiya, 2007).
Family Loxoconchidae Sars, 1925 Genus
Palmoconcha
Swain and Gilby, 1974
Palmoconcha
sp. 1
Figures 8.1–8.3
Remarks.—Specimens from Yatsuo are very similar to Palmoconcha parapontica (Zhou, 1995) from modern surface sediments off southwestern Japan in the Northwest Pacific realm, but they slightly differ by the wide area of the secondary reticulation on the carapace surface. The author considers that this species belongs to the Palmoconcha parapontica group defined by Yamaguchi and Kamiya (2007), from the existence of distinct posterodorsal muri and lack of distinct eye spots. This group is one of two morphological groups of this genus around the Pacific coastal region, and is endemic around Japan from the Oligocene to the present (Yamaguchi and Kamiya, 2007). Specimen of Figure 8.1 is an adult male, and specimens of Figure 8.2 and 8.3 are adult females.
Palmoconcha
sp. 2
Figures 8.4–8.6
Remarks.—Specimens from Yatsuo are very similar to Palmoconcha propontica (Hu, 1983) described from Plio-Pleistocene deposits of southern Taiwan in the Northwest Pacific realm, but they slightly differ by the fine reticulation in the anterior area and angular posterior margin. The author considers that this species belongs to the Palmoconcha laevimarginata group of Yamaguchi and Kamiya (2007), based on the absence of distinct postero-dorsal muri and existence of eye spots. This group is found in the Northwest and Northeast Pacific margins, and is present in Japan from the Eocene to the present (Yamaguchi and Kamiya, 2007). Specimen of Figure 8.4 is an adult male, specimens of Figure 8.5 and 8.6 are adult females.
Acknowledgements
I extend my thank to Y. Tanimura, M. Saito (National Museum of Nature and Science, Tsukuba, Japan), T. Kamiya (Kanazawa University, Japan) and H. Ohama (Toyama City, Japan) for assisting in various aspects of manuscript preparation and for giving the author the opportunity to use sediment samples and the information on fossil localities of the Yatsuo Group. I wish to thank B.-C. Zhou (Shanghai Science and Technology Museum, China) and T. Irizuki (Shimane University, Japan) for presenting the unpublished data of ostracod occurrences from the Pacific and Japan Sea coasts of southwestern Japan. I would like to thank T.M. Cronin (United States Geological Survey, USA) and K. Yamada (Shinshu University, Japan) for their constructive review and many useful suggestions. For use of their scanning electron microscopes, I thank the National Museum of Nature and Science, Tsukuba, Japan. This research was financially supported in part by a Grant-in-Aid from the Japan Society for the Promotion Science (No. 15K07199 to H. Ozawa).
