RESEARCH ARTICLE
Fossil ostracod faunas from the Katsuta Group of the Setouchi Geologic Province in southwestern Japan and their paleobiogeography during the mid-Miocene Climatic Optimum
2025 年 29 巻 p. 253-285
詳細
2025 年 29 巻 p. 253-285
A total of 52 ostracod species were found in 24 samples collected from four sites of the Yoshino Formation of the Katsuta Group, which were deposited in the Setouchi Geologic Province of southwestern Japan during the mid-Miocene Climatic Optimum (16.9–14.7 Ma). Q-mode cluster analysis revealed five sample clusters (ostracod biofacies), suggesting that the paleoenvironments included enclosed bay, middle to lower sublittoral bay, and upper bathyal sea, in ascending order, during the transgression. R-mode cluster analysis revealed seven species clusters (ostracod bioassociations) that reflect the paleoecology of ostracod species. Fossil ostracod assemblages are paleobiogeographically characterized by a mixture of tropical taxa (e.g. Cibotoleberis, Pacambocythere, and Paijenborchella), cosmopolitan taxa (e.g. Cytherella, Neonesidea, and Xestoleberis), and the taxa that survived or evolved around the Japanese islands since the Paleogene (e.g. Acanthocythereis, Munseyella, Pseudoaurila, and Trachyleberis). All taxa entered the study area after the opening of the Sea of Japan during 18–17 Ma. Cool-temperate species, such as Elofsonella, Hemicythere, and Laperousecythere, which have been reported from the Lower Miocene deposits in Japan, were not detected in the study area. Three new trachyleberidid species, Acanthocythereis kunihiroi sp. nov., Cibotoleberis tsuyamensis sp. nov., and Pacambocythere ishizakii sp. nov., are described.
ZooBank registration: urn:lsid:zoobank.org:pub:35693991-2F1D-414C-96EF-487CD8FBF96C
The mid-Miocene Climatic Optimum (MMCO) was a warming event that occurred from 16.9 to 14.7 Ma (late Burdigalian to early Langhian) and has been recognized worldwide based on geochemical and paleontological analyses (e.g. Flower and Kennett, 1994; Flower, 1999; Zachos et al., 2001; Böhme et al., 2007; Utescher et al., 2011; Kochhann et al., 2016; Song et al., 2018; Methner et al., 2020; Figure 1). It is inferred that the temperature of ocean bottom waters increased by approximately 4 to 6°C compared with present day temperatures during the MMCO, based on oxygen isotope records from deep sea benthic foraminifera (Zachos et al., 2001). Temperature anomalies of +10 to +20°C in the mid-latitudes of Central and East Asia during the MMCO have been inferred based on paleobotanical studies (Böhme et al., 2007; Utescher et al., 2011). Sea level rose during the MMCO due to a reduction in Antarctic ice volume. Miller et al. (2020) estimated that the maximum sea level during the MMCO was approximately 60 m above the present sea level. Therefore, a global marine transgression occurred during the MMCO (Figure 1). Concurrently, the Sea of Japan opened between approximately 18 and 15 Ma (Figure 1), following rifting and back-arc spreading along the northeastern margin of the Eurasian continent (e.g. Jolivet and Tamaki, 1992; Hosoi et al., 2023). As a result, upper Lower to lower Middle Miocene marine deposits are widely distributed across the Japanese islands. The MMCO appears to exhibit two peaks of warmth, interrupted by a positive excursion in oxygen stable isotope values known as the Miocene isotope event 2 (Mi2; Miller et al., 1991) at approximately 16 Ma (Irizuki et al., 1998b; Methner et al., 2020). Miller et al. (2020) inferred that sea level fell by about 40 m during the Mi2 event (Figure 1). During the first warming peak, from 16.9 to 16.5 Ma, before the Mi2 event, most of the Japanese islands were inundated due to global sea level rise and the opening of the Sea of Japan (e.g. Chinzei, 1991; Ogasawara, 1994; Otohuji, 2020, 2024). The region is thought to have been situated primarily within the tropical and subtropical marine climate zones influenced by strong warm water currents, except for Hokkaido in northernmost Japan (e.g. Chinzei, 1991; Ogasawara, 1994). During this period, several tropical to subtropical molluscan species of the Kadonosawa fauna (e.g. Otuka, 1939; Itoigawa, 1988) flourished in brackish and shallow-water areas with mangrove swamps from northeastern Japan, which is now in a cool-temperate marine climate zone (Matsubara et al., 2004), to southwestern Japan (e.g. Ogasawara et al., 2008). These subtropical to tropical shallow-water molluscan species are thought to have migrated northward and invaded the Japanese coast as a result of the development of the Kuroshio Current, corresponding to the closure of the Indonesian Seaway around 17 Ma (Ogasawara and Noda, 1996; Ogasawara, 2011). In northern Japan, a cooling trend around the time of the Mi2 event has been suggested based on paleontological studies (e.g. Irizuki and Matsubara, 1994, 1995). The second warming peak at approximately 15 Ma is also recognized in several areas of Japan, where subtropical to warm-temperate molluscan species of the Moniwa fauna (Figure 1) flourished (Ogasawara et al., 2008).
Ostracods are minute crustaceans (generally 0.5–1.0 mm in size) with two calcified valves that are easily preserved as microfossils. Most ostracods with thick calcified valves are adapted for a benthic lifestyle and do not have planktonic larval stages during their lifecycle (Boomer and Eisenhauer, 2002; Horne et al., 2002). Hence, their potential for dispersal is thought to be lower than that of molluscan species. Several studies on Miocene ostracods have been conducted in Japan (Ishizaki, 1963, 1966; Yajima, 1988, 1992; Irizuki and Matsubara, 1994, 1995; Tanaka et al., 2004, 2012; Takakuwa and Tsukagoshi, 2005; Goto et al., 2013; Tanaka and Hasegawa, 2013a; Ozawa, 2016; Goto and Irizuki, 2019; Ozawa et al., 2023; Irizuki et al., 2024), but only a few preliminary reports have focused on records from southwestern Japan despite the wide distribution of fossiliferous Miocene strata that were deposited during the MMCO there (e.g. Yajima, 1988; Goto et al., 2013; Goto and Irizuki, 2019). Here, we describe and classify ostracods from one such deposit in Okayama, the Lower–Middle Miocene Katsuta Group, and discuss their taxonomy, paleoecology, paleoenvironment, and paleobiogeography.
Upper Lower to lower Middle Miocene (~18–15 Ma) deposits are widely distributed throughout the Setouchi Geologic Province, which is a long and narrow east–west trending intra-arc basin located between central and southwestern Honshu, Japan (Figure 2, Ikebe, 1957; Shibata, 1985; Itoigawa, 1988; Irizuki et al., 2021). They contain abundant well-preserved shallow marine fossils that have been investigated by several researchers. The Katsuta Group (Kawai, 1957) is one of the key stratigraphic units in the western part of the Setouchi Geologic Province and is primarily distributed throughout the Tsuyama basin in Tsuyama, Okayama, southwestern Japan (Figures 2, 3). This group is composed of the Uetsuki, Yoshino, and Takakura formations, listed in ascending order (Kawai, 1957) (Figure 1). The Uetsuki Formation consists of non-marine conglomerates, sandstones, and mudstones containing terrestrial fossils. The Yoshino Formation consists of conglomerates, sandstones, and mudstones containing numerous tropical tidal marine molluscan fossils, such as Vicarya, Geloina, and Telescopium (e.g. Taguchi, 1981, 2002) and larger tropical foraminifers, such as Operculina complanata (e.g. Kawai, 1957). The Takakura Formation consists of massive black mudstones containing deep sea molluscan fossils and alternating beds of sandstone and mudstone (e.g. Taguchi, 2002). The age of the Katsuta Group has been constrained using planktonic microfossils (Saito, 1963; Yoshimoto, 1979; Watanabe et al., 1999). Watanabe et al. (1999) reported fossil diatoms and concluded that the Takakura Formation corresponds to either the upper Crucidenticula kanayae Zone (NPD3A: 16.9–16.3 Ma) or the lower Denticulopsis praelauta Zone (NPD3B: 16.3–15.9 Ma) described by Akiba (1986) and Yanagisawa and Akiba (1998). Therefore, the age of the underlying Yoshino Formation is estimated at approximately 16.9–16.3 Ma, which coincides with the MMCO and includes its first warming peak (Figure 1). Irizuki et al. (2021) correlated the Lower to Middle Miocene strata of the Setouchi Geologic Province with the global sea level curve of Miller et al. (2020), and the Katsuta Group was deposited during the interval III of Irizuki et al. (2021) (Figure 1).
A total of 24 rock samples were collected for fossil ostracod analysis from four sites in the westernmost part of the Tsuyama basin (Tsuboikami and Tsuboishimo, Tsuyama, Okayama, southwestern Japan: Figure 3), where the Katsuta Group is narrowly distributed, strikes generally east to west, and dips gently to the north in the southern part and south in the northern part. It dips more steeply near the unconformable contact with the underlying Mesozoic metamorphic basement rocks of the Suo belt (pelitic and mafic schists: Nishimura, 1998; Harada et al., 2024) (Figure 3). As the Katsuta Group in this study area is composed of conglomerates, sandstones, and sandy siltstones in ascending order (Figure 4), it may belong to the Yoshino Formation based on lithology; however, the stratigraphic implications will be discussed later. At site 1, the study sequence is composed of a 1.6-m-thick silty sandstone bed with pebbles, a 12-m-thick massive sandy siltstone bed with several thin layers of fine sandstone, a 3.4-m-thick conglomerate bed, and a 1.6-m-thick weathered siltstone bed, in ascending order (Figure 4). The conglomerate bed cuts into the underlying sandy siltstone bed. Its lower half exhibits an imbricated structure, suggesting a west-to-east trending paleocurrent. The upper half of the conglomerate bed is stratified and contains angular boulders (maximum 1.7 m in diameter) of basement rock. A total of 19 samples (sample nos. 1-1 to 1-19) were collected from the base to the horizon just below the conglomerate bed, of which 18 are composed of sandy siltstone to silty very fine sandstone. One sample (sample no. 1-10) was collected from a thin intercalated layer of fine sandstone. At site 2, the study sequence is composed of a 2.6-m-thick basal conglomerate bed and a 5.1-m-thick massive sandy siltstone bed, in ascending order (Figure 4). The basal conglomerate bed includes subrounded to subangular pebbles of basement rock and contains disarticulated shells of Crassostrea gravitesta, which lived in brackish or intertidal coastal environments. Three samples (2-1 to 2-3) were collected from a massive sandy siltstone bed. The remaining two samples (3-1 from site 3 and 4-1 from site 4) were collected from small outcrops of massive silty very fine sandstone along the road (Figure 3), and their stratigraphy is uncertain.
80 g of dried sediment from each sample was disaggregated using a saturated sodium sulfate solution and naphtha for the maceration of rocks (e.g. Maiya and Inoue, 1973; Yamasaki and Domitsu, 2013), washed through a No. 200 (75 μm) mesh sieve, and then dried again. These procedures were repeated until the entire sediment aliquot disintegrated. Under a binocular stereomicroscope, all ostracod specimens were picked up from the coarser sediment remaining after dry sieving using an No. 80 (180 μm) mesh sieve. If > 200 ostracod valves were found in a sample, a sample splitter was used to divide the sample. Each carapace (i.e., articulated valves) was counted as two valves (specimens). Scanning electron micrographs of uncoated specimens of selected ostracod species were digitally imaged using the low-vacuum mode of a JEOL JCM-5000 Neoscope at the Department of Earth Science, Interdisciplinary Faculty of Science and Engineering, Shimane University.
Two-way cluster analysis (Q-mode and R-mode) was conducted using all 24 samples and more than 10 specimens in total. The similarity is Horn’s overlap index (Horn, 1966), and clustering is the unweighted pair group method with arithmetic mean. The software PAST (Hammer et al., 2001) was used for calculations. Diversity (Shannon and Weaver, 1949) and equitability (Buzas and Gibson, 1969) of all samples were calculated.
A total of 52 ostracod species was identified among all samples from the four sites in this study (Table 1). Specimens were moderately to poorly preserved (Figures 5, 6, 7, 8). Figure 9 shows the vertical changes in the percentages of dominant ostracod taxa at site 1. The most dominant species was Acanthocythereis noriyukikeyai Tanaka in Tanaka and Hasegawa, 2013 (Tanaka and Hasegawa, 2013a). Acanthocythereis noriyukikeyai, Acanthocythereis kunihiroi Irizuki sp. nov., Palmoconcha irizukii Tanaka in Tanaka et al., 2002, and Palmenella limicola (Norman, 1865) exhibited relatively high percentages in samples from the lower and middle sequences at site 1 and decreased in abundance upward. Pectocythere cf. ishizakii Irizuki and Yamada in Irizuki et al., 2004 demonstrated high percentages in samples from the lower sequence at site 1 and low percentages in those from sites 2 and 4. Falsobuntonia taiwanica Malz, 1982 and Pacambocythere ishizakii Irizuki sp. nov. exhibited high percentages in samples from the middle sequence at site 1 and those from the upper sequence at site 1, respectively. Cytherella cf. donghaiensis Gou in Hou and Gou, 2007, Cytherella sp., and Krithe sp. were found only in samples from the upper sequence at site 1. The percentages of Loxoconcha cf. taiwanensis Zhao in Wang et al., 1988, Pseudoaurila spp., and Schizocythere sp. tended to increase upward at site 1. Cibotoleberis tsuyamensis Irizuki sp. nov. was abundantly found at site 4 but was not found or rare at other sites.
| Sample No. | 1-1 | 1-2 | 1-3 | 1-4 | 1-5 | 1-6 | 1-7 | 1-8 | 1-9 | 1-10 | 1-11 | 1-12 | 1-13 | 1-14 | 1-15 | 1-16 | 1-17 | 1-18 | 1-19 | 2-1 | 2-2 | 2-3 | 3-1 | 4-1 | Sum |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Acanthocythereis kunihiroi sp. nov. | 20 | 8 | 54 | 16 | 24 | 44 | 26 | 29 | 52 | 13 | 9 | 3 | 2 | 6 | 4 | 2 | 5 | 7 | 9 | 3 | 11 | 20 | 2 | 6 | 375 |
| Acanthocythereis noriyukikeyai Tanaka | 51 | 11 | 124 | 31 | 39 | 32 | 32 | 35 | 31 | 16 | 13 | 4 | 4 | 2 | 34 | 1 | 22 | 19 | 14 | 12 | 36 | 563 | |||
| Argilloecia sp. | 2 | 4 | 10 | 2 | 8 | 3 | 2 | 2 | 1 | 10 | 8 | 2 | 54 | ||||||||||||
| Australimoosella hanaii Yajima | 2 | 1 | 2 | 2 | 1 | 2 | 1 | 2 | 13 | ||||||||||||||||
| Bradleya sendaiensis Ishizaki | 1 | 1 | |||||||||||||||||||||||
| Bythocypris? sp. | 2 | 2 | |||||||||||||||||||||||
| Bythocythere? sp. | 2 | 2 | |||||||||||||||||||||||
| Callistocythere kyongjuensis Huh and Whatley | 2 | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 1 | 1 | 12 | ||||||||||||||
| Callistocythere subsetanensis Ishizaki | 2 | 10 | 2 | 2 | 2 | 2 | 3 | 2 | 4 | 6 | 1 | 4 | 4 | 44 | |||||||||||
| Callistocythere sp. | 2 | 1 | 3 | ||||||||||||||||||||||
| Cibotoleberis tsuyamensis sp. nov. | 4 | 2 | 3 | 2 | 6 | 2 | 86 | 105 | |||||||||||||||||
| Cornucoquimba moniwensis (Ishizaki) | 6 | 2 | 6 | 1 | 1 | 6 | 3 | 2 | 1 | 1 | 4 | 6 | 3 | 10 | 1 | 5 | 2 | 60 | |||||||
| Cornucoquimba saitoi (Ishizaki) | 2 | 2 | 3 | 4 | 12 | 8 | 7 | 1 | 6 | 5 | 3 | 5 | 2 | 5 | 2 | 1 | 10 | 78 | |||||||
| Cornucoquimba cf. tosaensis (Ishizaki) | 8 | 1 | 1 | 5 | 1 | 1 | 17 | ||||||||||||||||||
| Cornucoquimba sp. | 2 | 1 | 4 | 9 | 3 | 1 | 1 | 3 | 24 | ||||||||||||||||
| Cornucoquimba? sp. | 1 | 1 | |||||||||||||||||||||||
| Cythere omotenipponica Hanai | 1 | 1 | 1 | 3 | 2 | 1 | 1 | 5 | 1 | 1 | 2 | 2 | 21 | ||||||||||||
| Cytherella cf. donghaiensis Gou | 20 | 21 | 41 | ||||||||||||||||||||||
| Cytherella sp. | 2 | 8 | 10 | ||||||||||||||||||||||
| Cytheropteron sp. 1 | 1 | 2 | 4 | 6 | 2 | 15 | |||||||||||||||||||
| Cytheropteron sp. 2 | 1 | 1 | |||||||||||||||||||||||
| Cytheropteron sp. 3 | 1 | 1 | |||||||||||||||||||||||
| Falsobuntonia taiwanica Malz | 4 | 6 | 6 | 3 | 6 | 8 | 3 | 19 | 17 | 8 | 13 | 16 | 16 | 15 | 6 | 5 | 11 | 23 | 12 | 197 | |||||
| Hanaiborchella ikeyai (Irizuki and Yamada) | 1 | 1 | 2 | 4 | |||||||||||||||||||||
| Hemicythere paiki (Huh and Whatley) | 1 | 1 | 2 | ||||||||||||||||||||||
| Hemicytherura cf. taiwanensis Kaseda and Ikeya | 1 | 1 | |||||||||||||||||||||||
| Hirsutocythere? akatsukiborensis Yajima | 1 | 1 | 4 | 6 | 12 | ||||||||||||||||||||
| Krithe sp. | 2 | 2 | 3 | 1 | 2 | 1 | 5 | 2 | 18 | ||||||||||||||||
| Loxoconcha nozokiensis Ishizaki | 14 | 5 | 2 | 2 | 21 | 3 | 2 | 12 | 1 | 2 | 11 | 7 | 6 | 3 | 2 | 2 | 1 | 16 | 112 | ||||||
| Loxoconcha cf. taiwanensis Zhao | 1 | 1 | |||||||||||||||||||||||
| Loxoconcha? sp. | 2 | 1 | 3 | ||||||||||||||||||||||
| Loxocythere tetsurohanaii Tanaka | 4 | 4 | 2 | 6 | 5 | 2 | 12 | 35 | |||||||||||||||||
| Munseylla simplex Chen | 6 | 3 | 1 | 1 | 1 | 2 | 2 | 16 | |||||||||||||||||
| Neonesidea sp. | 3 | 1 | 2 | 1 | 2 | 3 | 1 | 2 | 2 | 1 | 13 | 1 | 3 | 1 | 36 | ||||||||||
| Nipponocythere? sp. | 2 | 8 | 10 | ||||||||||||||||||||||
| Pacambocythere ishizakii sp. nov. | 2 | 2 | 4 | 4 | 11 | 4 | 2 | 1 | 15 | 9 | 29 | 13 | 5 | 2 | 1 | 3 | 107 | ||||||||
| Pacambocythere sp. | 1 | 1 | |||||||||||||||||||||||
| Paijenborchella spinosa Hanai | 2 | 1 | 11 | 2 | 2 | 1 | 1 | 1 | 2 | 1 | 24 | ||||||||||||||
| Palmenella limicola (Norman) | 9 | 7 | 1 | 6 | 4 | 3 | 5 | 10 | 4 | 2 | 5 | 9 | 3 | 1 | 4 | 16 | 20 | 8 | 117 | ||||||
| Palmoconcha irizukii Tanaka | 1 | 27 | 6 | 2 | 2 | 27 | 26 | 18 | 3 | 5 | 2 | 8 | 10 | 2 | 5 | 4 | 15 | 47 | 210 | ||||||
| Paracytheridea neolongicaudata Ishizaki | 1 | 1 | 1 | 2 | 1 | 1 | 1 | 8 | |||||||||||||||||
| Pectocythere cf. ishizakii Irizuki and Yamada | 43 | 19 | 43 | 7 | 12 | 16 | 1 | 1 | 3 | 2 | 4 | 5 | 1 | 6 | 163 | ||||||||||
| Pontocythere sp. | 2 | 3 | 2 | 2 | 1 | 2 | 12 | ||||||||||||||||||
| Pseudoaurila ishizakii Irizuki and Yamada | 1 | 1 | 2 | 2 | 3 | 3 | 1 | 5 | 4 | 3 | 2 | 5 | 1 | 4 | 3 | 1 | 4 | 45 | |||||||
| Pseudoaurila okumurai (Yajima) | 2 | 6 | 7 | 2 | 1 | 5 | 5 | 1 | 2 | 16 | 3 | 2 | 8 | 7 | 9 | 4 | 4 | 25 | 9 | 2 | 8 | 128 | |||
| Schizocythere sp. | 7 | 3 | 10 | 2 | 1 | 6 | 2 | 3 | 8 | 1 | 3 | 7 | 10 | 3 | 4 | 6 | 6 | 22 | 9 | 1 | 4 | 118 | |||
| Semicytherura hanaii Ishizaki | 6 | 2 | 8 | ||||||||||||||||||||||
| Semicytherura pseudoundata Irizuki and Yamada | 1 | 3 | 2 | 2 | 1 | 9 | |||||||||||||||||||
| Spinileberis? sp. | 1 | 4 | 2 | 1 | 5 | 2 | 7 | 22 | |||||||||||||||||
| Trachyleberis leei Huh and Whatley | 5 | 4 | 10 | 3 | 2 | 5 | 1 | 1 | 2 | 1 | 1 | 6 | 23 | 64 | |||||||||||
| Trachyleberis mizunamiensis Yajima | 11 | 9 | 2 | 6 | 7 | 9 | 2 | 1 | 4 | 1 | 6 | 1 | 2 | 1 | 34 | 96 | |||||||||
| Xestoleberis sp. | 2 | 1 | 2 | 4 | 9 | ||||||||||||||||||||
| No. of valves | 185 | 113 | 315 | 75 | 126 | 161 | 222 | 127 | 168 | 91 | 55 | 40 | 79 | 96 | 135 | 74 | 35 | 60 | 197 | 48 | 141 | 165 | 50 | 273 | 3031 |
| No. of species | 18 | 19 | 21 | 14 | 21 | 19 | 25 | 13 | 22 | 15 | 15 | 8 | 17 | 19 | 20 | 20 | 11 | 12 | 30 | 15 | 21 | 21 | 14 | 21 | 51 |
| Diversity | 2.25 | 2.51 | 2.11 | 1.94 | 2.38 | 2.3 | 2.84 | 2.03 | 2.39 | 2.35 | 2.36 | 1.87 | 2.49 | 2.61 | 2.38 | 2.76 | 2.28 | 2.2 | 3.05 | 2.06 | 2.8 | 2.35 | 2.19 | 2.36 | |
| Equitability | 0.53 | 0.64 | 0.39 | 0.5 | 0.51 | 0.53 | 0.68 | 0.58 | 0.5 | 0.7 | 0.7 | 0.81 | 0.71 | 0.71 | 0.54 | 0.79 | 0.89 | 0.75 | 0.7 | 0.52 | 0.78 | 0.5 | 0.64 | 0.5 | |
| Sample mass (g) | 80 | 80 | 80 | 40 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 80 | 40 |
Q-mode cluster analysis revealed five sample clusters (Biofacies I–V) at a similarity of 0.65 (Figure 10). R-mode cluster analysis revealed seven species clusters (Bioassociations CN, SL, PT, AA, PS, PA, and CC; these abbreviations are derived from the first letters of the two primary species in each bioassociation) except one species (Cornucoquimba sp.) at a similarity of 0.55 (Figure 10). Biofacies I consists of 10 samples from the lower sequence at site 1 and three samples from site 2 (Figures 11, 12). It is characterized by moderate to high diversity and equitability and the dominance of species of Bioassociation AA with the subordinate species of Bioassociation PT. Biofacies II consists of one sample from the middle sequence at site 1 (Figure 11) and from site 3 and is characterized by the dominance of the species of Bioassociation AA with the subordinate species of Bioassociations PA and PS. Biofacies III comprises five samples from the middle and upper sequences at site 1 (Figure 11) and is characterized by high diversity and equitability and a mixture of species of Bioassociations AA, PS, and PA. Biofacies IV is composed of two samples from the uppermost sequence at site 1 (Figure 11) and is characterized by high diversity and equitability and the dominance of species of Bioassociation CC with the presence of species of Bioassociation PS. Biofacies V consists of two samples from a thin sandstone layer at site 1 (sample no. 1-10; Figure 11) and from site 4 and is characterized by species of Bioassociations CN, PT, AA, and PS.
Bioassociation CN consists of Cibotoleberis tsuyamensis sp. nov. and Nipponocythere? sp. (Figure 10). The genus Cibotoleberis Guan in Hubei Institute of Geosciences et al., 1978 was previously described and reported only from the Pliocene deposits of southern China, suggesting that the genus was a tropical taxon. Hou and Gou (2007) suggested that Ambtonia Malz, 1982 is a junior synonym of Cibotoleberis. Cibotoleberis and Ambtonia are indeed morphologically similar; however, the former differs from the latter in having a rim and furrow along the anterior margin, and they are generally accepted as distinct genera. The extant species of the genus Nipponocythere have primarily been reported from bay and inland sea deposits (e.g. Ishizaki, 1971; Irizuki et al., 2006, 2011; Jöst et al., 2022). Hence, the members of this ostracod bioassociation may have lived in tropical enclosed muddy inner bays.
Bioassociation SL is composed of Spinileberis? sp. and Loxoconcha nozokiensis Ishizaki, 1963 (Figure 10). Spinileberis is commonly found in brackish bay or estuary deposits (e.g. Ikeya et al., 1995; Tanaka et al., 2011). Loxoconcha nozokiensis has been reported from Lower to Middle Miocene deposits in Japan and is interpreted to have lived on warm shallow sandy bay coasts with seaweeds or seagrasses (Irizuki et al., 2004).
Bioassociation PT consists of six species (Figure 10), the most abundant of which are Pectocythere cf. ishizakii and Trachyleberis spp. Pectocythere cf. ishizakii is conferrable to P. ishizakii reported from Lower Miocene strata in central Japan (Irizuki et al., 2004, 2021). Pectocythere ishizakii may have been dominant in the innermost parts of brackish muddy bays (Irizuki et al., 2004). Trachyleberis leei Huh and Whatley, 1997 and T. mizunamiensis Yajima, 1992 are common throughout the Lower Miocene (Irizuki et al., 2004) and MMCO (e.g. Yajima, 1988, 1992; Huh and Whatley, 1997; Irizuki et al. 2024). According to Irizuki et al. (2004), T. leei may have lived in enclosed muddy bays, whereas T. mizunamiensis lived in sandy shorefaces under the influence of warm water currents. Therefore, this ostracod bioassociation is characterized by a mixture of species adapted to muddy enclosed bay and sandy bay coasts.
Bioassociation AA comprises seven species (Figure 10), the most abundant of which are Acanthocythereis noriyukikeyai and A. kunihiroi sp. nov. The former species was described from the lower Middle Miocene Obata Formation of the Tomioka Group in central Japan (Tanaka and Hasegawa, 2013a), which was deposited during the same time period as the formation examined in this study. Takakuwa and Tsukagoshi (2005) reported Acanthocythereis? sp. from the lower Middle Miocene Haratajino Formation of the Tomioka Group in central Japan. This species is closely related to Acanthocythereis quadrata Irizuki and Yamada in Irizuki et al., 2004, which has been reported from Lower Miocene deposits (Irizuki et al., 2004). Acanthocythereis kunihiroi sp. nov. is extremely similar to the extant species A. munechikai Ishizaki, 1981, which is rare to common at water depths of approximately 100 m in the East China Sea (Ishizaki, 1981) and >80 m in the Pacific Ocean off the eastern coast of Japan (Zhou, 1995). It is a member of the Tsushima Warm Current Core Water ostracod assemblage (TWCA), which is found at a water depth of approximately 100 m in the southwestern Sea of Japan (Ozawa, 2003). Species such as Falsobuntonia taiwanica, Palmoconcha irizukii, and Palmenella limicola are subordinate members of this bioassociation. Falsobuntonia taiwanica (= Buntonia hanaii of Zhao in Wang et al., 1988) lives at water depths of 50 to ~200 m in the East China Sea (Wang et al., 1988) and > 40 m in the Pacific Ocean off the eastern coast of Japan (Zhou, 1995). Palmoconcha irizukii was first described from Middle Miocene strata in southwestern Japan (Tanaka et al., 2002), and it may have lived in bays (Irizuki and Matsubara, 1994). Extant Palmoconcha species are common in middle sublittoral to middle bathyal seas (Zhou, 1995). Palmenella limicola is an extant circumpolar species (e.g. Cronin and Ikeya, 1987) that is commonly associated with F. taiwanica and P. irizukii in Lower to Middle Miocene strata (Irizuki and Matsubara, 1994; Tanaka et al., 2002; Irizuki et al., 2004). Therefore, the members of this association may have lived predominantly in the muddy bay bottom of the middle to lower sublittoral zone.
Bioassociation PS is composed of 11 species of the genera Pseudoaurila, Schizocythere, Cornucoquimba, Cythere, Neonesidea, Callistocythere, and Xestoleberis (Figure 10). Species of these genera have been abundantly reported from modern shallow marine sandy deposits (e.g. Ishizaki, 1968; Irizuki et al., 2006). Pseudoaurila ishizakii Irizuki and Yamada in Irizuki et al., 2004 and P. okumurai are common in Lower to Middle Miocene strata (Yajima, 1992; Irizuki et al., 2004, 2024) and may have lived on seaweeds and seagrasses (Irizuki et al., 2004). Extant species of Cythere, Neonesidea, and Xestoleberis are also common around seaweeds and seagrasses (e.g. Kamiya, 1988). Therefore, the members of this association likely inhabited open shallow sandy coastal environments along with seaweeds and seagrasses under the influence of the warm water current.
Bioassociation PA comprises Argilloecia sp. and Pacambocythere ishizakii sp. nov. (Figure 10), whose paleoecology remains uncertain because of the lack of species identification or new species. Pacambocythere ishizakii sp. nov. is morphologically related to P. similis Malz, 1982, which has not been reported from modern deposits but has been reported from the Pliocene Maanshan Formation in southwestern Taiwan (Malz, 1982). The Maanshan Formation contains upper bathyal ostracods (Tanaka et al., 2013). Therefore, the members of this ostracod bioassociation may have lived in the deep sea (probably the lower sublittoral to upper bathyal zone).
Bioassociation CC consists of the genera Cytherella, Cytheropteron, and Krithe (Figure 10). Cytherella and Krithe are commonly found in deep sea deposits worldwide (e.g. Zhou, 1995; Swanson et al., 2005). Cytherella is known from the modern seas around the Japanese islands (Cytherella leizhouensis Gou in Gou et al., 1983). It lives in the lower sublittoral to upper bathyal zones under the warm Kuroshio Current (Zhou, 1995) and is distributed from the Malay Peninsula to Borneo (Mostafawi, 1992). A similar bioassociation comprising Cytherella and Krithe was reported from the Fushikidani Mudstone Member of the Kurosedani Formation in central Japan, which was deposited in the outer shelf to upper bathyal zones during the MMCO around 16 Ma (Ozawa, 2016). Cytherella species have also been reported from the overlying Takakura Formation (Goto and Irizuki, 2019) and Makino Formation in central Japan (Irizuki, 2003), which were deposited in the upper bathyal zone. The members of this ostracod bioassociation are therefore interpreted to have inhavited offshore muddy bottom waters of the upper bathyal zone under the influence of warm water currents in Japan. Cytherella belongs to Platycopida, which is characterized by a filter-feeing lifestyle. Whatley (1991) reported that Cytherella is abundant in the oxygen minimum zone (OMZ) of the modern North Atlantic, which demonstrates that Cytherella can survive in oxygen-depleted water masses. Moreover, Whatley et al. (2003) proposed the use of relative abundances of filter-feeding platycopid ostracods including Cytherella as a proxy for past levels of dissolved oxygen in the ocean (the ‘Platycopid Signal Hypothesis’ or PSH). However, Brandão (2008) and Brandão and Horne (2009) concluded that previously published and widely applied calibrations of platycopid abundance against dissolved oxygen levels are inadequately justified by the available data. Wilson et al. (2014) also suggested that the PSH is not tenable based on the study of the Miocene oxygen minimum zone. On the other hand, Horne et al. (2011) proposed that ostracod assemblages that are overwhelmingly dominated by platycopids signify oligotrophy. Therefore, the PSH remains controversial and further studies may be required.
Paleoenvironment of the study sitesFigure 11 shows the vertical changes of each ostracod bioassociation and biofacies at site 1 where Biofacies I–IV are arranged from the base to top. Bioassociation PT (enclosed bay and bay coast) decreases upward from 37.3% to a few percentages in Biofacies I. Conversely, Bioassociation AA (muddy bay bottom of the middle (to lower) sublittoral zone) increases upward from 42.2% and peaks at 83.5% near the upper part of Biofacies I. The percentage of Bioassociation AA is higher than that of Bioassociation PT even at the lowest sampling horizon, suggesting a depositional environment in the middle sublittoral zone at the lower part of Biofacies I and a deeping upward trend due to transgression. High diversity > 2.0 and moderate equitability around 0.5 (Tables 1 and 2) suggest that the paleoenvironment of Biofacies I was an open bay.
| Bioassociation | Biofacies | Paleoecology | ||||
|---|---|---|---|---|---|---|
| I | II | III | IV | V | ||
| CN | 0–3.9 | 0–4.0 | 0 | 0 | 0–34.4 | Tropical enclosed inner bay |
| SL | 0–7.1 | 0 | 0–0.7 | 0 | 2.2–3.3 | Shallow sandy bay coast with seaweeds or seagrasses |
| PT | 0–37.3 | 0–2.0 | 0–10.8 | 0–8.1 | 14.3–28.9 | Bay coast and muddy bay |
| AA | 41.4–83.5 | 62.5–70.0 | 16.2–46.9 | 7.1–11.7 | 15.8–31.9 | Middle sublittoral bay |
| PS | 7.1–31.9 | 12.5–14.0 | 24.1–45.9 | 31.7–52.3 | 16.1–49.5 | Open shallow sandy coast with seaweeds and seagrasses |
| PA | 0–6.7 | 4.0–25.0 | 17.7–23.7 | 5.0–5.6 | 0 | Lower sublittoral sea |
| CC | 0–1.2 | 0 | 0–8.6 | 20.8–46.7 | 0 | Upper bathyal sea |
| Species No. | 14–25(19) | 8–14(11) | 11–20(17) | 12–30(21) | 15–21(18) | |
| Diversity | 1.94–2.84(2.33) | 1.87–2.19(2.03) | 2.28–2.76(2.50) | 2.20–3.05(2.62) | 2.35–2.36(2.35) | |
| Equitability | 0.39–0.78(0.57) | 0.64–0.81(0.73) | 0.54–0.89(0.73) | 0.70–0.75(0.72) | 0.50–0.70(0.60) | |
At the horizon of Biofacies II, the percentage of Bioassociation AA starts decreasing, whereas that of Bioassociation PA (lower sublittoral zone) abruptly increases, suggesting an increase in water depth from the middle sublittoral zone to the lower sublittoral zone.
Biofacies III is characterized by a decrease in the percentage of Bioassociation AA upward, an increase in the percentage of Bioassociation PS (open shallow sandy coast), and a relatively stable abundance of Bioassociation PA. Several thin layers of fine sandstone are intercalated within the horizons of Biofacies III and IV. Therefore, the species of Bioassociation PS, characterized by open coastal to upper sublittoral species, are inferred to have been transported from shallower areas to lower sublittoral to upper bathyal sea bottom waters probably by events such as tectonic activity associated with the spreading of the Sea of Japan. The mixture of shallow and deep water species leads to the high values of diversity and equitability in Biofacies III and IV.
Biofacies IV is characterized by a high percentage of Bioassociation CC (upper bathyal zone); however, the percentage of Bioassociation PS was also high, suggesting the transportation and deposition of ostracods from shallower areas. According to the paleogeography of Taguchi (2002), the present study site was influenced by the eastward paleocurrent and surrounded by numerous small islands. The conglomerate bed overlying the sample horizons exhibited an eastward direction of the paleocurrent based on the imbrication of gravels. This bed contained huge clasts of basement schist, suggesting strong eastward currents such as submarine debris flows and tsunamis; however, understanding the origin of this bed is beyond the scope of this study.
Sample 1-10 of Biofacies V was collected from a thin layer of fine sandstone intercalated among massive sandy siltstones. The percentages of shallower bioassociations (PT and PS) were higher than those of the overlying and underlying samples, whereas the percentage of deeper Bioassociation AA was lower than those samples. This suggests that Biofacies V reflects the transportation of shallower water species to deeper depositional environments during high energy events.
At site 2, all three samples (2-1 to 2-3) belonged to Biofacies I, and no distinct vertical change was observed (Figure 12). The percentage of Bioassociation AA was high, ranging from 51.8% to 77.0%, and the percentage of Bioassociation PS was subordinate, ranging from 9.7% to 31.9%. Hence, the water depth was inferred to have been in the middle sublittoral zone with some mixing from shallower waters.
Therefore, sites 1 and 2 represent the middle sublittoral zone, both sites were deposited distal to main sedimentary basin of the Katsuta Group where numerous molluscan fossils that lived in brackish and tidal areas and upper sublittoral seas have been reported from the lower Yoshino Formation (e.g. Taguchi, 1981, 2002). Cytherella sp. from the uppermost samples at site 1 is characteristic of the Takakura Formation (Goto and Irizuki, 2019). These findings suggest the stratigraphic sequence studied herein can be chronologically correlated with the upper Yoshino Formation or the overlying Takakura Formation and assigned to marginal lithofacies of the basin because the study area is inferred to have been narrowly surrounded by basement rock during deposition.
Only one sample each was collected from sites 3 (3-1) and 4 (4-1). Sample 3-1 belongs to Biofacies II, suggesting that the paleoenvironment was in the middle or lower sublittoral zone (Figure 10). Biofacies II is composed of this sample and sample 1-12, suggesting that sample horizon at site 3 can be correlated with the middle sequence of site 1. Sample 4-1 belongs to Biofacies V but was slightly different in species composition from sample 1-10 from the same biofacies (Figure 10). It is characterized by the dominance of C. tsuyamensis sp. nov., which is a member of Bioassociation CN. Therefore, the paleoenvironment at site 4 is inferred to have been a tropical enclosed muddy inner bay.
Ostracod biogeographical implicationsIn this study, the species belonging to the genera Acanthocythereis, Cytherella, Hanaiborchella, Munseyella, Pacambocythere, Palmoconcha, Pseudoaurila, Schizocythere, and Trachyleberis were common or abundant throughout the sequences. These genera have been reported from upper Eocene to Oligocene strata in eastern Asia, ranging from the East China Sea to southwestern Japan (e.g. Liu, 1989; Yamaguchi, 2004, 2006; Yamaguchi et al., 2006; Yamaguchi and Kamiya, 2007). Nevertheless, most Paleogene species differ from those observed in this study, with the exception of Munseyella simplex Chen in Yang et al., 1990.
Here, we reported abundant ostracods from Lower Miocene strata in Japan that were deposited just before the MMCO (e.g. Yamada et al., 2001; Irizuki et al., 2004), of which Callistocythere kyongjuensis Huh and Whatley, 1997, Cornucoquimba moniwensis (Ishizaki, 1966), C. saitoi (Ishizaki, 1963), Hanaiborchella ikeyai (Irizuki and Yamada) in Irizuki et al., 2004, Loxoconcha nozokiensis, Pseudoaurila ishizakii, P. okumurai, Trachyleberis leei, and T. mizunamiensis were especially dominant (Table 1). Some of these species are morphologically similar to aforementioned Paleogene taxa. Therefore, these taxa may have evolved in the eastern Asia during the Oligocene and Early Miocene and survived during the MMCO.
Ostracods from tropical western Pacific areas throughout the MMCO are poorly understood (Shin et al., 2019) due to limited reports from Southeast Asia. Shin et al. (2019) reported Middle to Late Miocene ostracods from the Java Island, Indonesia. Those assemblages are primarily composed of genera such as Pistocythereis, Neomonoceratina, Xestoleberis, Neonesidea, Actinocythereis?, Cytherella, Cytherelloidea, Lankacythere, Atjehella, and Neocyprideis. In our study, Xestoleberis, Neonesidea, and Cytherella were found; however, these are cosmopolitan genera (Benson et al., 1961). Furthermore, it is difficult to identify the species of these genera due to the smooth ornamentation on their valve surfaces. Cibotoleberis, Paijenborchella, and Pacambocythere have been abundant in modern subtropical to tropical regions since at least the Pliocene (e.g. Gou et al., 1983). Therefore, Pacambocythere ishizakii sp. nov., Paijenborchella spinosa Hanai, 1970, and Cibotoleberis tsuyamensis sp. nov. possibly invaded the seas around the Japanese islands during the MMCO from warmer southern seas due to the birth of the Kuroshio Current corresponding to the closure of the Indonesian Seaway at ca. 17 Ma (Ogasawara and Noda, 1996; Ogasawara, 2011). Ozawa (2016) discussed the migration of deep-water genera such as Cytherella and Krithe into the Sea of Japan around the Japanese islands during the Miocene. He proposed that these bathyal ostracods would have passed through the “Paleo-Sanin-Hokuriku Trough” or “Paleo-Fossa Magna Trough” (Iijima and Tada, 1990; Figure 13). As the present study site is located near the former trough, the bathyal ostracods reported herein could have migrated from the Pacific Ocean through the “Paleo-Sanin Hokuriku Trough” after the opening of the Sea of Japan. Seawater invasion into the Sea of Japan started from ca. 18 Ma. For instance, Nomura (2021) constrained the age of a horizon separating nonmarine deposits and marine deposits in Shimane, southwestern Japan to ca. 17.65 Ma. Numerous macrofossils of several faunas indicative of a tropical marine climate have been reported from the Katsuta Group (e.g. Taguchi, 1981, 2002). Mangrove pollen fossils have also been found from the Yoshino Formation (Yamanoi et al., 1980; Mori and Yamanoi, 2003). Hence, we infer the shallow marine conditions in the study area to have been tropical during the MMCO.
Only one cold-water species, Palmenella limocola, was commonly found in this study. It is a circumpolar species defined by Cronin and Ikeya (1987) that lives in the high-latitude North Atlantic, North Pacific, and Arctic seas (e.g. Brouwers, 1990; Stepanova, 2006; Gemery et al., 2015). It is also classified as a JSI-PA species by Ozawa (2003) and Ozawa and Kamiya (2005), which are species associated with the Japan Sea Intermediate Water (water temperature ranging from 0 to 10°C). The oldest record of this species is from the Lower Miocene in Japan (Irizuki et al., 2004). This species has been reported from the other MMCO deposits (e.g. Irizuki and Matsubara, 1994; Ozawa, 2016), Middle to Upper Miocene deposits (e.g. Ishizaki, 1966; Irizuki, 1994; Irizuki et al., 1998b; Tanaka et al., 2002; Tanaka and Nomura, 2009a; Tanaka and Hasegawa, 2013b; Ozawa et al., 2025; Tanaka et al., 2025), Upper Pliocene to Lower Pleistocene deposits containing cool-temperate to subarctic molluscan species (Omma–Manganji fauna: Otuka, 1939) (e.g. Ishizaki and Matoba, 1985; Cronin and Ikeya, 1987; Ozawa, 1996; Irizuki et al., 2007). Therefore, P. limocola is inferred to have lived in warm water seas during the Early and early Middle Miocene (MMCO) and adapted gradually to cooler conditions during the Middle to Late Miocene since the mid-Miocene climatic transition (MMCT) and the Northern Hemisphere glaciation (NHG) since the Late Pliocene. Cornucoquimba moniwensis is common in shallow-water strata deposited during the MMCO (Ishizaki, 1966; Yajima, 1988, 1992; Ozawa, 2016; Irizuki et al., 2024); however, it is also common in Middle to Upper Miocene strata deposited during the MMCT (Ishizaki et al., 1996; Irizuki et al., 1998b) and Upper Pliocene to Lower Pleistocene strata deposited during the NHG (Ishizaki and Matoba, 1985; Ozawa, 1996; Irizuki et al., 2007). Therefore, like P. limicola, C. moniwensis also adapted to the gradual cooling after the MMCT.
Cryophilic to circumpolar hemicytherids, such as Elofsonella, Finmarchinella, Hemicythere, and Laperousecythere, were not detected in this study. These taxa have been reported from Miocene strata deposited during cooling events such as MMCT and the Miocene isotope events (Irizuki and Matsubara, 1994, 1995; Irizuki et al., 1998b, 2004; Tanaka et al., 2025). Hence, the ostracod fauna in the present study is not indicative of cool-water conditions.
1. This study recognized 52 ostracod species in 24 samples from the Yoshino Formation of the Katsuta Group, which was deposited during the MMCO. Three new trachyleberidid species were described: Acanthocythereis kunihiroi sp. nov., Cibotoleberis tsuyamensis sp. nov., and Pacambocythere ishizakii sp. nov.
2. Q-mode cluster analysis revealed five distinct sample clusters (i.e., Biofacies I–V), suggesting that the depositional environments were enclosed bay, middle to lower sublittoral bay, and upper bathyal sea, in ascending order.
3. R-mode cluster analysis revealed seven ostracod bioassociations, each representing distinct paleoecological preferences.
4. Fossil ostracod assemblages are paleobiogeographically characterized by a mixture of tropical taxa (e.g. Cibotoleberis, Pacambocythere, and Paijenborchella) and cosmopolitan taxa (e.g. Cytherella, Neonesidea, and Xestoleberis) that were introduced after the opening of the Sea of Japan, the survivors or descendants of which have continued to live around the Japanese islands since the Paleogene (e.g. Acanthocythereis, Munseyella, Pseudoaurila, and Trachyleberis). No cool-temperate species were detected.
5. Bathyal ostracods of the genera Krithe and Cytherella migrated from the Pacific Ocean through the “Paleo-Sanin Hokuriku Trough” after the opening of the Sea of Japan.
6. Abundant species such as Palmenella limicola and Cornucoquimba moniwensis are inferred to have adapted to cooler-water conditions after the MMCO.
(By T. Irizuki)
In this section, primary ostracod species and three new trachyleberidid species are described and discussed. Terminology in the descriptions follows that of Scott (1961) and Athersuch et al. (1989). The order of taxonomic ranks higher than genus follows Hanai et al. (1977). All voucher specimens are reposited in the Shimane University Museum (SMU) in Matsue, Shimane. The number of specimens means the number of individual valves (articulated carapaces were counted as two valves). Abbreviations in Type series are as follows: RV = right valve, LV = left valve, C = carapace, L = length, and H = height.
Subclass Podocopa Sars, 1866
Order Platycopida Sars, 1866
Family Cytherellidae Sars, 1866
Genus Cytherella Jones, 1849
Cytherella cf. donghaiensis Gou in Hou and Gou, 2007
Materials.—41 specimens.
Occurrence.—Abundant to few in all samples of Biofacies IV.
Remarks.—This species is similar to Cytherella donghaiensis Gou in Hou and Gou, 2007 in valve outline. Cytherella donghaiensis was first described as C. elliptica Liu, 1989, which is a junior homonym of C. elliptica Brady, 1878, from the Eocene Oujiang Formation in the East China Sea (Liu, 1989). It has been reported widely from Eocene to lowest Oligocene strata in southwestern Japan (Yamaguchi et al., 2005, 2006). Several specimens were found in the samples from the upper sequence at site 1, but most of them were juvenile and/or broken valves. Several species of Cytherella have also been reported from Neogene strata in Japan or modern deposits off the coast of southwestern Japan: Cytherella leizhouensis Gou in Gou et al., 1983 (= Cytherella japonica Ishizaki, 1983) of Zhou (1995), Cytherella sp. of Yamaguchi et al. (2012), and Cytherella sp. of Ozawa (2016). Nevertheless, they differ valve morphology along the anterior margin.
Cytherella sp.
Materials.—10 specimens.
Occurrence.—Rare in a few samples of Biofacies III and IV.
Remarks.—The specimens of this species are poorly preserved or broken valves and carapaces. The figured specimen (Figure 5.2) is a carapace. Numerous foveola are distributed along the valve margin and numerous puncta are scattered in the median area of the valve surface. No distinct ridges are distributed on the valve surface, but a faint ridge is discernable in the ventromedial area. Shallow compressed areas are present along the anterior margin.
Order Podocopida Sars, 1866
Family Pontocyprididae Müller, 1894
Genus Argilloecia Sars, 1866
Argilloecia sp.
Materials.—54 specimens.
Occurrence.—Rare in a few samples of Biofacies I, rare to common in Biofacies II, rare to few in Biofacies III and IV.
Remarks.—Specimens of this species are relatively larger in valve size than other Argilloecia species. These specimens differ from Argilloecia toyamaensis Ishizaki and Irizuki, 1990, which has a more protruded posterior margin.
Family Pectocytheridae Hanai, 1957b
Genus Pectocythere Hanai, 1957b
Pectocythere cf. ishizakii Irizuki and Yamada in Irizuki, Yamada, Maruyama and Ito, 2004
Pectocythere cf. ishizakii Irizuki and Yamada in Irizuki et al. Goto and Irizuki, 2019, p. 9, fig. 5.7, 5.8.
Materials.—163 specimens.
Occurrence.—Common to rare in most samples of Biofacies I, rare in several samples of Biofacies III and IV and all samples of Biofacies V.
Remarks.—The type specimens of P. ishizakii have a reticulated ornamentation on the valve surface (Irizuki et al., 2004). However, the specimens reported here have a faint reticulated (Figure 5.4) or smooth (Figure 5.5–5.7) ornamentation on the valve surface. Thus, we tentatively assign these specimens to Pectocythere cf. ishizakii.
Genus Munseyella Bold van den, 1957
Munseyella simplex Chen in Yang, Chen and Wang, 1990
Munseyella sp. Yajima, 1988, table 2; Yajima, 1992, p. 255, pl. 31, figs. 5, 6.
Munseyella simplex Chen in Yang et al., 1990, p. 374, 385, 386, pl. 1, figs. 7, 8; Yamaguchi, 2004, p. 66, 69, fig. 7.4a, b; Yamaguchi, 2006, p. 911, 912, fig. 6.12; Yamaguchi et al., 2006, p. 92, 93, pl. 1, fig. 3a–3c.
Materials.—35 specimens.
Occurrence.—Rare in several samples of Biofacies I, IV, and V.
Remarks.—This species has been widely reported from Eocene and Oligocene strata in Japan and China (e.g. Yamaguchi, 2004, 2006). Therefore, this species is one of early invaders into Japanese marine coasts and survived until at least the Middle Miocene.
Family Leptocytheridae Hanai, 1957a
Genus Callistocythere Ruggieri, 1953
Callistocythere kyongjuensis Huh and Whatley, 1997
Callistocythere sp. A. Yun et al., 1990, p. 4, pl. 3, figs. 1–3; Huh and Paik, 1992b, pl. 3, fig. 11; Huh et al., 1994, pl. 1, figs. 5, 6.
Callistocythere kyongjuensis Huh and Whatley, 1997, p. 32, 34, pl. 1, figs. 1–6; Tanaka et al., 2002, p. 8, fig. 3.9; Irizuki et al., 2004, p. 120, pl. 2, figs. 5, 6; Irizuki et al., 2024, p. 88, pl. 1, fig. 4.
Materials.—12 specimens.
Occurrence.—Rare in several samples of Biofacies I, II, and III.
Remarks.—Callistocythere kyongjuensis was first reported from the Miocene of the Pohang Basin in eastern Korea. This species differs from Callistocythere subsetanensis Ishizaki, 1966 in lacking an arched posteroventral ridge and being larger in valve size.
Callistocythere subsetanensis Ishizaki, 1966
Callistocythere subsetanensis Ishizaki, 1966, p. 149, pl. 16, figs. 18, 19; Irizuki et al., 2001, fig. 18.5; Tanaka et al., 2004, p. 60, pl. 1, fig. 7; Tanaka, 2009, p. 37, pl. 3, fig. 14.
Materials.—44 specimens.
Occurrence.—Few to rare in most samples of Biofacies III and all samples of Biofacies IV, rare in several samples of Biofacies I and all samples of Biofacies V.
Remarks.—The specimens from the study area are identified as C. subsetanensis due to a distinct arched posteroventral ridge running from the posterodorsal area to the posteroventral area and an anterior oblique ridge. Callistocythere subsetanensis has been reported from Middle Miocene strata in central to northern Japan. This species differs from Callistocythere kyokoae Tanaka in Tanaka and Hasegawa, 2013 (Tanaka and Hasegawa, 2013b) in having more robust and thicker arched posteroventral ridge.
Family Cytheridae, Baird, 1850
Genus Cythere O. F. Müller, 1785
Cythere omotenipponica Hanai, 1959
Cythere lutea omotenipponica Hanai, 1959, p. 413, pl. 28, figs. 1, 3, 7a, text-fig. 3a, b; Ishizaki, 1966, p. 137, pl. 17, figs. 9–12; Ishizaki, 1968, p. 16, pl. 3, figs. 7, 8; Ishizaki, 1969, p. 215, 216, pl. 26, fig. 9; Hanai, 1970, text-figs. 10A–E, 11A, E, J, 12A, A’, 13B, D, D’, F, H, H’; Cai, 1982, pl. 4, fig. 46.
Cythere omotenipponica Hanai. Schornikov, 1975, p. 4, 5; Tsukagoshi and Ikeya, 1987, p. 214, figs. 4e, 5i, 9-1a, b; Ikeya and Suzuki, 1992, p. 127, pl. 4, fig. 1; Lee and Paik, 1992, p. 149, pl. 2, fig. 10; Kamiya and Nakagawa, 1993, p. 125, pl. 2, fig. 10; Irizuki et al., 1998a, p. 7, fig. 2.6; Irizuki et al., 1998b, p. 35, fig. 5.6; Irizuki et al., 2001, p. 67, fig. 18.6; Nakao et al., 2001, p. 133, fig. 5.8; Yamaguchi and Hayashi, 2001, p. 247, fig. 5.10; Tanaka et al., 2004, p. 60, pl. 1, fig. 8; Sasaki et al., 2007, p. 524, fig. 5.16; Yamada, 2007, p. 53, fig. 7E, p. 54, fig. 8D; Irizuki et al., 2009, p. 320, fig. 3.2; Tanaka and Nomura, 2009b, p. 61, fig. 3.19; Tanaka and Hasegawa, 2013b, p. 144, 160, 161, fig. 4.19; Ozawa, 2016, p. 133, fig. 9.7.
Cythere sp. Cheong et al., 1986, p. 44, pl. 1, fig. 17.
Materials.—21 specimens.
Occurrence.—Rare in most samples of Biofacies I, several samples of Biofacies III and IV, and all samples of Biofacies V.
Remarks.—This species is widely distributed along recent marine coasts in central to southwestern Japan under the warm Kuroshio Current and Tsushima Current (e.g. Tsukagoshi and Ikeya, 1987). It has been reported from strata deposited after the MMCO at several localities in Japan (e.g. Ozawa, 2016).
Genus Schizocythere Triebel, 1950
Schizocythere sp.
Schizocythere sp. Yamada et al., 2001, pl. 1, fig. 2; Irizuki et al., 2004, p. 120, pl. 2, figs. 7, 8; Goto et al., 2013, fig. 9.10.
Schizocythere sp. 1. Ozawa, 2016, p. 139, fig. 9.16, 9.17.
Materials.—118 specimens.
Occurrence.—Common to rare in all samples of Biofacies II, few in all samples of Biofacies II and IV, few to rare in most samples of Biofacies I.
Remarks.—This species has reported from other Lower to Middle Miocene deposits in Japan (e.g. Schizocythere sp. of Irizuki et al., 2004; Schizocythere sp. 1 of Ozawa, 2016). It differs from the Pliocene to Recent Schizocythere okhotskensis Hanai, 1970 in having less prominent ridges. It is distinguishable from Schizocythere sakanouei Tanaka, 2003 from the Middle Miocene Omori Formation in southwestern Japan in having different reticulation and ridge patterns. It differs from Schizocythere asagao Yajima, 1982 in having less prominent reticulated ornamentation. It somewhat resembles Schizocythere kishinouyei (Kajiyama, 1913), which has more distinct robust ridges.
Genus Paijenborchella Kingma, 1948
Paijenborchella spinosa Hanai, 1970
Paijenborchella spinosa Hanai, 1970, p. 726, pl. 108, fig. 1a–e; Iwatani and Irizuki, 2008, table 1; Iwatani et al., 2011, table 1.
Materials.—24 specimens.
Occurrence.—Rare in several samples of all biofacies except for Biofacies IV.
Remarks.—Paijenborchella spinosa was first described from the Pleistocene tropical terrace deposits of Ishigaki Island in southern Japan (Hanai, 1970). It has also been reported from the Pliocene deposits of Kyushu in southwestern Japan (Hanai, 1970; Iwatani and Irizuki, 2008; Iwatani et al., 2011).
Genus Hanaiborchella Gründel, 1976
Hanaiborchella ikeyai (Irizuki and Yamada in Irizuki, Yamada, Maruyama and Ito, 2004
Neomonoceratina ikeyai Irizuki and Yamada in Irizuki et al., 2004, p. 120, 121, pl. 2, figs. 9–13; Tanaka et al., 2012, p. 35, pl. 1, fig. 7.
Hanaiborchella ikeyai (Irizuki and Yamada). Irizuki et al., 2024, p. 88, pl. 1, fig. 6.
Materials.—16 specimens.
Occurrence.—Rare in several samples of Biofacies I, IV, and V.
Remarks.—Yamaguchi et al. (2005) suggested that this species belongs to the genus Hanaiborchella rather than Neomonoceratina based on the presence of a short caudal process in the middle of the carapace. These specimens have somewhat coarser reticulation than that of the type specimens of H. ikeyai.
Genus Palmenella Hirschmann, 1916
Palmenella limicola (Norman, 1865)
Cythereis limicola Norman, 1865, p. 20, 21, pl. 6, figs. 1–4.
Palmenella limicola (Norman). Elofson, 1941, p. 277, 278, text-fig. 21; Swain, 1963, p. 830, 831, pl. 99, fig. 3a–3d; Ishizaki, 1966, p. 156, pl. 19, fig. 8; Hanai, 1970, p. 704, text-figs. 6B, 7G, H; Neale and Howe, 1975, pl. 5, figs. 7, 8; Siddiqui and Grigg, 1975, pl. 2, fig. 14; Rosenfeld, 1977, p. 15, 16, pl. 1, figs. 3–6; Lord, 1980, pl. 3, fig. 6; Cronin, 1981, p. 412, pl. 11, figs. 1, 2, 4; Ishizaki and Matoba, 1985, pl. 5, figs. 16, 17; Cheong et al., 1986, pl. 2, fig. 1; Cronin and Ikeya, 1987, p. 86, pl. 2, fig. 17; Brouwers, 1988, figs. 5, 6; Yajima, 1988, pl. 2, fig. 8; Athersuch et al., 1989, p. 82, 83, pl. 1.2, fig. 28; Brouwers, 1990, p. 19, 20, pl. 1, fig. 15, pl. 4, figs. 9, 12, 17, fig. 1; Brouwers et al. 1991, pl. 1, fig. 7; Cronin, 1991, fig. 7.11; Ikeya and Suzuki, 1992, pl. 7, fig. 2; Huh and Paik, 1992a, pl. 1, fig. 10; Huh and Paik, 1992b, pl. 1, fig. 10; Lee and Paik, 1992, pl. 3, fig. 2; Hartmann, 1993, p. 241, pl. 1, figs. 2–4; Brouwers, 1994, fig. 10.7; Irizuki, 1994, p. 8, pl. 1, fig. 4; Irizuki and Matsubara, 1994, pl. 1, fig. 7; Ozawa et al., 1995, pl. 1, fig. 4; Kamiya et al., 1996, pl. 2, fig. 1; Ozawa, 1996, pl. 7, fig. 9; Kim et al. 1998, pl. 1, fig. 11; Irizuki et al., 1998b, fig. 5.8; Irizuki et al., 2001, fig. 18.7; Tanaka et al., 2002, p. 10, fig. 7.3; Irizuki et al., 2004, p. 121, pl. 2, figs. 14, 15; Kamiya et al., 2001, fig. 16.11; Yamaguchi and Hayashi, 2001, fig. 6.8; Ozawa, 2004, p. 251, pl. I, fig. 8; Ozawa et al., 2004, p. 44, pl. 2, fig. 8; Stepanova, 2006, p. S169–S171, pl. 4, figs. 1–3; Irizuki et al., 2007, p. 427, fig. 5.5; Tanaka and Nomura, 2009a, p. 263, fig. 3.C; Ozawa and Domitsu, 2010, p. 6, fig. 5.8; Gemery et al., 2015, fig. 22.2; Ozawa, 2016, p. 131, fig. 7.15; Ozawa et al., 2025, pl. 2, figs 3, 4a–f; Tanaka et al., 2025, fig. 3.g.
Palmenella sp. – Hanai, 1961a, p. 369, text-fig. 11, fig. 4a, b.
Kyphocythere limicola (Norman, 1865). Sars, 1925, p. 181, 182, pl. 82; Tada et al., 2015, fig. F24.12.
Materials.—117 specimens.
Occurrence.—Common to few in all samples of Biofacies II, few to rare in most samples of Biofacies I and III.
Remarks.—Palmenella limicola has been widely reported from the modern cold waters of the northern high latitude Atlantic, North Pacific, and circumpolar seas (e.g. Brouwers, 1988, 1990; Ozawa, 2004; Stepanova, 2006). The oldest fossils were recorded from the Lower Miocene (Irizuki et al., 2004). Specimens reported here show several variations in surface ornamentation: distinct reticulation (Figure 5.19), delicate reticulation with smaller puncta (Figure 5.20, 5.21), and weak reticulation (Figure 5.22). Several tubercles were less developed in comparison with those of Late Miocene to Recent specimens. These characteristics are the same as those of other specimens reported from the Lower to lower Middle Miocene in Japan, which occur with numerous warm water species (e.g. Irizuki and Matsubara, 1994; Irizuki et al., 2004; Ozawa, 2016).
Genus Spinileberis Hanai, 1961b
Spinileberis? sp.
Spinileberis? sp. Yajima, 1988, pl. 2, fig. 1.
Materials.—22 specimens.
Occurrence.—Rare in several samples of Biofacies I and V.
Remarks.—The specimens reported here have valve surface ornamentation and morphology characteristic of species of the genus Spinileberis, but the detailed structure of the hingement is not discernable.
Family Hemicytheridae Puri, 1953
Genus Pseudoaurila Ishizaki, 1983
Pseudoaurila ishizakii Irizuki and Yamada in Irizuki, Yamada, Maruyama and Ito, 2004
Aurila aff. convexa (Baird). Ishizaki, 1966, p. 142, pl. 16, figs. 3, 4.
Aurila sp. – Hanai et al., 1977, p. 44.
Pseudoaurila okumurai (Yajima). Miyazawa, 1997, pl. 1, figs. 5, 6.
Pseudoaurila sp. – Irizuki et al., 1998b, fig. 5.13.
Pseudoaurila ishizakii Irizuki and Yamada in Irizuki et al., 2004, p. 132, 133, pl. 6, figs. 9–14; Tanaka et al., 2012, p. 36, pl. 1, fig. 18; Goto et al., 2013, p. 329, fig. 9.8; Ozawa, 2016, p. 133, fig. 9.19; Irizuki et al., 2024, p. 88, pl. 1, fig. 9.
Materials.—45 specimens.
Occurrence.—Few to rare in most samples of Biofacies III, rare in most samples of Biofacies I, II, IV, and all samples of Biofacies V.
Remarks.—This species is discernable from Pseudoaurila okumurai but in its smaller valve size and different posterior ornamentation pattern.
Pseudoaurila okumurai (Yajima, 1992)
Aurila sp. Yajima, 1988, pl. 1, fig. 19; Huh and Paik, 1992a, pl. 1, figs. 12, 13.
Aurila sp. A. Huh and Paik, 1992b, pl. 1, figs. 13, 14.
Aurila okumurai Yajima, 1992, p. 261, 262, pl. 29, figs. 3, 4, pl. 30, figs. 3–6.
Pseudoaurila okumurai (Yajima). Miyazawa, 1997, pl. 1, fig. 4; Irizuki et al., 1998b, fig. 5.12; Yamada et al., 2001, pl. 1, fig. 5; Irizuki et al., 2001, fig. 7.7; Irizuki and Maruyama, 2001, fig. 4.1; Irizuki et al., 2004, p. 134, pl. 7, figs. 1–6; Tanaka et al., 2004, p. 60, pl. 1, fig. 11; Tanaka et al., 2012, p. 36, pl. 1, fig. 19; Goto et al., 2013, p. 329, fig. 9.9; Irizuki et al., 2024, p. 88, pl. 1, fig. 10.
Materials.—128 specimens.
Occurrence.—Common to rare in all samples of Biofacies V, few in all samples of Biofacies IV, few to rare in most samples of Biofacies I and all samples of Biofacies III.
Remarks.—This is a common shallow-water species in Lower to Middle Miocene strata together with Pseudoaurila ishizakii Irizuki and Yamada in Irizuki et al., 2004 (e.g. Yajima, 1992; Irizuki et al., 2004). Pseudoaurila? sp. of Yamaguchi (2006) from the uppermost Eocene–lowermost Oligocene Kishima Formation in southwestern Japan is similar to P. okumurai as suggested by Yamaguchi (2006), who noted that Pseudoaurila? sp. differs from P. okumurai in its smaller carapace, indistinct posterodorsal corner, undeveloped caudal process, and rectangular fossae.
Genus Cornucoquimba Ohmert, 1968
Cornucoquimba moniwensis (Ishizaki, 1966)
Hermanites moniwensis Ishizaki, 1966, p. 158, 159, pl. 18, figs. 1–3; Ruan and Hao, 1988, p. 367, 368, pl. 67, figs. 9–12.
Cornucoquimba moniwensis (Ishizaki). Hanai et al., 1977, p. 47; Ishizaki and Matoba, 1985, pl. 5, figs. 16, 17; Yajima, 1988, pl. 1, fig. 6; Huh and Paik, 1992a, pl. 2, figs. 2, 3; Huh and Paik, 1992b, pl. 2, figs. 2, 3; Ishizaki et al., 1996, pl. 1, figs. 1–3; Irizuki et al., 1998b, fig. 6-1; Tanaka et al., 2004, pl. 1, fig. 13; Irizuki et al., 2007, p. 429, fig. 6.6; Irizuki and Ishida, 2007, p. 16, fig. 3.24; Tanaka, 2009, p. 38, p. 44, pl. 3, figs. 11, 12; Ozawa, 2016, p. 133, fig. 9.4; Irizuki et al., 2024, p. 88, pl. 1, fig. 11.
Cornucoquimba cf. moniwensis (Ishizaki). Ozawa, 1996, p. 108, l. 2, fig. 8.
Materials.—60 specimens.
Occurrence.—Rare in most samples of Biofacies I and III, few in most samples of Biofacies IV.
Remarks.—This species is abundant in Lower Miocene and younger strata in Japan (e.g. Ishizaki, 1966; Yajima, 1988, 1992; Irizuki et al., 1998b, 2004). This species occurs with cold-water taxa in Pliocene deposits in northeastern Japan (Ishizaki and Matoba, 1985; Irizuki et al., 2007; Irizuki and Ishida, 2007). Thus, it adapted to cooler seas after the MMCO throughout events such as the MMCT and NHG.
Cornucoquimba saitoi (Ishizaki, 1963)
Bradleya saitoi Ishizaki, 1963, p. 29, 30, pl. 2, figs. 11, 13–19.
Hermanites saitoi (Ishizaki). Ishizaki, 1966, p. 159, pl. 18, figs. 7, 8.
Therocythere? saitoi (Ishizaki). Benson, 1972, p. 33.
Cornucoquimba saitoi (Ishizaki). Hanai et al., 1977, p. 47. Irizuki and Matsubara, 1994, pl. 1, fig. 11; Irizuki et al., 1998b, fig. 6.2; Irizuki et al., 2001, fig.18.13; Yamada et al., 2001, pl. 1, fig. 6; Yamaguchi and Hayashi, 2001, fig. 5.8; Irizuki et al., 2004, Ozawa, 2016, p. 133, figs. 9.5, 9.6.
Cornucoquimba cf. kyokoae Tanaka et al., 2013, p. 164, 166, fig. 5.10.
Materials.—78 specimens.
Occurrence.—Few to rare in most samples of Biofacies I, III, IV and all samples of Biofacies V, rare in a sample of Biofacies II.
Remarks.—The specimens reported here have the same morphology as that of the type specimens (Ishizaki, 1963). Irizuki et al. (2004) suggested several morphological variations from robust ridges to delicate ones on the valve surface of C. saitoi. Cornucoquimba kyokoae may be a variation of C. saitoi.
Cornucoquimba cf. tosaensis (Ishizaki, 1968)
Cornucoquimba cf. tosaensis (Ishizaki). Irizuki et al., 2004, p. 134, 136, pl. 7, figs. 13–16.
Materials.—17 specimens.
Occurrence.—Rare in several samples of Biofacies I, III, and IV.
Remarks.—This species resembles and is related to C. tosaensis (Ishizaki, 1968), but it has more delicate marginal rim and ventral ridge and rectangular valve. Cornucoquimba cf. tosaensis also occurs in Lower Miocene strata in central Japan (Irizuki et al., 2004).
Cornucoquimba sp.
Cornucoquimba saitoi (Ishizaki). Tanaka et al., 2004, pl. 1, figs. 14, 15.
Materials.—24 specimens.
Occurrence.—Rare in several samples of Biofacies I, III, and IV.
Remarks.—This species differs from C. saitoi in having more inflated valves and a more weakly developed curved ridge in the posterodorsal area.
Family Trachyleberididae Sylvester-Bradley, 1948
Genus Trachyleberis Brady, 1898
Trachyleberis leei Huh and Whatley, 1997
Acanthocythereis sp. Huh and Paik, 1992a, pl. 2, fig. 10. – Huh and Paik, 1992b, pl. 2, fig. 10.
Trachyleberis sp. Irizuki and Matsubara, 1994, pl. 1, fig. 12.
Trachyleberis mizunamiensis Yajima. Miyazawa, 1997, pl. 4, figs. 1–6.
Trachyleberis leei Huh and Whatley, 1997, p. 37, pl. 2, figs. 10–15; Yamada et al., 2001, pl. 1, fig. 8; Irizuki et al., 2004, p. 122, pl. 3, figs.1–3; Goto and Irizuki, 2019, p. 9, fig. 5.9.
Materials.—64 specimens.
Occurrence.—Few to rare in all samples of Biofacies V, rare in several samples of Biofacies I, III, and IV.
Remarks.—Irizuki et al. (2004) reported this species in abundance from muddy sediments that were inferred to have been deposited in enclosed bay areas. Trachyleberis leei is extremely similar to Trachyleberis inouei Yamaguchi in Yamaguchi, Nagao and Kamiya, 2006 from the uppermost Eocene–lowermost Oligocene Kishima Formation in southwestern Japan, and both may be related to each other. Nevertheless, T. leei differs from T. inouei in having finer reticulation in the median area, a fainter oblique carina in the anterodorsal area, and a fainter carina along the anteroventral margin (Yamaguchi et al., 2006).
Trachyleberis mizunamiensis Yajima, 1992
Trachyleberis sp. 1. Yajima, 1988, pl. 1, fig. 1.
Trachyleberis mizunamiensis Yajima, 1992, p. 257, 258, pl. 32, figs. 7–10; Irizuki et al., 2004, p. 122, pl. 3, figs. 4–7; Tanaka et al., 2004, p. 60, pl. 1, fig. 17; Tanaka et al., 2012, p. 35, pl. 1, figs. 9, 10; Irizuki et al., 2024, p. 88, pl. 1, figs. 13, 14.
Trachyleberis leei Huh and Whatley. Ozawa, 2016, p. 133, fig. 9.20.
Materials.—96 specimens.
Occurrence.—Few in all samples of Biofacies V, few to rare in several samples of Biofacies I and III, rare in several samples of Biofacies II and IV.
Remarks.—These specimens can be identified as Trachyleberis mizunamiensis Yajima, 1992, but they show less or no development of spines on the valve surface and have more delicate reticulation than the type specimens, similar to the specimens reported from the Lower Miocene Akeyo and Toyama formations in central Japan (Irizuki et al., 2004).
Genus Acanthocythereis Howe, 1963
Type species.—Acanthocythereis araneosa Howe, 1963.
Acanthocythereis kunihiroi sp. nov.
ZooBank lsid: urn:lsid:zoobank.org:act:5EFA911E-F1CA-4C22-8ACC-8A54383D96AA
Acanthocythereis munechikai Ishizaki. Yajima, 1988, p. 1078, 1079, pl. 2, fig. 14.
Acanthocythereis sp. 1. Goto et al., 2013, p. 329, fig. 9.1.
Acanthocythereis sp. Goto and Irizuki, 2019, p. 9, fig. 5.1.
Etymology.—In honor of Kunihiro Ishizaki, who is a pioneering Japanese ostracod worker.
Types series.—Holotype: SMU-IC-F0102, female RV (Figure 7.4a, b), L = 0.683 mm, H = 0.420 mm. Paratypes: SMU-IC-F0099, female LV (Figure 7.1), L = 0.772 mm, H = 0.542 mm; SMU-IC-F0100, female RV (Figure 7.2), L = 0.777 mm, H = 0.491 mm; SMU-IC-F0101, male LV (Figure 7.3), L = 0.751 mm, H = 0.458 mm; SMU-IC-F0103, female C (Figure 7.5), L = 0.697 mm, H = 0.478 mm; SMU-IC-F0104, male RV (Figure 7.6a, b), L = 0.672 mm, H = 0.416 mm; SMU-IC-F0105, male LV (Figure 7.7a, b), L = 0.692 mm, H = 0.449 mm; SMU-IC-F0106, male LV (Figure 7.8), L = 0.679 mm, H = 0.448 mm.
Other materials.—366 specimens.
Type locality.—An outcrop of the Yoshino Formation along Okayama Prefectural Route 341, 160 m E of Tsuyama City Kyosho Elementary School, Tsuboi-kami, Tsuyama, Okayama, Japan (site 1-10; 35°03′41″N, 133°51′19″E).
Diagnosis.—Characterized by its overall reticulation, subtriangular in right valve, subquadrate in left valve, and distinct short ventromedian ridge and spines in posterior area.
Description.—Valve moderate to large in size, slightly inflated, subtriangular in the right valve and subrectangular in left valve, tapering posteriorly, in lateral view. Maximum height at anterior cardinal angle. Anterior margin broadly rounded with numerous marginal spines. Dorsal margin almost straight with several conical spines. Ventral margin almost straight or convex in left valve, slightly concave in right valve. Posterior margin slightly concave in its upper one-third, meeting dorsal margin at posterior cardinal angle. Lower two-thirds of posterior margin obliquely arched with clavate or conical spines, gradually meeting ventral margin. Valve surface consisting of polygonal reticulation, ridge, eye tubercle, subcentral tubercle, and several spines. Reticulation covers almost entire lateral surface and distributed radially in anterior area. Marginal rim with denticulation and spines developed nearly along outer margin from anterodorsal to posteroventral area. Ventromedian ridge distinct, starting below subcentral tubercle, running posteriorly, terminating at posterior half of valve length. Oblique ridge running from eye tubercle to subcentral tubercle. Three or four distinct clavate spines present along dorsal margin. A node-like spine presents in posteromedian area. Normal pores primarily scattered on muri. Marginal infold broad anteriorly and posteriorly. Hingement holamphidont with anterior stepped tooth with subround socket, straight smooth groove and elongate posterior tooth in right valve. Hingement in left valve complementary. Muscle scars not observed because of poor preservation. Sexual dimorphism distinct. Males slenderer than females.
Occurrence.—Abundant to few in all samples of Biofacies I, few to rare in all samples of Biofacies II, III, IV, and V.
Remarks.—This new species exhibits variations in valve size. Specimens from lower horizons are smaller in size (holotype and some paratypes: Figure 7.4–7.7) than those from upper horizons (some paratypes: Figure 7.1–7.3). As their ornamentations are similar, we consider them the same species. This new species is extremely similar to and may be ancestral to Acanthocythereis munechikai Ishizaki, 1981 from recent sea-surface deposits in the East China Sea and the Sea of Japan (e.g. Ishizaki, 1981; Tanaka, 2009). Acanthocythereis kunihiroi sp. nov. differs from A. munechikai in having a short, robust ventral ridge. This new species differs from Acanthocythereis volubilis (Liu, 1989), which has been frequently reported from the Paleogene in Japan (e.g. Yamaguchi, 2004, 2006; Yamaguchi et al., 2006), in the posteroventral valve shape. The new species differs from Acanthocythereis abei Iwatani and Irizuki in Iwatani et al., 2011 in its smaller valve size and different ornamentation. It also has a different valve outline to Acanthocythereis takanabensis Iwatani and Irizuki in Iwatani et al., 2011. Acanthocythereis inouei Yamaguchi in Yamaguchi et al., 2012 differs from Acanthocythereis kunihiroi in having blunt ornamentation in the anterior areas. The species of the genus Abrocythereis Gou in Gou et al., 1983, are similar to Acanthocythereis kunihiroi in terms of surface ornamentation, but they are different from the latter in having a smooth anterior marginal rim.
Acanthocythereis noriyukikeyai Tanaka in Tanaka and Hasegawa, 2013a
Acanthocythereis? sp. Takakuwa and Tsukagoshi, 2005, p. 89, 90, fig. 3.C–3.H.
Acanthocythereis quadrata Irizuki and Yamada. Tanaka et al., 2012, p. 35, pl. 1, fig. 5; Ozawa, 2016, p. 131, fig. 7.2, 7.3; Goto and Irizuki, 2019, p. 9, fig. 5.2, 5.3.
Acanthocythereis cf. quadrata Tanaka et al., 2013, p. 21, fig. 5.
Acanthocythereis noriyukikeyai Tanaka in Tanaka and Hasegawa, 2013a, p. 6, 7, pl. 5, figs. 1–15.
Acanthocythereis sp. 2. Goto et al., 2013, p. 329, fig. 9.2.
Materials.—563 specimens.
Occurrence.—Abundant to few in all samples of Biofacies I, common to few in all samples of Biofacies II and V, common to rare in most samples of Biofacies III.
Remarks.—Acanthocythereis noriyukikeyai is known from the Middle Miocene of central Japan (Tanaka and Hasegawa, 2013a) and is closely related to Acanthocythereis quadrata from the Lower Miocene of Japan (Irizuki et al., 2004). Acanthocythereis quadrata has a reticulate ornamentation and broadly rounded posterior margin, but A. noriyukikeyai has many conjunctive and disjunctive spines with pores on the valve surface and nearly straight posterior margin. However, the ornamentation patterns on the valve surface are extremely similar, so the two species may have an ancestor-descendant relationship. Acanthocythereis noriyukikeyai also resembles Acanthocythereis fujinaensis Tanaka in Tanaka et al., 2002 but has larger valves. Hirsutocythere? nozokiensis Ishizaki, 1963 differs from Acanthocythereis noriyukikeyai in its larger valves, more elongated morphology, and larger tubercles on the valve surface.
Genus Hirsutocythere Howe, 1951
Hirsutocythere? akatsukiborensis Yajima, 1992
Hirsutocythere? sp. Yajima, 1988, pl. 1, figs. 10, 13.
Hirsutocythere? akatsukiborensis Yajima, 1992, p. 259, 260, pl. 32, figs. 12–14.
Material.—1 specimen.
Occurrence.—Only one specimen in sample 1-15 of Biofacies III.
Remarks.—These specimens have more distinct spines on the valve surface than those of the type specimens. This species differs from Hirsutocythere? nozokiensis Ishizaki, 1963 in having few numbers of spines on the valve surface. Legitimocythere sp. of Goto et al. (2013) is similar to Hirsutocythere? akatsukiborensis but it may be identified as Hirsutocythere? nozokiensis.
Genus Cibotoleberis Guan in Hubei Institute of Geosciences, Department of Geology of Henan Province, Bureau of Geology of Hubei Province, Bureau of Geology of Hunan Province, Bureau of Geology of Guangdong Province and Geological Bureau of Guangxi Zhuang Autonomous Region, 1978
Type species.—Cibotoleberis quadrata Guan in Hubei Institute of Geosciences et al., 1978.
Cibotoleberis tsuyamensis sp. nov.
ZooBank lsid: urn:lsid:zoobank.org:act:D2A67A68-E395-49F2-8FA9-FAC6FD80ED15
Etymology.—Named after the city of Tsuyama, Okayama in southwest Japan, where the study sites are located.
Type series.—Holotype: SMU-IC-F0108, female C (Figure 7.10), L = 0.513 mm, H = 0.300 mm. Paratypes: SMU-IC-F0109, female C (Figure 7.11), L = 0.455 mm, H = 0.270 mm; SMU-IC-F0110, male C (Figure 7.12), L = 0.495 mm, H = 0.266 mm.
Other materials.—99 specimens.
Type locality.—A roadcut outcrop of the Yoshino Formation 250 m west-southwest of Kasamori Shrine, Tsuboi-kami, Tsuyama, Okayama, Japan (site 4-1, 35°03′32″N, 133°50′24″E) .
Diagnosis.—Characterized by reticulation in the posteromedian area and anterior marginal rim and furrow.
Description.—Valve small in size. Subelliptical in lateral view. Maximum height at a half of valve. Anterior margin broadly and evenly rounded. Dorsal margin gently arched. Ventral margin almost straight. Posterior margin truncated or slightly convex with irregular outline, meeting dorsal margin at posterior cardinal angle. Valve surface smooth with several fossa or puncta in anterior and central areas, weak reticulation in posteromedian area, approximately four weak marginal ridges lying from posterodorsal to midventral margin. Marginal broad rim with deep furrow lying from anterodorsal to midventral margin, connecting to marginal ridge in posterior margin. Normal pores scattered on valve surface. Hindgement and muscle scars not observed because of carapace. Sexual dimorphism distinct. Males slenderer than females.
Occurrence.—Abundant in sample 4-1 of Biofacies V, rare in several samples of Biofacies I and II.
Remarks.—The genus Cibotoleberis was established by Guan in Hubei Institute of Geosciences et al. (1978) as a type species of C. quadrata Guan in Hubei Institute of Geosciences et al. (1978), which was obtained from the Pliocene of central southern China. Hou and Gou (2007) argued that Ambtonia Malz, 1982 is a junior synonym of Cibotoleberis; however, Ambtonia does not have an anterior marginal rim and furrow but a smooth margin, suggesting that they are distinct genera.
Cibotoleberis tsuyamensis is similar to Cibotoleberis quadrata in having an anterior marginal rim and narrow furrow and a general shape but is distinguishable from the latter in different ornamentation in the posterior area. Cibotoleberis tsuyamensis is similar to Pacambocythere ucarinata (Ishizaki, 1983) in general shape and has an anterior marginal rim and furrow, but Pacambocythere ucarinata has a distinct hook-shaped ridge.
Genus Falsobuntonia Malz, 1982
Falsobutonia taiwanica Malz, 1982
Falsobuntonia taiwanica Malz, 1982, p. 392, 393, pl. 8, figs. 51–56; Huh and Paik, 1992a, pl. 2, fig. 15; Huh and Paik, 1992b, p. 111, pl. 2, fig. 15; Ikeya and Suzuki, 1992, p. 127, pl. 4, fig. 9; Irizuki and Matsubara, 1994, pl. 1, fig. 16; Irizuki et al., 2001, p. 53, fig. 7.16; Tanaka et al., 2002, p. 18, fig. 9.5; Iwatani et al., 2011, p. 280, fig. 5.19; Maehama et al., 2021, p. 371, fig. 6.15.
Materials.—197 specimens.
Occurrence.—Abundant to common in all samples of Biofacies II, common to few in all samples of Biofacies III, few to rare in most samples of Biofacies I.
Remarks.—Some specimens have distinct reticulated ornamentation in the posterior area (Figure 7.14).
Genus Pacambocythere Malz, 1982
Type species.—Pacambocythere cytherelloidae Malz, 1982.
Pacambocythere ishizakii sp. nov.
ZooBank lsid: urn:lsid:zoobank.org:act:5F856485-2F47-437E-AAE2-44F7BB126371
Etymology.—In honor of Kunihiro Ishizaki, who is a pioneering Japanese ostracod worker.
Type series.—Holotype: SMU-IC-F0114, female LV (Figure 8.1), L = 0.529 mm, H = 0.341 mm. Paratypes: SMU-IC-F0115, female LV (Figure 8.2), L = 0.518 mm, H = 0.332 mm; SMU-IC-F0116, male C (Figure 8.3), L = 0.525 mm, H = 0.300 mm.
Other materials.—103 specimens.
Type locality.—An outcrop of the Yoshino Formation along Okayama Prefectural Route 341 500 m west-southwest of Tsuyama City Kyosho Elementary School, Tsuboi-kami, Tsuyama, Okayama, Japan (site 2-3, 35°03′39″N, 133°50′54″E) .
Diagnosis.—Characterized by its many puncta in the anterior half and reticulation in the posterior half.
Description.—Valve moderate in size, suboblong with truncated posterior end. Maximum height at somewhat before midlength in lateral view. Anterior margin broadly rounded. Dorsal margin arched. Ventral margin almost straight or slightly concave in midventral area. Posterior margin truncated. Valve surface consisting of distinct reticulation in a posterior half that changing gradually into numerous puncta or second reticulation in an anterior half. Marginal narrow rim, particularly raised along anterior margin with compressed marginal area, starts from midlength of dorsal area, running along anterior and ventral margins, curving obliquely upward in posterior one-sixth of valve length, and terminating at midposterior area. Hook-shaped ridge distinct in posteromedian area, starting below faint subcentral tubercle, running posteriorly, curving in midposterior area, and terminating near median area. Normal pores scattered on entire valve. Marginal infold moderately broad along anterior margin and narrower along ventral and posterior margins. Hindgement holamphidont, but its details not observed because of poor preservation. Muscle scars not observed because of poor preservation. Sexual dimorphism distinct. Males slenderer than females.
Occurrence.—Common to few in all samples of Biofacies III, rare in most samples of Biofacies I and all samples of Biofacies IV.
Remarks.—Pacambocythere ishizakii differs from Pacambocythere aff. similis described by Tanaka and Hasegawa (2013a) and Tanaka et al. (2013) in its more delicate reticulation, ridge, and marginal rim on the valve surface than the latter species. Nevertheless, both may be related to each other. This new species differs from Pacambocythere similis Malz, 1982 from the Pliocene Maanshan Formation of Taiwan in its less developed anterior marginal rim, radial ridges, and reticulation in the anterior area and its different valve morphology in female specimens. Pacambocythere ishizakii differs from Pacambocythere buntoniae Malz, 1982 in lacking the latter’s robust fish hook-shaped ridge from the ventromedian to posteroventral area. Pacambocythere reticulata (Jiang and Wu) in Gou et al., 1981 is also similar to Pacambocythere ishizakii in valve morphology but differs from it in having distinct coarse reticulations on the valve surface.
Family Cytheruridae Müller, 1894
Genus Semicytherura Wagner, 1957
Semicytherura hanaii Ishizaki, 1981
Semicytherura hanaii Ishizaki, 1981, p. 53, 54, pl. 11, figs. 3, 4, 6, 7a, b, pl. 14, fig. 6; Wang and Zhao, 1985, pl. 8, fig. 11; Ruan and Hao, 1988, p. 302, pl. 53, figs. 1–3; Wang et al., 1988, p. 262, pl. 51, figs. 1, 2; Tanaka, 2003, p. 121, fig. 4b.
Materials.—8 specimens.
Occurrence.—Rare in samples 1-9 and 1-11 of biofacies I.
Remarks.—This species is widely distributed in the East China Sea and South China Sea (e.g. Ishizaki, 1981).
Semicytherura pseudoundata Irizuki and Yamada in Irizuki, Yamada, Maruyama and Ito, 2004
Semicytherura pseudoundata Irizuki and Yamada in Irizuki et al., 2004, p. 140, 142, pl. 9, figs. 5–10.
Materials.—9 specimens.
Occurrence.—Rare in a few samples of biofacies I and IV.
Remarks.—This species is characterized by its broadly sigmoidal ventral ridge.
Genus Cytheropteron Sars, 1866
Cytheropteron sp. 1
Materials.—15 specimens.
Occurrence.—Rare in a sample of biofacies I and rare to few in biofacies III and IV.
Remarks.—Characterized by several vertical ridges and furrows in the posterior area. These specimens are similar to Cytheropteron liue Yamaguchi, 2006 reported from the Eocene Iojima Group (Yamaguchi, 2006) and Cytheropteron rectocostum Zhou, 1995 from the modern Pacific off southwest Japan (Zhou, 1995) in having vertical ribs in median area but could not be unambiguously referred to either because they comprise only broken valves or juveniles.
Family Loxoconchidae Sars, 1925
Genus Loxoconcha Sars, 1866
Loxoconcha cf. taiwanensis Zhao in Wang, Zhang, Zhao, Min, Bian, Zheng, Cheng and Chen, 1988
Loxoconcha optima Ishizaki. Huh and Paik, 1992a, p. 283, pl. 2, fig. 17; Huh and Paik, 1992b, p. 111, pl. 2, fig. 17.
Loxoconcha cf. taiwanensis Zhao. Tanaka, 2003, p. 114, fig. 4e; Tanaka et al., 2004, p. 62, pl. 2, figs. 12, 13, 15; Tanaka and Hasegawa, 2013b, p. 153, fig. 4.9.
Loxoconcha cf. pulchra Ishizaki. Goto and Irizuki, 2019, p. 9, fig. 5.4.
Materials.—112 specimens.
Occurrence.—Few in all samples of Biofacies V, few to rare in most samples of Biofacies I and III, rare in a sample of Biofacies II and IV.
Remarks.—Loxoconcha cf. taiwanensis was reported from Lower to Middle Miocene strata in eastern Asia.
Genus Palmoconcha Swain and Gilby, 1974
Palmoconcha irizukii Tanaka in Tanaka, Seto, Mukuda and Nakano, 2002
Palmoconcha sp. Irizuki and Matsubara, 1994, pl. 1, fig. 19.
Palmoconcha irizukii Tanaka in Tanaka et al., 2002, p. 18, figs. 5.10, 9.7–9.9; Tanaka et al., 2025, p. 4, fig. 3.o.
Materials.—210 specimens.
Occurrence.—Common to rare in all samples of Biofacies I, few in a sample of Biofacies II, few to rare in most samples of Biofacies III, rare in a sample of Biofacies IV.
Remarks.—This species is known from muddy facies of Middle Miocene strata.
Genus Nipponocythere Ishizaki, 1971
Nipponocythere? sp.
Materials.—10 specimens.
Occurrence.—Rare in a few samples of Biofacies I and V.
Remarks.—These specimens may be referrable to juveniles of Nipponocythere based on valve outline, but their hingement shape is not known because of poor preservation.
We would like to thank G. Tanaka (Kumamoto University, Japan) and H. Ozawa (Nihon University, Japan) for their constructive review and many useful suggestions. Thanks are extended to T. Haga and A. Yabe (National Museum of Nature and Science) for helpful suggestions and improvement of the main text. We thank enago (https://www.enago.jp) for English language editing.
K. S. initiated the study. T. I. conducted ostracod taxonomy. All authors contributed to the geological aspects of the study and the writing of the paper.
