Eighty-seven species of molluscs were obtained from the uppermost lower-lowest middle Miocene Higashibessho Formation at Shimo-sasahara, Yatsuo Town in Toyama Prefecture, central Japan. Among them, Pagodula shojii is new to science. Judging from the autochthonous species, the Higashibessho Formation was deposited at the lower sublittoral to upper bathyal depth. Both the deep-sea and the derived shallow-water species include many warm-water dwellers. During the latest early-earliest middle Miocene, the deep-sea species migrated from the Pacific side of central Honshu to the Japan Sea through deep-sea pathways.
Introduction
In the Oligocene, the “Japanese Islands” were located at the eastern margin of the Asian Continent. The Japan Sea began to open as a marginal sea in the early Miocene (Iijima and Tada, 1990). No deep-sea molluscan faunas have been recorded from the uppermost lower-lower middle Miocene deposits in the central Japan Sea borderland, other than the upper part of the Kurosedani and the lower part of the Higashibessho Formation in Toyama Prefecture (Tsuda, 1960).
Tsuda (1960) reported a deep-sea assemblage IV represented by Solemya tokunagai,Portlandia (Megayoldia) aff. thraciaeformis, Propeamussium transnipponica, and Dentalium (Fissidentalium) yokoyamai from the upper part of the Kurosedani Formation. Saito (1988) also recorded ill-preserved deep-water molluscs including Propeamussium cf. tateiwai Kanehara from the Nanamagari Formation in Ishikawa Prefecture and correlated them with Tsuda's assemblage IV. However, the exact age of the formation and most of the species were uncertain. When they recorded 19 species including warm-water planktonic species such as Aturia sp. and Clio itoigawai Shibata from the Higashibessho Formation, Shimizu et al.(2000) noted that the formation was deposited in the lower sublittoral to bathyal zones. They also inferred the existence of a cold deep-water mass below a warm-water current. Amano et al. (2000) found a deep-water dweller, Neilo (Multidentata) multidentata (Khomenko), in the Higashibessho Formation. This species is widely distributed in the northwestern Pacific in Oligocene to middle Miocene rocks. From the Higashibessho Formation, Amano et al. (2001) described the oldest vesicomyids in the Japan Sea borderland including Calyptogena sp. and Vesicomya kawadai (Aoki).
Thus, the Higashibessho fauna (Kaseno, 1964) consists of deep-water species, including some characteristics of the chemosynthetic community. The Kurosedani and Higashibessho Formations were deposited during the Mid-Neogene Climatic Optimum (Tsuchi, 1987; Amano et al., 2001). During this warm age, the shallow-water tropical Kurosedani fauna (Itoigawa, 1988) distributed to Yamagata Prefecture while the subtropical shallow-sea Kadonosawa fauna (originally Otuka, 1939, redefined by Itoigawa, 1988) prevailed to the southwestern part of Hokkaido. Therefore, it is reasonable that shallow warm-water species have been described from the Kurosedani and Higashibessho formations.
Chinzei (1978, 1981, 1986) proposed the subsurface cold-current “Oyashio senryu” at the Mid-Neogene Climatic Optimum. Probably taking account of this proposal, Shimizu et al. (2000) considered that the deep water was cold during the time of deposition of the Higashibessho Formation as mentioned above. On the other hand, when they proposed a new Pliocene taxodont species of Acilana, Noda et al. (1989) denied the existence of the “Oyashio senryu” in the Japan Sea borderland at the Mid-Neogene Climatic Optimum. Karasawa et al. (1992, 1995) also suggested that the lower sublittoral to bathyal environment in the Japan Sea borderland was warm, based on the distribution of the crustacean Bathynomus, which occurred also from the Higashibessho Formation. To resolve this confusion, it is necessary to examine the temperature implied by the deep-water fauna during the latest early-earliest middle Miocene in the Japan Sea borderland in detail.
Moreover, Ozawa et al. (1986) pointed out that the muddy bottom associations of the upper part of the Kurosedani Formation are closely related to that of the Oidawara Member of the Mizunami Group in Gifu Prefecture. However, no other comparisons have been made between the Higashibessho and the deep-sea faunas in the central Pacific side of Japan.
Fortunately, we were able to collect many well-preserved specimens from the horizon of the Higashibessho Formation slightly higher than the level from which Amano et al. (2001) described the vesicomyids at Shimo-sasahara. In this paper, we list all these species including one new gastropod and discuss the climatic condition and the biogeographic significance of the fauna.
Geological settings
In and around Yatsuo Town, the Neogene deposits consist of the Nirehara, Iwaine, Iozen, Kurosedani, Higashibessho, Tenguyama, Otokawa and Mita Formations in ascending order (Sakamoto and Nozawa, 1960; Hayakawa and Takemura, 1987; Ogasawara et al., 1989). The Higashibessho Formation (Fujita and Nakagawa, 1948) consists mainly of muddy sediments overlying the Kurosedani Formation (Tsuda and Chiji, 1950). On the other hand, the upper part of the Kurosedani Formation is predominantly siltstone or silt-stone-dominated alternation. Sometimes, it is difficult to separate lithologically the Kurosedani from the Higashibessho Formation. The upper limit of the Kurosedani Formation is usually defined as the upper surface of the Yamadanaka Tuff (Tsuda, 1953).
We collected many molluscan specimens from the Higashibessho Formation at about 250 m west of Shimo-sasahara, Yatsuo Town, Toyama Prefecture (Figure 1). This locality is the same as that of Amano et al. (2001). The large outcrop consists of a 2.3 m – thick alternation (L) of fine-grained sandstone and mudstone in the lower part yielding vesicomyid fossils, and a 21.2 m-thick black mudstone (UM) bearing calcareous concretions in the upper part (Figure 2). Many well-preserved fossils occur in the upper mud-stone of the cliff. In the middle part of the mudstone, a 60 cm-thick sandstone lens (US) including many packed shells can be traced for 20 m. In this area, the Yamadanaka Tuff is exceptionally intercalated in the lower part of the Higashibessho Formation (Sakamoto and Nozawa, 1960). Stratigraphically, the upper mudstone of the fossil locality is situated about 75 m lower than the Yamadanaka Tuff. Therefore, the Higashibessho Formation here is a contemporaneous heterotopic facies of the upper part of the Kurosedani Formation to the west of Yatsuo Town.
Figure 1.
Locality of fossils (from Amano et al., 2001, fig. 1; using the topographical map of “Yatsuo”, scale 1 : 25,000, published by Geographical Survey Institute of Japan).
Figure 2.
Columnar section of the fossil locality. A. Black mudstone, B. Sandstone, C. Alternation of fine-grained sandstone and mudstone, D. Calcareous concretions. L, UM, US show horizons of fossils (see the text).
As already reported by Hasegawa and Takahashi (1992), Yanagisawa (1999) and Amano et al. (2001), the planktonic microfossils from the lowest part of the Higashibessho Formation indicate the diatom NPD 3A and 3B of Akiba (1986), the calcareous nannofossils CN3 of Okada and Bukry (1980) and the planktonic foraminiferal zone N8 of Blow (1969) and the radiolarian Calocyletta costata zone of Riedel and Sanfilippo (1978). Thus, the Higashibessho Formation at Shimo-sasahara can be assigned to the latest early-earliest middle Miocene. However, based on the magnetostratigraphic data, Itoh et al. (1999) assigned the Kurosedani and the lower part of the Higashibessho Formations to the C5Br Chron which is slightly younger than the age based on the microfossils. Although this problem for now remains unsolved, we use the age based on the microfossils in this paper.
Occurrences of molluscs
Molluscan fossils were obtained from three horizons: the lower alternation (L), upper mudstone (UM), and the sandstone lens intercalated in the mudstone (US) (Figure 2; Table 1). In the lower alternation (L), six bivalve species including two vesicomyids sporadically occur as articulated shells. As described by Amano et al. (2001), the commissures of vesicomyids are parallel to the bedding plane despite the shells being articulated.
Table 1.
Molluscan fossils from the Higashibessho Formation at Shimo-sasahara. Number shows total number of specimens. Number in parentheses shows number of articulated specimens. * Warm-water genera or species, ** after Higo et al. (1999).
In the upper mudstone (UM), 51 species of bivalves, gastropods and scaphopods (Figures 3, 4) occurred sporadically, including one new species. Most specimens are well preserved, including many articulated bivalves. In contrast, 35 species of bivalves and gastropods (Figure 5) were obtained from the sandstone lens (US) intercalated in the upper mudstone. All bivalve specimens here are disarticulated.
Figure 3.
Bivalves and scaphopods from the lower alternation (L) and the upper mudstone (UM) of the Higashibessho Formation. 1. Lamellinucula hokoensis (Kanehara), ×2, JUE no. 15746, L. 2. Portlandia (Portlandella) lischkei (Smith), ×1, JUE no. 15747, UM. 3. Neilonella tsukigawaensis Kurihara, ×2.3, JUE no. 15748, UM. 4. Bathymalletia inermis (Yokoyama), ×1.5, JUE no. 15749, UM. 5. Delectopecten sp., ×3, JUE no. 15750, UM. 6. Propeamussium tateiwai Kanehara, ×2, JUE no. 15751, UM. 7. Ennucula osawanoensis (Tsuda), ×1.5, JUE no. 15752, UM. 8. Acila (Acila) submirabilis Makiyama, ×1.2, JUE no. 15753, UM 9. Bathymodiolus? sp., ×1, JUE no. 15754, UM. 10. Solamen fornicatum (Yokoyama), ×1.6, JUE no. 15755, UM. 11. Thyasira tokunagai Kuroda and Habe, ×2, JUE no. 15756, L. 12. Lucinoma acutilienatum (Conrad), ×1, JUE no. 15757, UM. 13. Lyonsiella mitsuganoensis Shibata, ×3.25, JUE no. 15758, UM. 14. Periploma mitsuganoense Araki, ×1, JUE no. 15759, UM. 15. Periploma yokoyamai Makiyama, ×1, JUE no. 15760, UM. 16. Poromya osawanoensis Tsuda, ×2, JUE no. 15761, UM. 17. Cardiomya mitsuganoensis Shibata, ×3.1, JUE no. 15762, UM. 18. Neilo (Multidentata) multidentata (Khomenko), ×1, JUE no. 15763, UM. 19. Acesta goliath (Sowerby), ×0.75, JUE no. 15764, UM. 20. Gloripallium izurense Masuda, ×1, JUE no. 15765, UM. 21. Dentalium sp., ×1.2, JUE no. 15766, UM. 22. Fissidentalium yokoyamai (Makiyama), ×0.75, JUE no. 15767, UM.
Figure 4.
Gastropods from the upper mudstone (UM) of the Higashibessho Formation. 1, 2, 4, 5. Pagodula shojii sp.nov.; 1a, ×1.6, 1b, ×1.5, JUE no. 15745-1, paratype; 2a, ×1.5, 2b, ×1.55, JUE no. 15745-3, paratype; 4a,b, ×1.55, JUE no. 15744, holotype; 5a, ×1.6, 5b, ×1.5, JUE no. 15745-2. 3, 7. Boreotrophon osawanoensis (Tsuda); 3, ×2.1, JUE no. 15768-1; 7, ×1, JUE no. 15768-2. 6. Tomopleura osawanoensis Tsuda, ×1, JUE no. 15769. 8. Cochlespira osawanoensis (Tsuda), ×1.3, JUE no. 15770. 9. Megasurcula yokoyamai (Otuka), ×1, JUE no. 15771. 10. Comitas sp., ×2, JUE no. 15772. 11, 13. Neadmete nakayamai Habe; 11, ×2, JUE no. 15773-1; 13, ×2, JUE no. 15773-2. 12. Eoscaphander corpulenta (Yokoyama), ×1.5, JUE no. 15774. 14.Clio sp., ×1.7, JUE no. 15775. 15, 16. Bowdenatheca sp.; 15, ×1.3, JUE no. 15776-1; 16, ×1.3, JUE no. 15776-2. 17. Sinum ineptum (Yokoyama), ×1, JUE no. 15777. 18. Ginebis osawanoensis (Tsuda), ×2, JUE no. 15778. 19. Calliostoma (Calotropis) simane Nomura and Hatai, ×2, JUE no. 15779. 20. Babylonia kokozurana Nomura, ×1, JUE no. 15780. 21. Cryptonatica ichishiana (Shibata), ×1.5, JUE no. 15781. 22. Echinophoria etchuensis (Hatai and Nisiyama), ×2, JUE no. 15782. 23. Liracassis japonica (Yokoyama), ×1, JUE no. 15783.
Figure 5.
Molluscan fossils from the sandstone lens (US) of the Higashibessho Formation. 1. Barbatia (Savignyarca) osawanoensis Tsuda, ×2, JUE no. 15784. 2. Veremolpa minoensis Itoigawa, ×2.4, JUE no. 15785. 3. Notomyrtea sp., ×3, JUE no. 15786. 4. Saccella kongiensis (Otuka), ×2.1, JUE no. 15787. 5. Polinices mizunamiensis Itoigawa, ×3, JUE no. 15788. 6. Anadara (Anadara) watanabei (Kanehara), ×1.2, JUE no. 15789. 7. Anadara (Scapharca) makiyamai Hatai and Nisiyama, ×1, JUE no. 15790. 8. Nipponocrassatella osawanoensis (Tsuda), ×1.5, JUE no. 15791. 9, 10. Gyrineum osawanoense (Tsuda); 9, ×3, JUE no. 15792-1; 10, ×3, JUE no. 15792-2. 11. Conus (Asprella) toyamaensis Tsuda, ×1.5, JUE no. 15793. 12. Siphonalia osawanoensis Tsuda, ×1.65, JUE no. 15794. 13. Inquisitor kurodai (Tsuda), ×1.5, JUE no. 15795. 14. Vicaryella atukoae (Otuka), ×1, JUE no. 15796. 15. Gemmula osawanoensis Tsuda, ×1, JUE no. 15797. 16. Mitrella (Indomitrella) mizunamiensis Itoigawa, ×3, JUE no. 15798. 17. Subula osawanoensis Tsuda, ×1, JUE no. 15799.
Judging from these occurrences, the assemblages from the lower alternation and upper mudstone are autochthonous while that of the upper sandstone lens is allochthonous.
Systematic description of a new species
Family Muricidae Rafinesque, 1815
Subfamily Trophoninae Cossmann, 1903
Genus Pagodula Monterosato, 1884
Type species.—Murex vaginata Cristofori and Jan, 1832. This fossil species was subsequently designated as the type species by Radwin and D'Attilio (1976).
Remarks.—According to Houart (2001), this genus is characterized by a fusiform shell with shouldered whorls, distinct spines, no or few spiral cords, and long, open siphonal canal. He distinguished Boreotrophon Fischer, 1884 from this genus based on the former's lacking spiral cords and having adapically pointed axial ribs. We have chosen to assign our new species to Pagodula on the basis of shell characters outlined by Houart (2001).
Most species of Pagodula live in the lower sublittoral to bathyal depths of the Mediterranean Sea and the northeastern Atlantic Ocean (Bouchet and Warén, 1985; Houart, 2001). Recently, P. kosunorum was found in 200–250 m depth, off Northeast Taiwan (Houart and Lan, 2003). Geologically, the oldest species of Pagodula has been recorded from the middle Miocene in Europe (La Perna, 1996).
Pagodula shojii sp. nov.
Trophon (Boreotrophon) osawanoensis Tsuda, 1959, pl. 4, fig. 12 (non pl. 4, figs. 11, 13).
Type specimen.—Holotype, JUE (abbreviation of Joetsu University of Education) no. 15744, 27.6 mm high, 11.0 mm wide; paratype, JUE no. 15745-1, 22.0 mm high, 10.7 mm wide; paratype, JUE no. 15745-2, 23.5 mm high, 9.8 mm wide; paratype, JUE no. 15745-3, 26.0 mm high, 12.2 mm wide; paratype, JUE no. 15745-4, 7.6 mm high, 4.8 mm wide.
Type locality.—Shimo-sasahara, Yatsuo Town, Toyama Prefecture (36° 33′54″N, 137°8′45″E).
Diagnosis.—Medium-sized Pagodula characterized by rather weak spines, long siphonal canal, few spiral cords (two on last whorl) and numerous axial ribs (11-14 on last whorl).
Description.—Shell medium-sized for genus, attaining 27.6 mm in height, narrow and fusiform; proto-conch partly preserved, of more than one smooth angulate whorl; teleoconch of six whorls. Spire rather high, occupying about two-fifths of shell height. Suture rather deep. Axial ribs located at shoulder, 12–15 on penultimate whorl and 12–15 on last whorl, ending in rather weak spine. One spiral cord at shoulder of early whorls; one at shoulder and another distinct cord below shoulder of last whorl. Aperture ovate; inner lip covered by thin callus; outer lip thin and smooth on inner side. Siphonal canal open, very long and slightly curved.
Remarks.—Pagodula shojii closely resembles the Recent species, P. kosunorum Houart and Lan, 2003 from Taiwan in its size (height = 23:47 mm), number of whorls (six and a half) and number of spiral cords (two on the last whorl). However, P. shojii has more numerous (10 on the last whorl in P. kosunorum) as well as weaker spines than in P. kosunorum. P. shojii also resembles the Recent species P. cossmanni (Locard, 1897) in shell outline, number of whorls (six), weak spines on the shoulder and number of axial ribs on the last whorl. P. shojii can be distinguished from P. cossmanni in having less numerous spiral cords on the last whorl than in P. cossmanni (three-five).
It is possible that this new species might be confused with Boreotrophon osawanoensis (Tsuda, 1959) (Figure 4.3, 4.7). B. osawanoensis can be distinguished easily from the new species by having no spiral cord, a more flattened area above the shoulder, and more prominent, lamellated axial ribs whose ends are pointed adapically. From this point of view, one of the paratypes of B. osawanoensis (Tsuda, 1959, pl. 4, fig. 12) can be identified with the new species because of its two spiral cords and 12 axial ribs on the last whorl.
Distribution.—Known from the type locality and Kashio in the Higashibessho Formation.
Etymology.—This new species is named after Dr. Shoji Fujii, who has contributed to the Neogene stratigraphy and paleontology of Toyama Prefecture.
Paleoenvironmental significance
Judging from the autochthonous molluscan fossils from both the lower alternation and the upper mudstone, the Higashibessho Formation at Shimosasahara was deposited at lower sublittoral to upper bathyal depths (Table 1). This is supported by estimations using benthic foraminifers from the upper part of the Kurosedani Formation (upper to middle bathyal depths; Hasegawa and Takahashi, 1992) and crustacean Bathynomus from the Higashibessho Formation (lower sublittoral to bathyal depths; Karasawa et al., 1992). In contrast, the upper sandstone lens includes an allochthonous assemblage consisting of shallow marine molluscs (Table 1).
The lower alternation yields two warm-water genera, Lamellinucula and Tindaria. Many warm-water genera can be found in the upper mudstone assemblage: Tindaria, Neilonella, Propeamussium, Lyonsiella,Sinum, Echinophoria,Comitas, Bowdenatheca andClio. This is the oldest record of Neadmete nakayamai Habe, now living in Tosa Bay and the southern Japan Sea (Higo et al., 1999). It is noteworthy that no cold-water genera can be recognized in the assemblage from the upper mudstone. As discussed by Amano et al. (2001),Neilo (Multidentata)multidentata from the northwestern Pacific is a nonboreal species. From the upper sandstone lens, 14 warm-water genera and subgenera can be recognized among 32 genera: Bellucina, Notomyrtea, Nipponocrassatella, Vicaryella, Calyptraea, Polinices, Gyrineum, Chicoreus, Mitrella (Indomitrella),Conus, Gemmula, Bathytoma, Strioterebrum andSubula.
From this composition, we infer that the deep water in the latest early-earliest middle Miocene was not so cold as in the Recent Japan Sea and a warm-water current affected the sea surface. Such climatic conditions deny the existence of the cold undercurrent “Oyashio senryu” here and may support the claim of Ogasawara (1994) who denied the existence of a cold current such as the Recent Oyashio during the Miocene.
Biogeographical significance
Similar deep-sea molluscan fossils to the lower alternation and upper mudstone have been found from the upper lower to lower middle Miocene deposits in the Setouchi, Sanin-Hokuriku, and Saikai Provinces of Southwest Japan and in the Pohang Basin of South Korea (Figure 6).
Figure 6.
Sites where late early to early middle Miocene deep-water faunas have been collected. Paleogeographic map based on Ogasawara and Nagasawa (1992). 1. Higashibessho Formation (present study). 2. Nanamagari Formation (Saito, 1988). 3. Uchiura Group (Nakagawa and Takemura, 1985). 4. Tottori Group (Akagi et al., 1992). 5. Katsuta Group (Taguchi, 2002). 6. Bihoku Group (Itoigawa and Nishikawa, 1976; Okamoto et al., 1989, 1990; Okamoto, 1992). 7. Susa Group (Okamoto et al., 1983). 8. Hagjeon, Heunghae, Duho formations in Pohang Basin, Japan Sea side of South Korea (Kanehara, 1936; Yoon, 1979; Lee, 1992). 9. Ichishi Group (Shibata, 1970). 10. Morozaki Group (Shikama and Kase, 1976; Yamaoka, 1993). 11. Oidawara Formation (Itoigawa et al., 1981).
In the Sanin-Hokuriku Province, Japan Sea side of central to western part of Honshu, the deep-water fauna is known from the Nanamagari Formation in Ishikawa Prefecture (Saito, 1988), the Kounoura Shale Member of Uchiura Group in Fukui Prefecture (Nakagawa and Takemura, 1985) and the Fuganji Mudstone Member of Tottori Group in Tottori Prefecture (Akagi et al., 1992). Among them, AcilaSaccella and Limatula-Propeamussium associations of the Kounoura Member include many common species and genera (five species and nine genera) with the Higashibessho fauna (Table 2).
Table 2.
Comparison between the deep-sea assemblages of the Higashibessho Formation with those of other upper lower to lower middle Miocene formations. A closed circle in the list indicates the same species while an open one means same genus. * Uc = Uchiura Group, Oi = Oidawara Formation, Ic = Ichishi Group, Mo = Morozaki Group, Bh = Bihoku Group, Kt = Katsuta Group, SK = Formations in Pohang Basin, South Korea.
In the eastern part of Setouchi Province, Pacific side of central Honshu, such a deep-water fauna has been recorded from the Oidawara Formation of the Mizunami Group in Gifu Prefecture (Itoigawa et al., 1981), the Morozaki Group in Aichi Prefecture (Shikama and Kase, 1976; Yamaoka, 1993) and the Ichishi Group in Mie Prefecture (Shibata, 1970). The fauna from the Oidawara Formation contains 10 species and 10 genera in common with the Higashibessho fauna (Table 2). The Morozaki Group yields 10 species and nine genera in common with the Higashibessho fauna. Nine species and seven genera of the Ichishi Group are in common.
In the western part of Setouchi Province, inland of western Honshu, the upper part of the Bihoku Group in Hiroshima Prefecture (Itoigawa and Nishikawa, 1976; Okamoto et al., 1989, 1990; Okamoto, 1992) and the Takakura Formation of the Katsuta Group in Okayama Prefecture (Taguchi, 2002) yield a similar fauna to the Higashibessho. Seven species and 13 genera of the upper part of the Bihoku Group are common to it and the Higashibessho fauna, while the Takakura Formation shares six species and seven genera with it.
From the Maeji Sandstone Member of the Susa Group (Okamoto et al., 1983) in Saikai Province, only two species and three genera are common with the Higashibessho fauna herein treated. There are four species in common between the Higashibessho and the middle Miocene faunas from the Hagjeon, Heunghae, and Duho Formations in Pohang Basin, Japan Sea side of South Korea (Kanehara, 1936; Yoon, 1979; Lee, 1992): Lamellinucula hokoensis,Acila (Acila)submirabilis, Propeamussium tateiwai and Lucinoma acutilineatum (Table 2). L. hokoensis has been recorded only from the type locality in Korea and the Higashibessho Formation and is an endemic species of the Japan Sea side.
The lower part of the Higashibessho Formation is a contemporaneous heterotopic facies of the upper part of the Kurosedani Formation. Therefore, it is reasonable that derived shallow-water species in the upper sandstone lens share many species with the shallow-water assemblage of the Kurosedani Formation (Table 3). Of the 25 species in the coarse deposits inter-calated in the Yamadanaka Tuff at Tsuzara (Kaneko and Goto, 1992), to the east of Shimo-sasahara, 16 species are shared with the Higashibessho Formation. In the shallow-water assemblages I–III of the Kurosedani Formation (Tsuda, 1960), nine species are shared with the Higashibessho Formation. It is noteworthy that the molluscs from the Shukunohora Facies of the Mizunami Group in Gifu Prefecture, on the central Pacific side of Honshu (Itoigawa et al., 1981) contain 11 species and seven genera in common with the Higashibessho Formation.
Table 3.
Comparison between the shallow-water species of the Higashibessho Formation with those of other upper lower to lower middle Miocene formations. A closed circle in the list indicates the same species while an open one means same genus. *Ky = Kurosedani Formation at Iguridani, Ku = Kurosedani assemblages I–III, Sh = Shukunohora Formation.
Up to this time, few detailed comparisons have been carried out especially on the relationship between the Higashibessho and the upper lower to lower middle Miocene deep-water faunas in the Setouchi Province because of the scarcity of fossil records from the Higashibessho Formation. In consequence of our huge collection, it has become clear that both deep- and shallow-water assemblages of the Higashibessho Formation resemble the fauna in Setuchi Province. The deep-sea species have in common direct development or a short pelagic larval stage, such as the protobranchs, Propeamussium, Lucinoma, Thyasira, Calyptogena, Lyonsiella, Cardiomya, buccinids and so on (Kasyanov et al., 1990; Bouchet and Warén, 1994; Van Dover, 2000). This leads us to infer that the Japan Sea was connected with the Pacific by deep-sea pathways at least in the latest early-earliest middle Miocene. Moreover, diatom study has revealed that the age of the Morozaki Group on the Pacific side can be assigned to NPD2B of Akiba (1986) (Gladenkov, 1998; Ito et al., 1999). Thus, the deep-water species migrated from the Pacific side into the Japan Sea borderland just after the formation of a deep-sea basin in the latest early-earliest middle Miocene.
Acknowledgments
We are grateful to Geerat J. Vermeij (University of California at Davis) for his critical reading of the manuscript. We thank Roland Houart (Institut Royal des Sciences naturelles de Belgique) for his information and his help in examining the genus Pagodula. We also thank Hiroshi Saito (National Science Museum, Tokyo) and Masanori Shimamoto (Tohoku University) for their help in examining some Recent and fossil specimens. We are grateful to Takashi Matsubara (Museum of Nature and Human Activities, Hyogo) and Alan G. Beu (Institute of Geological and Nuclear Sciences) for reviewing our manuscript and providing many useful comments.
