Many molluscan fossils have been found from the turbidite deposits of the lower Miocene Kurosedani Formation at Kakuma in Toyama City. They include six chemosymbiotic bivalves, namely Solemyidae gen. et sp. indet., Nucinella sp., Luinoma cf. acutilineatum (Conrad), Conchocele yatsuoensis sp. nov., Pliocardia kawadai (Aoki) and Adulomya chitanii Kanehara. This is the oldest record of chemosymbiotic species in the Japan Sea region. Nucinella has been unknown from the Cenozoic deposits in the Japan Sea borderland and from the Recent Japan Sea. Moreover, the vesicomyids P. kawadai and A. chitanii are shared with those from the lower Miocene Honya Formation in Fukushima Prefecture and from the middle Miocene Nupinai Formation in eastern Hokkaido. This supports our hypothesis that the invasion of chemosymbiotic species from the Pacific side to the Japan Sea took place soon after the formation of a deep-sea basin in the Japan Sea.
Introduction
Chemosynthetic communities inhabit deep-sea hydrothermal vents, hydrocarbon seeps and organic falls by relying on chemoautotrophic bacteria for nutrition (Van Dover, 2000). These communities have been recorded from the Cretaceous to Recent in Japan (Majima et al., 2005). Among them, four whale-fall communities and two wood-fall communities have been also recognized (Amano and Little, 2005; Amano et al., 2007b; Kiel et al., 2008; Jenkins et al., 2018; Amano et al., 2018). Other than these communities, most faunas are hydrocarbon seep faunas in Japan. Moreover, no Cretaceous and Paleogene seep communities have been recorded on the Japan Sea side in contrast with several Cretaceous and Paleogene records in Hokkaido and on the Pacific side (Majima et al., 2005; Amano and Jenkins, 2011; Amano et al., 2013).
As the Japan Sea was formed as a marginal sea at least by ca. 18 Ma (Ninomiya et al., 2014), only two hydrocarbon seep communities are recorded from the lower Miocene Taishu Group in Tsushima Island, westernmost part of the sea (Ninomiya, 2011, 2012) and from the uppermost lower to lowermost middle Miocene Higashibessho Formation in Toyama, central Honshu (Amano et al., 2001, 2004). Four chemosymbiotic species occurred from the Higashibessho Formation at Shimo-sasahara; the lucinid Lucinoma acutilineatum (Conrad), the thyasirid Thyasira tokunagai Kuroda and Habe, the vesicomyids Pliocardia kawadai (Aoki) and Adulomya hamuroi Amano and Kiel (Amano et al., 2001, 2004; Amano and Kiel, 2011).
We newly found molluscan fossils including chemosymbiotic bivalves from the lower Miocene Kurosedanai Formation 600 m southeast of Kakuma nearby Shimo-sasahara in Toyama City. This fauna includes six chemosymbiotic species such as a large Nucinella and a new species of Conchocele and corresponds to the oldest record of chemosymbiotic bivalves from the main part of the Japan Sea region. Hence, we shall describe the fauna and discuss its biogeographic significance.
Material and methods
Many molluscan fossils occurred from a large outcrop located 600 m southeast of Kakuma in Toyama City, Toyama Prefecture (Figure 1). Turbidite deposits are exposed at this locality and consist of alternating medium-grained sandstones (about 20 cm in thickness) and dark gray mudstones (about 5 cm in thickness). Both rocks yield many small plant fragments. This horizon is stratigraphically about 75 m below the locality of the Higashibessho Formation at Shimo-sasahara and is included in the Monmyoji Alternation Member which is the uppermost part of the lower Miocene Kurosedani Formation (Sakamoto and Nozawa, 1960; Yanagisawa, 1999).
Figure 1.
Locality of the fossils (using the topographical map of “Yatsuo”, scale 1:25,000, published by the Geospatial Information Authority of Japan).
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Twenty-two species of bivalves, six species of gastropods and one species of scaphopod have been identified from both mudstone and sandstone at this locality (Table 1). Among them, six chemosymbiotic bivalves are included, namely the nucinellid Nucinella sp., Solemyidae gen. et sp. indet., the lucinid Lucinoma cf. acutilineatum (Conrad), the thyasirid Conchocele yatsuoensis sp. nov., and the vesicomyids Pliocardia kawadai (Aoki) and Adulomya chitanii Kanehara. Other than one specimen of Lucinoma found in a small calcareous concretion, other specimens of chemosynthetic species were collected from sandstone or mudstone.
Most species from the mudstone are deep-water dwellers such as Lamellinucula hokoensis (Kanehara), Acila (Acila) submirabilis Makiyama, Bathymalletia inermis (Yokoyama), Portlandia (Portlandella) lischkei (Smith), Delectopecten sp., Periploma yokoyamai Makiyama, Poromya osawanoensis Tsuda and Ginebis osawanoensis (Tsuda). On the other hand, some shallow-sea species such as Anadara sp., Mactra sp., Donacilla sp., Cardilia toyamaensis Tsuda, Veremolpa minoensis Itoigawa and Protorotella depressa Makiyama co-occurred with deep-water shells such as Bathymalletia inermis (Yokoyama), Propeamussium sp. and some chemosymbiotic species in the sandstone (Table 1). These shallow-water species can be interpreted as having been transported from shallow water to the deep sea by turbidity currents. On the other hand, although Adulomya chitanii occurred as aggregations of articulated valves from the sandstone (Figure 2D), this species might have lived near the deep-sea depositional site because of its deep-sea paleoecology and occurrence. Pliocardia kawadai occurred from both mudstone and sandstone. In both cases, this species includes some articulated valves. Several articulated specimens of this species were locally crowded together even in the sandstone (Figure 2B). From this occurrence, this species also might have lived in the deep-sea depositional site.
Table 1.
List of molluscan fossils from the Kurosedani Formation at the fossil locality near Kakuma. Numbers mean number of individuals (number in parenthesis = articulated specimens). *, chemosymbiotic bivalve; °, shallow-water species; M, mudstone; S, sandstone.
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Figure 2.
Occurrence of the chemosymbiotic bivalves. A, outcrop of the fossil locality; B, occurrence of Pliocardia kawadai (Aoki) showing local aggregation in sandstone; C, D, occurrence of Adulomya chitanii Kanehara from sandstone; C, occurrence in the deposits; D, cross section of aggregation of Adulomya chitanii from sandstone.
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We measured the following dimensional characters of the fossils: shell length, height, and anterior length. Moreover, for Conchocele species, we also measured the apical angle. All illustrated and measured specimens are stored at the National Museum of Nature and Science, Tsukuba (NMNS). Other institutional abbreviations are as follows: TKD, Department of Geology and Mineralogy, Faculty of Science, Tokyo Kyoiku Daigaku, Tokyo (now University of Tsukuba); IGPS, Institute of Geology and Paleontology, Faculty of Science, Tohoku University, Sendai.
Systematic description
Family Nucinellidae Vokes, 1956
Genus
Nucinella
Wood, 1851
Type species.—Pleurodon ovalis Wood, 1840; Recent North Atlantic.
Nucinella
sp.
Figure 3A, B, D, E
Material examined.—One left valve, NMNS PM 27995 was examined.
Dimension.—Length, 13.6 mm+; height, 13.2 mm.
Description.—Shell large for genus, thin-walled, moderately inflated, nuculiform, inequilateral. Umbo produced above hinge line; beak opisthogyrate and located at anterior two-thirds of shell length. Anterodorsal margin short, nearly straight and anterior margin broadly arcuated; ventral margin well rounded; posterior margin nearly straight. Hinge plate rather wide and narrow triangular flat area existing below beak; anterior four cardinal teeth straight, long and slightly oblique anteriorly; two middle teeth short, nearly perpendicular to base of hinge plate; posterior four teeth long and oblique posteriorly; anterior lateral tooth long. Surface smooth except for regular growth lines and very weak fine radial striations. Inner surface of ventral margin smooth. Anterior muscle scar and pallial line unknown.
Remarks.—The present species is closely similar to the Recent species Nucinella maxima (Thiel and Jackel, 1931) from the Zanzibar Channel, off Tanzania in having a large nuculiform shell (12.5 mm in height) with a produced umbo above the hinge line. However, the Kurosedani species differs from N. maxima by its numerous cardinal teeth (seven in N. maxima) and smaller middle cardinal teeth.
Nucinella viridis Matsukuma, Okutani and Tsuchi, 1982 living off Nojimazaki, Boso Peninsula, central Honshu also has a similar shape and size (up to 10.4 mm) to the Kurosedani species. However, Nucinella viridis can be easily distinguished from the Kurosedani species in having less numerous teeth (three anterior and two posterior teeth).
Nucinella gigantea Amano, Jenkins and Hikida, 2007 from the Campanian Omagari Formation can be separated from the Kurosedani species in having a larger shell (up to 18.4 mm in length) and fewer cardinal teeth (six in N. gigantea).
Some species of Nucinella over 10 mm have been recorded from the seep carbonates from uppermost Jurassic to lowermost Cretaceous deposits on Spitsbergen Island (Hryniewicz et al., 2014), mid-Cretaceous deposits at Awanui II on the North Island of New Zealand (Kiel et al., 2013), Campanian Omagari Formation in Hokkaido (Amano et al., 2007a), and the upper Eocene to lower Oligocene Tanamigawa Formation in Honshu (Amano et al., 2013). The Kurosedani species is the first record of the Miocene Nucinella found with some chemosymbiotic bivalves.
Stratigraphic and geographic distribution.—Early Miocene: Kurosedani Formation in Toyama Prefecture.
Figure 3.
Chemosymbiotic bivalves except for the vesicomyids from the Kurosedani Formation at Kakuma. A, B, D, E, Nucinella sp., NMNS PM 27995; A, dorsal view; B, hinge area of left valve; D, frontal view of left valve; E, silicone rubber cast of inner mold after extracting shells; C, Solemyidae gen. et sp. indet., NMNS PM 27996, outer cast of right valve (silicone rubber); F, G, J–P, Conchocele yatsuoensis sp. nov.; F, N–P, holotype, NMNS PM 27999; F, dorsal view (silicone rubber); N, right valve; O, dorsal view; P, left valve; G, J, M, paratype, NMNS PM 28000; G, silicone rubber cast of inner mold, showing edentulous hinge; J, dorsal view of inner mold; M, inner mold of left valve; K, paratype, NMNS PM 28001, right valve; L, paratype, NMNS PM 28002, right valve showing many fine radial riblets; H, I, Lucinoma cf. acutilineatum (Conrad); H, NMNS PM 27998, left compressed valve; I, NMNS PM 27997, left valve. Abbreviations: aams, anterior adductor muscle scar; pams, posterior adductor muscle scar. All scale bars show 5 mm.
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Family Thyasiridae Dall, 1900
Genus
Conchocele
Gabb, 1866
Type species.—Thyasira bisecta Conrad, 1849; Miocene, Astoria Formation, Oregon, USA.
Conchocele yatsuoensis
sp. nov.
Figure 3F, G, J–P
Etymology.—The present new species is named for the old town name around the type locality.
Type specimens.—Holotype, NMNS PM 27999; para-types, NMNS PM 28000–28002.
Type locality.—Outcrop located 600 m southeast of Kakuma in Toyama City, Toyama Prefecture; Kurosedani Formation.
Material.—All of the nine specimens treated here were collected from the type locality.
Dimensions.—See Table 2.
Diagnosis.—Small-sized and thin-shelled Conchocele with deep posterior sulcus, deeply excavated lunule demarcated by distinct groove, weakly developed medial flat area, posterior adductor scar confined to inside of posterior fold, and edentulous hinge.
Description.—Shell small for genus, up to 39.8 mm, thin, subtriangular in shape (height/length = 0.75–1.19), equivalve and inequilateral; apical angle ranging from 91° in small specimen to 111° in adult. Anterodorsal margin slightly concave, making right angle with rounded ventral margin; posterodorsal margin long and nearly straight. Umbo situated at anterior one-fourth to two-fifths of shell length. Posterior sulcus wide and deep; posterior fold wide and medially swelled. Lunule very deep, long and broad. Medial flat area from beak to ventral margin distinct in larger specimens. Hinge plate very narrow and edentulous. Anterior adductor muscle scar moderately elongate, slightly detaching with pallial line; posterior adductor scar ovate, confined to inner part of posterior sulcus and fold, not extending to main disc. Shell surface sculptured by irregular commarginal undulations; weathered surface sometimes sculptured by many very weak radial striations.
Remarks.—Conchocele yatsuoensis sp. nov. is most closely similar to C. excavata (Dall, 1901) living in the deep sea (800–2520 m) from off Juan de Fuca to Baja California (Coan et al., 2000) and shallow sea (9–120 m) from Guaymas to Costa Rica (Coan and Valentich-Scott, 2012) in its thin and small shell, medial flat area, excavated lunule and medially swelled posterior sulcus. However, the new species can be distinguished by having a lower shell and a less swelled posterior fold.
Conchocele compacta minor Omori, 1954 described from the middle Miocene Kobana Formation in Tochigi Prefecture has also a rather small shell (up to 20.8 mm in length) and an excavated lunule. However, C. compacta minor has 27 to 30 distinct radial ribs on the surface which are not observed in the new species. When Kamada (1962) described C. compacta minor from the lower Miocene Kokozura Formation in northern Ibaraki Prefecture, he pointed out the similarity between this subspecies and Thyasira subexcavata Yabe and Nomura, 1925 probably from the same formation. As a matter of fact, the latter species shares small size, medial flat area and excavated lunule with Conchocele yatsuoensis sp. nov. and C. compacta minor. However, based on our observations, the holotype of T. subexcavata (IGPS 6973) has a short auricle above the posterior fold which is a characteristic feature of the genus Thyasira Lamarck, 1818.
Conchocele yatsuoensis sp. nov. can be easily distinguished from C. bisecta (Conrad, 1849), the type species of Conchocele, by having a smaller and subtriangular shell. The new species is also different from Conchocele taylori Hickman, 2015 reexamined and recorded from Paleogene deposits on both sides of the northern Pacific by Hryniewicz et al. (2017) by having a subtriangular shell despite sharing small shell size.
Stratigraphic and geographic distribution.—Early Miocene: Kurosedani Formation in Toyama Prefecture.
Table 2.
Measurements of Conchocele yatsuoensis sp. nov. H/L = height/length; AL/L = anterior length/length.
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Family Vesicomyidae Dall and Simpson, 1901
Subfamily Pliocardiinae Woodring, 1925
Genus
Pliocardia
Woodring, 1925
Type species.—Anomalocardia bowdeniana Dall, 1903; Pliocene, Bowden Formation in Jamaica.
Pliocardia kawadai
(Aoki, 1954)
Figure 4C, E, J–O
Lamelliconcha kawadai Aoki, 1954, p. 36, pl. 2, figs. 1, 10, 12–15, 22.
Vesicomya kawadai (Aoki). Kamada, 1962, p. 88, pl.8, fig. 2a, b; Amano et al., 2001, p. 192, figs. 3–5, 8–11; Amano et al., 2007b, fig. 3D, E, G, J.
Pliocardia kawadai (Aoki). Amano and Kiel, 2012, p. 80, figs. 2–4, 6, 7, 9–12.
Type specimen.—Holotype, TKD 5909.
Type Locality.—Donosaku, Kamikatayose (= Taira-Kamikatayose-Donosaku), Iwaki City, Fukushima Prefecture; lower Miocene Kabeya (= Honya) Formation.
Material examined.—Eight specimens were examined. All dimensions are shown in Table 3.
Remarks.—The hinge structure of Kurosedani specimens is consistent with the type specimens of this species. These specimens are characterized by their medium-sized (up to 43.1 mm), veneriform shell with shallow lunular incision. Although Amano and Kiel (2012) described the pallial sinus as shallow and V-shaped, the depth of the sinus varies from specimen to specimen. The sinus of the Kurosedani specimen is very shallow just before the posterior muscle scar (Figure 4N).
Pliocardia? tanakai Miyajima, Nobuhara and Koike, 2017 from the middle Miocene Bessho Formation in Nagano Prefecture is distinguished from P. kawadai by having a lower shell and by lacking a pallial sinus.
Stratigraphic and geographic distribution.—Early Miocene: Honya Formation in Fukushima Prefecture and Kurosedani Formation in Toyama Prefecture. Latest early Miocene to earliest middle Miocene: Higashibessho Formation in Toyama Prefecture. Middle Miocene: Nupinai Formation in Hokkaido.
Genus Adulomya Kuroda, 1931
Type species.—Adulomya uchimuraensis Kuroda, 1931; Middle Miocene Bessho Formation, Nagano Prefecture, central Honshu.
Figure 4.
Vesicomyid bivalves from the Kurosedani Formation at Kakuma. A, B, D, F–I, Adulomya chitanii Kanehara; A, B, NMNS PM 28019, hinge of left (A) and right (B) valve; D, NMNS PM 28016, right valve; F, G, NMNS PM 28017; F, right valve showing a V-shaped pallial sinus (white arrow); H, NMNS PM 28020, right valve; I, NMNS PM 28018, hinge area of both valves (silicone rubber); C, E, J–O, Pliocardia kawadai (Aoki); C, NMNS PM 28013, hinge plate of left valve; E, NMNS PM 28015, hinge plate of right valve (silicone rubber); J, NMNS PM 28011, right valve; K, M, NMNS PM 28014, hinge plate of left valve (K) and dorsal view (M) showing demarcated lunule by the groove; L, NMNS PM 28012, right valve; N, NMNS PM 28008, inner surface of left valve, having a shallow V-shaped pallial sinus; O, NMNS PM 28010, inner surface of right valve. Abbreviations: aams, anterior adductor muscle scar; apms, anterior pedal muscle scar; pams, posterior adductor muscle scar. All scale bars show 5 mm.
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Table 3.
Measurements of Pliocardia kawadai (Aoki). H/L = height/length; AL/L = anterior length/length.
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Adulomya chitanii
Kanehara, 1937
Figure 4A, B, D, F–I
Adulomya chitanii Kanehara, 1937, p. 19, pl. 5, figs. 1, 6–9; Kamada, 1962, p. 39, pl. 1, figs. 4–7; Amano and Kiel, 2011, p. 80, figs. 3–12.
“Adulomya” chitanii Kanehara. Aoki, 1954, p. 31, pl. 1, figs. 9–11.
Calyptogena chitanii (Kanehara). Kanno and Akatsu, 1972, pl. 8, figs. 13, 14; Amano and Little, 2005, fig. 6B–D; Amano and Jenkins, 2007, fig. 2C; Amano et al., 2007, fig. 3C, F, H.
Calyptogena sp., Yamaoka, 1993, pl. 4, figs. 1, 6, 7.
? Calyptogena chitanii (Kanehara). Shikama and Kase, 1976, pl. 2, fig. 6; Hirayama, 1973, p. 175, pl. 15, figs. 12, 13; Yamaoka, 1993, pl.4, figs. 2, 3.
? Akebiconcha chitanii (Kanehara). Hayashi and Miura, 1973, pl. 1, fig. 26; Hayashi, 1973, pl. 5, fig. 6.
non Akebiconcha chitanii (Kanehara). Kanno and Ogawa, 1964, pl. 1, figs. 17, 18; Kanno and Arai, 1964, pl. 1, figs. 19–22; Kanno, 1967, p. 401, pl. 1, figs. 9–11, 15.
Type specimen.—IGPS 87339 as neotype designated by Amano and Kiel (2011).
Type locality.—Tangozawa, Iwaki City, Fukushima Prefecture; Miocene, Honya Formation.
Material examined.—Although 48 specimens were obtained, most specimens are fragments. Only seven imperfect specimens were examined.
Remarks.—The Kurosedani specimens have a rather small size for the genus, up to 55.5 mm in length, elongate-quadrate shape (height/length = 0.29). Two cardinal teeth (1, 3b) in the right valve and three ones (2a, 2b, 4b) in the left valve can be observed. Also a small and deep pedal retractor scar was observed just above the anterior adductor muscle scar. Very small and V-shaped pallial sinus can be recognized just before the posterior adductor muscle scar. Based on these characters, the Kurosedani specimens are safely identified with Adulomya chitanii Kanehara, 1937.
Adulomya hamuroi Amano and Kiel, 2011 from the uppermost lower to lowermost middle Miocene Higashibessho Formation at Shimo-sasahara is similar to A. chitanii. However, A. hamuroi is different from A. chitanii by having a higher shell (height/length = 0.38–0.47), no pallial sinus, thick anterior cardinal tooth (1) and slightly bifid posterior cardinal tooth (3b) in the right valve.
Stratigraphic and geographic distribution.—Early Miocene: Honya, Mizunoya and Kamenoo formations in Fukushima Prefecture, the Toyohama Formation of Morozaki Group in Aichi Prefecture and the Kurosedani Formation in Toyama Prefecture. Middle Miocene: Nupinai Formation in Hokkaido.
Discussion
From the Kurosedani Formation, many molluscan fossils have been described by Tsuda (1959, 1960). Based on the occurrence of molluscs, Tsuda (1960) estimated that the upper part of the Kashio Member (Tsuda, 1953) which mostly overlaps with the Monmyoji Member (Sakamoto and Nozawa, 1960) was deposited in deep water with some transported species from shallow water. As mentioned above, the assemblage from Kakuma also shows the same pattern of occurrence. Six chemosymbiotic bivalves have been first recognized from the turbidite deposits of the lower Miocene Kurosedani Formation, namely, Nucinella sp., Solemyidae gen. et sp. indet., Lucinoma cf. aculilineatum, Conchocele yatsuoensis sp. nov., Pliocardia kawadai and Adulomya chitanii. They correspond to the earliest records of chemosymbiotic bivalves in the Japan Sea region. From the lower Miocene Middle Formation (ca. 16 Ma) of the middle part of the Taishu Group on Tsushima Island, westernmost part of the Japan Sea, some ill-preserved chemosynthetic species such as the solemyid Acharax spp., the bathymodiolines Bathymodiolus sp. and Adipicola sp., the vesicomyid Calyptogena spp. [ = Adulomya? spp.] were described and illustrated (Ninomiya, 2011; Table 4). However, the identifications of these taxa are somewhat problematic even at the genus level because that author did not make them based on the hinge and inner structure. According to Yanagisawa (1999), the Crucidenticula kanayae zone (16.3–16.9 Ma; Yanagisawa and Akiba, 1998) was recognized in the lower part of the Higashibessho Formation conformably overlying the Kurosedani Formation. From these data, the chemosymbiotic species from the Kurosedani Formation at Kakuma are probably the oldest record in the Japan Sea region.
As discussed by Amano et al. (2004), both shallow- and deep-water assemblages of the Higashibessho Formation conformably overlying the Kurosedani Formation are similar to those of Setouchi Province, which is connected to the the Pacific Ocean. During the latest early to earliest middle Miocene, the Japan Sea was connected to the Pacific Ocean by deep-sea pathways. The age of the Kurosedani fauna from Kakuma corresponds to the timing of the formation of a deep-sea basin in the Japan Sea, and is just after the invasion of warm shallow-water fauna into the Japan Sea (Chinzei, 1991; Sato et al., 2010).
Table 4.
Comparison of the chemosymbiotic bivalves from the Kurosedani Formation with the other Japanese early to middle Miocene hydrocarbon-seep bivalves.
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The early to middle Miocene hydrocarbon seep faunas in Japan have few common species with each other (Table 4). The Kurosedani fauna from Kakuma shares the vesicomyid Pliocardia kawadai with the Higashibessho fauna at Shimo-sasahara but is more similar to the early to middle Miocene faunas from the Pacific side of northern Japan. Occurrences of the vesicomyids Pliocardia kawadai and Adulomya chitanii from the Kurosedani Formation are common to the lower Miocene Honya Formation, Fukushima Prefecture, northeastern Honshu and the middle Miocene Nupinai Formation in eastern Hokkaido (Table 4). These data support our hypothesis that the invasion of chemosymbiotic species to the Japan Sea took place soon after its formation (ca. 18 Ma) along a migration route from northern Japan through central Honshu, in contrast with the Higashibessho normal deep-sea fauna. In the late early Miocene, the Pacific Ocean and the Japan Sea were connected through central Honshu (see Iijima and Tada, 1990; Kano et al., 1991).
Table 5.
Species of the genus Nucinella found from fossil hydrocarbon-seep sites.
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The occurrence of Nucinella sp. from the Kurosedani Formation is the first record from the Cenozoic deposits in the Japan Sea region as well as the Recent Japan Sea. The Cenozoic occurrence of this genus was previously confined to the Pacific side of southern Honshu and Kyushu (Matsukuma et al., 1982; Amano et al., 2013). Moreover, there is no record of this genus from the Recent Japan Sea (Higo et al., 1999). According to Higo et al. (1999), Nucinella ovalis (Jeffreys, 1879) is distributed in the Korea Strait (= Tsushima Strait, westernmost part of the Japan Sea). However, Habe (1977) had already suggested that this species might be a synonym of Hux-leyia sulcata A. Adams, 1860. Recently, it is becoming clear that two small nucinellids living off Oman possess chemosymbiotic bacteria in their gills (Oliver and Taylor, 2012). This finding confirms the speculation on the symbiosis of some Nucinella species with sulphide-oxidizing bacteria based on their gutless condition (Reid, 1990) and fossil occurrences (Amano et al., 2007a). Except for one doubtful record from the Late Triassic (Peckmann et al., 2011), four large-sized species of Nucinella have been recorded from the latest Jurassic to the early Oligocene seep sites (Table 5). It is plausible that the Kurosedani species (13.6 mm in length) is the youngest fossil record of this genus from a hydrocarbon seep site, judging from its shell size and associated fauna.
Acknowledgments
We are very grateful to Alan Beu (GNS Science) for his critical reading of this manuscript, T. Sasaki (University Museum of the University of Tokyo) and H. Nishi and J. Nemoto (Tohoku University) for their help in examining the fossils. We thank R. Majima (Yokohama National University) and an anonymous reviewer for their review and useful comments. We also thank members of Toyama Paleontological Research Club for helping us to collect the fossil specimens. This study was partly supported by a Grant-in-aid for Scientific Research of the Japan Society for the Promotion of Science (C, 17K05691, 2017–2019) to KA.
References
Appendices
Author contributions
Kazutaka Amano initiated the study and was primarily responsible for the taxonomic aspects. Yusuke Miyajima extracted fossils from rocks and found Nucinella sp. in this process. Kenyu Nakagawa first found this fauna including a new species of Conchocele and informed this occurrence to K. A. Masui Hamuro and Toshikazu Hamuro collected plenty of fossils and sent to K. A. and Y. M. All authors contributed to the writing of the paper.