Corbiculoid fauna from the Lower Jurassic Iwamuro Title Formation, Gunma Prefecture, ( fulltext )

MATSUKAWA,Masaki; SHIBATA,Kenichiro; Author(s) KOARAI,Kazuto; NISHIDA,Naohisa

Citation 東京学芸大学紀要. 自然科学系, 66: 149-158

Issue Date 2014-09-30

URL http://hdl.handle.net/2309/136941

Publisher 東京学芸大学学術情報委員会

Rights Bulletin of Gakugei University, Division of Natural Sciences, 66: pp．149～158，2014

Corbiculoid fauna from the Lower Jurassic Iwamuro Formation, Gunma Prefecture, Japan

Masaki MATSUKAWA*, Kenichiro SHIBATA**, Kazuto KOARAI***, Naohisa NISHIDA****

Department of Environmental Sciences

(Received for Publication; May 23, 2014)

MATSUKAWA, M., SHIBATA, K., KOARAI, K. and NISHIDA, N.: Corbiculoid fauna from the Lower Jurassic Iwamuro Formation, Gunma Prefecture, Japan. Bull. Tokyo Gakugei Univ. Div. Nat. Sci., 66: 149-158 (2014) ISSN 1880-4330

Abstract

Two corbiculoid species from the Lower Jurassic Iwamuro Formation are described in this paper. Characteristics of the fauna and the strategy of early Jurassic corbiculoid species during marine transgression are discussed. Based on biogeographical and biostratigraphical distribution of both Eomiodon vulgaris and Crenotrapezium kurumense, Eomiodon vulgaris appeared to be exterminated by marine transgression in the southern Kitakami area. In the Hokuriku area, both Eomiodon vulgaris and Crenotrapezium kurumense were interpreted to have retreated into a refuge during the first marine transgression, but they were exterminated by the second marine transgression.

Keywords: Eomiodon vulgaris, Crenotrapezium kurumense, refuge, extinction, marine transgression

Department of Environmental Sciences, Tokyo Gakugei University, 4-1-1 Nukuikita-machi, Koganei-shi, Tokyo 184-8501, Japan

* Tokyo Gakugei University (4-1-1 Nukuikita-machi, Koganei-shi, Tokyo, 184-8501, Japan) ** Yokosuka City Museum, 95 Fukadadai, Yokosuka, Kanagawa, 238-0016, Japan *** Keio Shonan Fujisawa Junior and Senior High School, Fujisawa, 252-0816, Japan **** Geological Survey of Japan, AIST, 1-1-1 Central 7, Higashi, Tsukuba, Ibaraki, 305-8567, Japan

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Introduction that from the Kuruma Group. Since the Kuruma Group is assigned to Pliensbachian and Toarcian with Early Jurassic The corbiculoids are one of the representative prosperous- ammonite indices (Sato, 1955; Kobayashi et al., 1957), the groups in Mesozoic brackish-water environments. Although Iwamuro Formation can be correlated to the Early Jurassic. they mainly inhabit changeable fresh water and brackish From sedimentary facies and occurrence of plant and bivalve water environments, they have inhabited these environments fossils, the lower member of the formation indicates an without change in shell design since Mesozoic time. To alluvial fan to meandering river, and the middle and upper understand the adaptive strategy and evolution of corbiculoids, members indicate a bay setting (Kimura, 1952; Takizawa, Matsukawa and Nakada (2003) and Nishida et al. (2013) 1985; Kamikubo and Takeuchi, 2011). discussed the origin and change of corbiculoids by ecological and environmental analyses of fossil assemblages from the Occurrence of bivalve fossils various horizons in the Japanese Jurassic and Cretaceous. As for the corbiculoids which they treated, their occurrence Our corbiculoid fossil specimens were collected from localities, characteristics of species composition in outcrop at the cliff on the right bank of the Katashina River assemblage and species described with systematics have in 1975. The fossil site named IW is situated near loc. 5 of been mostly shown in previous work. However, regarding Kimura (1952). This site produced more bivalve fossil the Iwamuro Formation in Gunma Prefecture, the Joetsu specimens than loc. 4 of Kimura (1952). Unfortunately, area, central Japan, Hayami (1958) listed five species when checked in 2001 the fossil site had been destroyed including two corbiculoid species. Afterward, Matsukawa artificially. and Nakada (2003) showed three species of corbiculoids and Corbiculoid fossils were obtained from both sandstone and their mode of occurrence. To date, the corbiculoid specimens mudstone beds of middle alternating beds of sandstone and from the Iwamuro Formation have not been described with mudstone member. As for those from the mudstone beds, the illustrations as yet. shells tend to be a somewhat flat. However, the surface So, in this paper, we describe corbiculoid species using ornamentation of shells and internal structures of shell are systematics, characteristics of assemblages, and discussion well preserved. On the other hand, as for shells found from of the fauna. the sandstone, the curvature of shell is preserved well. But internal structures of shell are not preserved. Most specimens Geological setting of shells of Crenotrapezium kurumense came from sandstone beds, while most specimens of shell of Eomiodon vulgaris The Iwamuro Formation, consisting of conglomerate, came from mudstone beds. This indicates that curvature of sandstone, alternating beds of sandstone and mudstone and shell of Crenotrapezium kurumense tends to be preserved, black mudstone, crops out along the river floor of the but internal structures tend to be not preserved. On the other Katashina River in Gunma Prefecture (Fig. 1). Based on hand, in the case of Eomiodon vulgaris, the curvature of Kimura (1952), the Iwamuro Formation is divided into shells is not preserved but an internal structure tends to be three members in ascending order: lower conglomerate preserved. member, middle alternating beds of sandstone and black Matsukawa and Nakada (2003) described the mode of mudstone member and upper black mudstone member. occurrence of shells using Kidewell et al. (1986)’ s Afterwards, Kamikubo and Takeuchi (2011) showed their classification. The shell assemblages of the Iwamuro stratigraphy with a geological map, but they basically followed Formation can be classified as Type II which is exceptional Kimura (1952)’ s stratigraphy. They added observations of occurrence of shells in black mudstone to very fine sedimentary structures such as trough cross-bedding and scour sandstone beds. Then, shell orientation and geometry can structures in the lower member. Plant leaf fossils occur in all be classified as concordant in cross-section, with pavement three members, and corbiculoid fossils also occur in the black geometry, and matrix-supported packing. These characters mudstone of the middle alternating beds of the sandstone and indicate the shells deposited and concentrated by mudstone member. Kimura (1952) concluded that the fossil discontinuous unidirectional currents. Then, the mode of flora from the Iwamuro Formation can be closely compared to occurrence of shells can be classified as the current

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138°E 139°E 140°E 141°E 37°N 139°E N

37°N

N

36°N Gunma Tokyo Prefecture

35°N 100 km 25 km 36°N

Iwamuro Lake Sonohara Anahara

Sonohara Dam 36°38’30”N

139°12’00”E

Katashina River Hikage Aoki nango Neri River

36°37’30”N36°37’30”

139°10’30”E Kakidaira N

KEY Andesitic volcanic rocks Felsic volcanic rocks Mafic rocks Iwamuro Formation Ultramafic rocks 01 km Upper part Quartz porphyry dike Middle part Fault Lower part Fossil locality

Fig. 1. Fossil locality in the Iwamuro area. Geological map referred from Kamikubo and Takeuchi (2011).

concentration type (Fürsich, 1995), and it was caused by Description of specimens the influence of a stream in a river or by a tidal current. Since a high-density shell layer is thin, the frequency where Abbreviations. L, shell length, H, shell height, L/H, thanatocoenosis shells were carried is low, the shells were length/height ratio. All specimens illustrated are deposited probably accumulated in an environment influenced by a in the Department of Environmental Sciences, Tokyo weak stream. It seems that it was a low energy environment Gakugei University, Koganei, Tokyo (TGUSE). Specimens because of abundant juvenile shells. referred to in the University Museum of the University of Tokyo are abbreviated as UMUT.

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Class: Bivalvia Linné, 1758 spaced strong concentric ribs among which numerous faint Subclass: Heterodonta Neumayr, 1884 concentric growth-lines are presented; anterior lateral teeth Order: Veneroida H. Adams and A. Adams, 1856 long (TGUSE-MM 5588) . Superfamily: Arcticacea Newton, 1891 Remarks. The present specimens are similar to the type Family: Neomiodontidae Casey, 1955 specimens (UMUT-MM 2844, 2845, 2846, 2847) of Subfamily: Eomiodontinae Hayami, 1965 Eomiodon vulgaris from the Lower Jurassic of the Kuruma Genus: Eomiodon Cox, 1935 Group (Hayami, 1958, p. 19-21, pl. 2, figs. 15-21, pl. 3, figs. 1-3) in having Eomiodon-like trigonally suboval to Eomiodon vulgaris Hayami, 1958 orbicular in outline and ornamented shell surface with wide-spaced concentric ribs. Lucinoid-type dentition, Fig. 2A-H, J-L lunule and escutcheon are shown as generic characters of the genus by Cox et al. (1969). They are not observed in the 1958 Eomiodon vulgaris; Hayami, p. 19-21, pl. 2, figs. 15- present specimens, but it is due to different state of 21, pl. 3, figs. 1-3. preservation and principally to the flattened shells. 2003 Eomiodon vulgaris; Matsukawa and Nakada, p. 167, The present specimens are almost the same size as the type listed. specimens from the Kuruma Group, , and 2003 Eomiodon sp.; Matsukawa and Nakada, p. 167, listed. Toyama prefectures, ranging from 6.9 to 24.3 mm in length while the Kuruma specimens range from 21.0 to 27.5 mm. Material. 52 specimens (TGUSE-MM 5579-5586, 5588, 5589, 5591-5634, 5647). They consist of one external cast Subfamily: Neomiodontinae Casey, 1955 of disjointed open valves, 12 external casts of right valve, Genus: Crenotrapezium Hayami, 1958 13 external casts of left valve, eight external molds of right valve, six external molds of left valve, seven internal molds Crenotrapezium kurumense Hayami, 1958 of right valve, and five internal molds of left valves (M. Matsukawa, Coll.). Fig. 2 I, M-Q

Measurements (in mm except for L/H) 1958 Crenotrapezium kurumense; Hayami, p. 14-16, pl. 2, Specimen L H L/H figs. 22-28. TGUSE-MM 5579 14.4 12.8 1.13 1958 Crenotrapezium kurigata: Hayami, p. 16-17, pl. 2, TGUSE-MM 5606 23.3 17.5 1.33 figs. 29,30. TGUSE-MM 5607 11.8 8.8 1.34 1961 Crenotrapezium kurumense grossum; Hayami, p. TGUSE-MM 5627 24.3 18.5 1.31 115-116, pl. 16, fig. 4. TGUSE-MM 5632 6.9 4.4 1.57 1975 Crenotrapezium kurumense kurumense; Hayami, p. TGUSE-MM 5633 18.4 15.6 1.18 140. 1975 Crenotrapezium kurumense grossum; Hayami, p. Description. Shell large for the genus, inequilateral, 140-141. variable outline from trigonally suboval to orbicular, much 2003 Crenotrapezium kurumense; Matsukawa and Nakada, longer than high, moderately inflated; antero-dorsal margin p. 167, listed. slightly concave in front of umbo, smoothly sloping down into the anterior margin; postero-dorsal margin gently Material. 19 specimens (TGUSE-MM 5587, 5590, 5635- convex, forming obtuse angle at the junction with the 5646, 5648-5656). They consist of one external cast of siphonal margin; siphonal margin rounded; ventral margin disjointed open valves, three internal casts of right valve, broadly arched, and ventro-anterior curvature weaker than seven internal casts of left valve, one external mold of right ventro-posterior one; umbo prominent, slightly incurved, valve, four external casts of right valve and three external prosogyrous, placed at about one-third to half of shell- casts of left valve (M. Matsukawa Coll.). length from the anterior end; surface marked with wide-

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Fig. 2. A-H, J-L, Eomiodon vulgaris Hayami; A, TGUSE-MM 5632, external cast of left valve; B, TGUSE-MM 5609, external cast of right valve; C, TGUSE-MM 5611, external cast of left valve; D, TGUSE-MM 5606, external cast of left valve; E and F, TGUSE-MM 5588, internal mold of right valve (E) and its rubber mold (F); G, TGUSE-MM 5627, external cast of right valve; H, TGUSE-MM 5592, external cast of left valve; J, TGUSE- MM 5618, external cast of left valve; K, TGUSE-MM 5617, external cast of right valve; L, TGUSE-MM 5633, external cast of right valve; I, M-Q, Crenotrapezium kurumense Hayami; I, TGUSE-MM 5646, external cast of right valve; M, TGUSE-MM 5643, internal mold of right valve; N, TGUSE-MM 5654, external cast of left valve; O, TGUSE-MM 5640, external cast of left valve; P, TGUSE-MM 5644, external cast of right valve; Q, exclusive occurrence of Crenotrapezium kurumense, Q1, TGUSE-MM 5656, external cast of right valve, Q2, TGUSE-MM 5655, external cast of open valves, Q3, TGUSE-MM 5645, external cast of open valves.

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Measurements (in mm except for L/H) Formation in Prefecture (Hayami, 1961, p. 115- Specimen L H L/H 116, pl. 16, fig. 14), because its specimen is larger than TGUSE-MM 5641 11.5 9.5 1.21 specimens of Crenotrapezium kurumense (s.s.) from the TGUSE-MM 5644 33.6 29.2 1.15 Kuruma Group, ranging from 36.0 to 42.0 mm in shell TGUSE-MM 5645 35.6 16.4 2.17 length, while the Kuruma specimens range from 25.5 to TGUSE-MM 5649 16.6 13.8 1.20 28.5 mm. Then, Ratio of length/height of Crenotrapezium TGUSE-MM 5654 30.5 17.2 1.77 kurumense grossum is somewhat smaller than that of TGUSE-MM 5655 30.2 21.2 1.42 Crenotrapezium kurumense (s.s.); the former ranges from 1.10 to 1.35, while the latter ranges from 1.35 to 1.62+. Description. Shell large-sized, inequilateral, variable in Ratio of length/height of our specimens from the Iwamuro outline from trigonal to trigonally ovate, much longer than Formation ranges from 1.15 to 2.17. This indicates our high, provided with a sharp posterior carina, and specimens have characters of two subspecies. moderately inflated; test fairly thick; umbo prominent, high Regarding to shell outline of the recent Corbicula above hinge-margin, slightly prosogyrous, placed at from fluminea, Sousa et al. (2007) showed that variable shell two-fifth to one-sixth from the anterior end; antro-dorsal shape including both elongated shell and short and rounded margin short, slightly concave in front of beak, passing shells belong to the same species with genetic analysis in gradually into anterior margin; postero-dorsal margin long, the River Minho estuary (NW Portugal), and Zhou et al. nearly straight, turning somewhat abruptly into posterior (2011) demonstrated that elongated shells in the species margin; siphonal margin straight to round and well defined inhabit in high flow rate water current, but short and from postero-dorsal and ventral margin; ventral margin rounded shells in the species inhabit in low flow rate water broadly arched and vetro-anterior curvature weaker than current in Yellow, Huaihe, Hanshui, Yangtze and Xijiang ventro-posterior one; surface marked with fine concentric rivers in China. Based on the study of recent Corbicula growth-lines. Internally, anterior scar small and ovate; fluminea, shell outline variation of Crenotrapezium posterior scar large and ovate; pallial line simple. Dental kurumense is probably controlled by water current power. formula is AI, AII, 3a, 3b, PI, PIII / AII, 4b, PII (TGUSE- So, Crenotrapezium kurumense with various shell forms MM 5643 and 5648 for right and left valve, respectively), should be classify as the same species. AI long; AII long and arcuate along the anterior margin; 3a trigonal form, opisthocline; 3b trigonal from prosocline; PI Characteristics of fauna and discussion about strategy long, slightly curved, parallel to PIII; PIII long; AII long, of early Jurassic corbiculoid species during marine curved along the anterior margin; 2 stout trigonal form; 4b transgression ridge-like form, prosocline; PIII long. Remarks. Because of the lucinoid type dentition and Two bivalve species from the Iwamuro Formation are sharp posterior carina, the present specimens belong to the identified and described above. Since Hayami (1961) listed genus Crenotrapezium. However, fine crenulation on lateral three more species, Bakevellia magnissima, Eopecten (?) sp. teeth cannot be observed, but it may be due to the different and Cardinioides (?) sp., bivalve fossil fauna of the Iwamuro state of preservation and principally to the coarse-grained Formation is characterized by five species consisting matrix. The present specimens are similar to type predominantly of Eomiodon vulgaris, Crenotrapezium specimens (UMUT-MM 2823, 2824, 2825, 2826) of kurumense, poor Bakevellia magnissima, Eopecten (?) sp. and Crenotrapezium kurumense (Hayami, 1958, p. 14-16, pl. 2, Cardinioides (?) sp. Eomiodon vulgaris is also reported from the figs. 22-28) in their shell outline. However, the present Kitamatadani, Negoya, Shinatani and Tsuschizawa specimens are larger than the type specimens from the formations of the Kuruma Group in Hokuriku area, the Kuruma Group, Nagano, Niigata and Toyama prefectures, Niranohama Formation in , southern ranging from 11.5 to 33.6 mm in length, while the Kuruma Kitakami area, northeast Japan and the Yamaoku Formation specimens range from 25.5 to 28.5 mm. Hayami (1961) in , Chugoku area, west Japan. established Crenotrapezium kurumense grossum, as new Crenotrapezium kurumense occurs in the Kitamatadani, subspecies, from the Y1 member of the Yamaoku Negoya, Shinatani and Tsuchizawa formations of the

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Kuruma Group in Hokuriku area. So, the bivalve fossil spatial scale enable the formation of a restricted brackish- fauna of the Iwamuro Formation is similar to it of the water environment, thereby maintaining the same corbiculoid Niranohama Formation, Kuruma Group and Yamaoku species. So, we can interpret that a corbiculoid species run Formation as Hayami (1961) mentioned. into a refuge and we can interpret it as having survived the Based on ammonites, the Niranohama Formation yielding environmental change of the marine transgression. Alsatites onoderai can be assigned to the Hettangian In the southern Kitakami area, Eomiodon vulgaris does (Matsumoto, 1956), and the Kitamatadani and Negoya not occur in formations overlying the Niranohama Formation formations yielding Canavaria sp. ex gr. geyeriana and (Fig. 3). Then, a corbiculoid, Crenotrapezium kitakamiense, Amaltheus sp. can be assigned to the Pliensbachian (Sato, occurs in the Lower Cretaceous Tategami Formation which 1955). Then, the Shinatani Formation can be assigned to the overlies the Niranohama Formation. This indicates Eomiodon Toarcian, because Grammoceras sp. ex. Pseudogrammoceras vulgaris was exterminated by the marine transgression, and muelleri and Hammatoceratidae gen. et sp. indet. are Crenotrapezium kitakamiense appeared as a newcomer obtained from its overlying the Otakidani Formation species. However, in Hokuriku area, Eomiodon vulgaris and (Kobayashi et al., 1957; Sato, 1962). So, the Iwamuro Crenotrapezium kurumense are interpreted to have run into a Formation can be correlated to the Hettangian to the refuge, because they occur in brackish formations which Toarcian through Sinemurian and Pliensbachian. sandwich a marine formation. On the basis of Matsukawa and Nakada (2003) and In chronological distribution, in the southern Kitakami Nishida et al. (2013), the corbiculoids which appeared in the area in northeast Japan, Eomiodon vulgaris occurs only in Early Jurassic were derived from the Arcticidae, moved to the Niranohama Formation correlating to the Hettangian brackish-water environments during marine transgression, Stage, but in Hokuriku area, Eomiodon vulgaris occurs in and inhabited in the environments. Since different the Kitamatadani, Negoya, Shinatani and Tsuchizawa corbiculoid species occur in two brackish formations which formations of the Kuruma Group which correlates to the sandwich a marine formation, it is interpreted that the species Pliensbachian to Toarsian stages. The occurrence of from the formation below the marine formation was Eomiodon vulgaris from the Niranohama Formation is the exterminated by the marine transgression, and the species oldest record of corbiculoids in the World. Although from the upper formation is the newcomer species. However, Eomiodon vulgaris has a wider geographical distribution when the same species occur in two brackish-water than Crenotrapezium kurumense, E. vulgaris has a reduced formations which sandwich a marine formation, it is geographical distribution. Since both Eomiodon vulgaris interpreted that environmental changes at a relatively small and Crenotrapezium kurumense were not found in the

Age / Area Hokuriku Joetsu southern Kitakami Chugoku

Aalenian

Otakidani Fm Toarcian

Shinatani Fm e aris g p mens ul u Teradani Fm v Hosoura Fm ur k on s

Pliensbachian d i m o umense iu

Negoya Fm r ez p Eomi vulgar ra n n vulgaris t um ku i o z Iwamuro Fm do Yamaoku Fm

Kitamatadani no e od Early Jurassic e Kuruma Grou p r

Fm mi C Sinemurian omio E Eo s ri a g l Jyogodani Fm Crenotra n vu o d

Niranohama io

Hettangian m

Fm o E

Fig. 3. Biogeographical and biostratigraphical distribution of both Eomiodon vulgaris and Crenotrapezium kurumense.

－ 155 － Bulletin of Tokyo Gakugei University, Division of Natural Sciences, Vol. 66 (2014) MATSUKAWA, et al.: Corbiculoid fauna from the Lower Jurassic Iwamuro Formation, Gunma Prefecture, Japan formation overlying the marine Otakidani Formation in the Fürsich, F. T., 1995. Shell concentrations. Ecologae geologicae Kuruma Group, we can consider that both species were Helvetie 88, 643-655. exterminated by a marine transgression. Unfortunately, we Hayami, I., 1958. A review of the so-called Liassic “Cyrenoids” in do not discuss the above problem for the Iwamuro Japan. Japanese Journal of Geology and Geography 29, 11-27, Formation, because there is no marine intercalation in the pls. 2-3. formation. We can merely understand that both species Hayami, I., 1961.Pelecypods from the Liassic Yamaoku Formation in expanded geographical distribution to the Joetsu area, west Japan. Transactions and Proceedings of the central Japan. Palaeontological Society of Japan 43, 113-116, pl. 16. Hayami, I., 1965. Lower Cretaceous marine pelecypods of Japan, Conclusions part II. Memoirs of the Faculty of Science, Kyushu University. Series D, 17(2), 73-150, pls.7-21. 1. Bivalve specimens were collected from the Lower Hayami, I., 1975. A systematic survey of the Mesozoic bivalvia from Jurassic Iwamuro Formation in Gunma Prefecture in the Japan. The University Museum of the University of Tokyo Joetsu area, central Japan. They are identified as Bulletin 10, 228 pp. 10 pls. Eomiodon vulgaris and Crenotrapezium kurumense, and Kamikubo, H., Takeuchi, M., 2011. Detrital heavy minerals from are described with systematics. Lower Jurassic clastic rocks in the Joetsu area, central Japan: 2. The bivalve fossil fauna of the Iwamuro Formation is Paleo-Mesozoic tectonics in the East Asian continental margin similar to that of the Niranohama Formation and Kuruma constrained by limited chloritoid occurrences in Japan. Island Group. Arc 20, 221-247.iar_762 221..247 3. Based on biogeographical and biostratigraphical distribution Kidewell, S. M., Fürsich F. T., Aigner, T., 1986. Conceptual of both Eomiodon vulgaris and Crenotrapezium kurumense, framework for the analysis and classification of fossil Eomiodon vulgaris was exterminated by a marine concentrations. Palaios 1, 228-238. transgression in the southern Kitakami area. In the Hokuriku Kimura, T., 1952. On the geological study of the Iwamuro Formation area, both Eomiodon vulgaris and Crenotrapezium Tone Gun, Gunma Prefecture, series 1. Journal of the kurumense are interpreted to have run into a refuge during Geological Society of Japan 58, 457-468 (in Japanese with the first marine transgression, but they were exterminated English abstract). by the second marine transgression. Kobayashi, T., Konishi, K., Sato, T., Hayami, I., Tokuyama, A., 1957. On the Lower Jurassic Kuruma Group. Journal of the Acknowledgements Geological Society of Japan 63, 182-194 (in Japanese). Linné, C., 1758. Systema Naturae per regna tria natuae . . ., edt. 10, We thank Drs I. Obata and M.G. Lockley for their critical 1, iv + 824 p. and errata page, Laurentius Salvius, Stockholm. reading of an early version of the manuscript. Matsukawa, M., Nakada, K., 2003. Adaptive strategy and evolution of corbiculoids based on the Japanese Mesozoic fossils. Bulletin References of the Tokyo Gakugei University, section 4, 55, 161-189 (in Japanese with English abstract). Adams, H., Adams, A., 1856. The genera of Recent Mollusca (b) v. 2 Matsumoto, T., 1956. Yebisites, a new Lower Jurassic ammonite (1854-58), 661 p., v. 3 (1858), 136 pl. (). from Japan. Transactions and Proceedings of the paleontological Casey, R., 1955. The pelecypod family Corbiculidae in the Mesozic Society of Japan 23, 205-212, pl. 30. of Europe and Near East. Journal of Washington Academy of Neumayr, M., 1884. Zur Morphologie des Bivalvenschlosses. Science 45, 366-372. Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften Cox L. R., 1935. The Triassic, Jurassic and Cretaceous Gastropoda in Wien, 88, 1, 385-418. and Lamellibranchia of the Attock District. Memoirs of the Newton, R.B., 1891. Systmatic list of the F.E. Edwards collection of Geological Survey of India, Paleontologia Indica N.S. 20, 1-27. British Eocene and Oligocene Mollusca in the British Museum Cox, L.R., et al., 1969. Mollusca 6. Bivalvia. In: Moore, R. C. ed., (Natural History). British Museum, London. Treatise on Invertebrate Paleontology. Part N. University of Nishida, N., Shirai, A., Koarai, K., Nakada, K., Matsukawa, M., Kansas Press and Geological Society of America. Inc., 952p. 2013. Paleoecology and evolution of Jurassic-Cretaceous

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corbiculoids from Japan. Palaeogeography, Palaeoclimatology 1774) in two Portuguese estuaries. Estuarine, Coastal and Shelf and Palaeoecology 369, 239-252. Science 74, 166-174. Sato, T. 1955. Les Ammonites recueillies dans le groupe de Kuruma, Takizawa, F., 1985. Tuff and sandstone composition of tne Iwamuro Nord du Japon central. Transactions and Proceedings of the Formation, Joetsu belt. Collaborative Research ‘Joetsu and Palaeontological Society of Japan, New Series, 20:111-118. Ashio Belts’ 2, 141-149 (in Japanese) Sato, T. 1962. Études biostratigraphiques des ammonites du Zhou, H., Liu, C.Q., Yan, H., Ding, W.C., Wang, B., Jiang, W., Zhao, Jurassique du Japon. Mémoirs de la Société Géologique de T. L., 2011. Shell morphology of Corbicula fluminea (Müller, France N. S. 94, 1-122. 1774) and its implication for the adaptation to environmental Sousa, R., Freire, R., Rufino, M., Méndez, J., Gaspar, M., Antunes, change in the major drainage basins of China. Chinese Journal C., Guilhermino, L., 2007. Genetic and shell morphological of Ecology 30, 1497-1503 (in Chinese with English abstract). variability of the invasive bivalve Corbicula fluminea (Müller,

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群馬県の下部ジュラ系の岩室層から産出したシジミ貝類化石動物群

松川 正樹・柴田健一郎・小荒井千人・西田 尚央

環境科学分野

要 旨

群馬県の下部ジュラ系の岩室層から産出したシジミ貝類化石をEomiodon vulgarisとCrenotrapezium kurumenseとし て記載した。これまで，Eomiodon vulgarisは，本地域のほか，北陸地域の下部ジュラ系の来馬層群，南部北上地域の 韮の浜層と中国地方の山奥層から，Crenotrapezium kurumenseは本地域と北陸地域の下部ジュラ系の来馬層群から産 出が報告されていた。Eomiodon vulgarisは，南部北上地域では韮の浜層からのみ産出し，それより上位の汽水成層か ら産出しない。韮の浜層の上位に重なる細浦層は海成層なので，Eomiodon vulgarisは海進により，南部北上地域では 絶滅したと解釈される。Eomiodon vulgarisとCrenotrapezium kurumenseは，来馬層群では，海成層の寺谷層を挟んで 産出するので，海進期には避難所に退避した解釈される。しかし，この 2 種は，さらに上位の海成層の大滝谷層より 上位に重なる地層から産出しないので，この海進により絶滅したと解釈される。岩室層は，海成層を含まないので， この 2 種の海進に対する避難所の議論はできない。なお，これまで，岩室層からこれらの種が産出することは知られ ていたが，標本写真の学術的な提示は無かった。また，本標本が産出した崖は人工的な改変により消失したので，記 載された標本の記録性は極めて高い。

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