This study describes a brachiopod fauna, consisting of 15 species in 15 genera, from the lower part of the Takakurayama Formation at the locality T1 in the Takakurayama area, Abukuma Mountains, South Kitakami Belt, northeastern Japan. The fauna is considered to be late Permian (Wuchiapingian) in age based on the occurrence of Haydenella wenganensis, Costatumulus tazawai and Pterospirifer alatus. In terms of palaeobiogeography, the Takakurayama (T1) fauna is a mixed Boreal—Tethyan fauna with Chonetinella, Lamnimargus, Megousia, Costalumulus, Yakovlevia, Neospirifer, Gypospirifer, Alispiriferella and Pterospirifer as the Boreal (anti-tropical) elements, and Haydenella, Transennatia and Echinauris as the Tethyan (tropical) elements. The fauna is allied with the middle—late Permian brachiopod fauna of South Primorye, Far Eastern Russia.
Introduction
The Takakurayama area in the Abukuma Mountains. South Kitakami Belt, northeastern Japan (Figure 1) is a renowned Pennian fossil locality in Japan. Many fossils of marine invertebrates and land plants have been previously described from the Pennian Takakurayama Fonnation, including foraminifers (Ueno, 1992), brachiopods (Yanagisawa, 1967; Nakamura, 1972; Tazawa, 1999b, 2008b), pelecypods (Yanagisawa, 1967; Nakazawa and Newell, 1968). cephalopods (Hayasaka, 1957, 1965; Yanagisawa, 1967; Tazawa et al., 2005; Ehiro, 2008; Fujikawa and Suzuki, 2011), trilobites (Yanagisawa, 1958, 1967; Endo and Matsumoto, 1962; Koizumi, 1972, 1974; Kobayashi and Hamada, 1984) and plants (Asama, 1974).
However, there is little agreement on the age of the Takakurayama Formation, especially of the lower part, owing to a lack of systematic studies of the fossils. Until now, only six brachiopod species (Yanagisawa, 1967; Tazawa, 1999b; Tazawa, 2008b) and four ammonoid species (Fujikawa and Suzuki, 2011) have been described from locality T1, a single fossil locality of the lower part of the Takakurayama Formation (= Iriishikura Formation of Yanagisawa and Nemoto, 1961). The lower part of the formation has variously been assigned to the lower Permian (Sakmarian—Kungurian) by Yanagisawa (1967) and Nakazawa and Newell (1968), the middle Permian (Wordian) by Ehiro and Okami (1989) and the upper Permian (Wuchiapingian) by Tazawa (2008b).
The present study describes the brachiopod species from the lower part of the Takakurayama Formation at the locality T1 in the B-sawa Valley (named by Yanagisawa and Nemoto, 1961), a tributary of the Sodetamayamagawa River, in the Takakurayama area, and discusses the age and palaeobiogeography of the brachiopod fauna. In this study, C. Suzuki prepared the brachiopod specimens; N. Kaneko and S. Hasegawa studied the stratigraphy of the Takakurayama Formation; and J. Tazawa studied the systematics of the brachiopods. The specimens described herein are registered and housed in the Geological Museum, Geological Survey of Japan, AIST, Tsukuba with prefix GSJ F.
Stratigraphy
The Takakurayama Fonnation (named by Onuki, 1966) is distributed on an area of about 3 km2 on the northeastern slope of Mt. Takakurayama. with a general NNE-SSW strike and dip of 30°–58° toward the WNW (Figure 2). The fonnation consists mainly of black shale with subordinate sandstone and conglomerate, and exceeds 805 m in total thickness (Figure 3). The black shale of the Takakurayama Fonnation is lithologically similar to that of the upper Permian Toyoma Fonnation (Onuki. 1969) in the southern Kitakami Mountains, northeastern Japan, and the conglomerate is correlated with the Usuginu-type conglomerate (Kano. 1971). which is widely distributed in the upper Pennian of Japan.
The fossil locality T1 (Figures 1, 3) occurs in the upper stream of B-sawa Valley (lat. 37°7′57″N. long. 140°55′8″E), where there occurs alternation of dark grey, fine-grained sandstone and dark grey shale in the lower part of the Takakurayama Formation, about 255 m above its base.
Brachiopod fauna from the lower Takakurayama Formation
The brachiopod fauna from the locality T1 consists of the following 15 species in 15 genera: Chonetinella krotovi (Fredericks, 1925), Rugoconcha sp., Haydenella wenganensis (Huang, 1932), Transennatia gratiosa (Waagen, 1884), Lamnimargus peregrinus (Fredericks. 1924b), Echinauris sp., Megousia auriculata Muir-Wood and Cooper. 1960, Costatumulus tazawai Shen, Archbold, Shi and Chen, 2000, Yakovlevia mammatiformis (Fredericks, 1926), Hustedia ratburiensis Waterhouse and Piyasin, 1970, Neospirifer sp., Gypospirifer kobiyamai Tazawa and Araki, 2013, Alispiriferella sp., Pterospirifer alatus (von Schlotheim. 1813) and Dielasma sp.
The geographical and stratigraphical distributions of the brachiopod species of the Takakurayama (T1) fauna are summarized in Figure 4. It is noteworthy that 6 species (Chonetinella krotovi, Haydenella wenganensis, Transennatia gratiosa, Lamnimargus peregrinus, Costatumulus tazawai and Gypospirifer kobiyamai) also occur in the upper part of the Takakurayama Formation at the localities T7 and T8 (Figures 1. 3, 4).
Age and correlation
Chonetinella krotovi, Transennatia gratiosa and Lamnimargus peregrinus are known from the Wordian— Changhsingian; Haydenella wenganensis is known from the Wuchiapingian—Changhsingian; Gypospirifer kobiyamai is known from the Wordian—Wuchiapingian; and Pterospirifer alatus is known from the Capitanian— Wuchiapingian. Costatumulus tazawai is restricted to the Wuchiapingian. Megousia auriculata, Yakovlevia mammatiformis and Hustedia ratburiensis are long-ranging species.
Figure 3.
Generalized columnar section of the Takakurayama Formation in the Takakurayama area, showing the fossil horizons of T1, T7, T8.
In summary, the age of the Takakurayama (T1) fauna is considered to be Capitanian—Wuchiapingian, most probably Wuchiapingian. The brachiopods from the upper part of the Takakurayama Formation at localities T7 and T8 indicate a Wuchiapingian—Changhsingian age, probably Wuchiapingian (Tazawa, 2008b). Therefore, the whole of the Takakurayama Formation is assigned to the upper Permian (Wuchiapingian), and is correlated with the lower part of the Toyoma Formation, including the Usuginu-type conglomerate, in the southern Kitakami Mountains, northeastern Japan.
Sedimentary environment of the Daido Fonnation (equivalent of the Toyoma Formation) and the Usuginu Conglomerate is considered to be a deep-water turbidite basin (Yoshida and Machiyama, 1997, 1998). The early Pennian (Sakmarian-Artinskian) ammonoids (Thalassoceras? sp., Agathiceras sp., Paragastrioceras? sp. and Artinskia sp.). described by Fujikawa and Suzuki (2011) from the locality T1, are poorly preserved and may have been reworked into these late Pennian (Wuchiapingian) turbidites, as was stated by the authors (p. 64), “we cannot say for certain whether these fossils are reworked or not”.
Palaeobiogeography
The Takakurayama (T1) fauna includes the Boreal (anti-tropical) elements Chonetinella krotovi, Lamnimargus peregrinus, Megousia auriculata, Costatumulus tazawai, Yakovlevia mammatiformis, Neospirifer sp., Gypospirifer kobiyamai, Alispiriferella sp. and Pterospirifer alatus, and also the Tethyan (tropical) elements Haydenella wenganensis, Transennatia gratiosa and Echinauris sp. Consequently, the fauna is a mixed Boreal—Tethyan fauna, with a predominance of Boreal elements. In terms of the specific composition, the Takakurayama (T1) fauna is closest to the middle—upper Pennian brachiopod fauna of South Primorye. Far Eastern Russia (Fredericks. 1924b. 1925; Licharew and Kotlyar, 1978; Kotlyar, 1989). The following five species are common to both faunas: Chonetinella krotovi, Transennatia gratiosa, Lamnimargus peregrinus, Yakovlevia mammatiformis and Gypospirifer kobiyamai.
These data suggest that in the late Pennian (Wuchiapingian) the South Kitakami region, including the Takakurayama area, was located in the mid-latitudes of the Northern Hemisphere, the Inner Mongolia—Japan Transitional Zone (Tazawa, 1991, 1998, 2007) and/or the Sino-Mongolian—Japanese Province (Shi and Tazawa, 2001; Shi, 2006; Shen et al., 2009), a transitional zone between the Boreal and Tethyan realms in East Asia, and close to South Primorye. This conclusion is in agreement with Tazawa's (2008c, 2011, 2012) late Permian reconstruction of Proto-Japan (i.e., the Hida Gaien, South Kitakami and Kurosegawa belts).
Systematic descriptions
(by J. Tazawa)
Order Productida Sarytcheva and Sokolskaya, 1959
Suborder Chonetidina Muir-Wood, 1955
Superfamily Chonetoidea Bronn, 1862
Family Rugosochonetidae Muir-Wood, 1962
Subfamily Rugosochonetinae Muir-Wood, 1962
Genus Chonetinella Ramsbottom, 1952
Type species.—Chonetes flemingi Norwood and Pratten, 1855
Chonetinella krotovi (Fredericks, 1925)
Figure 5.1
Chonetes (Chonetina) krotovi Fredericks, 1925, p. 6, pl. 1, figs. 54–57.
Chonetina substrophomenoides forma A Shimizu, 1961, p. 317, pl. 16, figs. 11–14.
Chonetes uralicns Moeller. Yanagisawa, 1967, p. 86, pl. 1, fig. 11.
Waagenites aff. striata Liao. Yanagida, 1996, fig. 2.10.
Chonetinella krotovi (Fredericks). Tazawa, 2008b, p. 19, figs. 3.1–3.7; Tazawa, 2009, p. 69, fig. 4.1.
Material.—One specimen, a ventral valve, GSJ F18045.
Remarks,—The single ventral valve specimen from Takakurayama is small in size (length 6 mm, width 7 mm), slightly transverse, subquadrate in outline, strongly convex in lateral profile, and having a deep and narrow sulcus bordered by a pair of high, broad lateral ridges. External surface of the ventral valve is ornamented by numerous capillae on the whole valve except for the posterior half of the ears. The specimen is referred to Chonetinella krotovi (Fredericks, 1925), from the Chandalaz Formation of South Primorye, Russian Far East, in size and shape of the ventral valve and in having a deep and narrow sulcus bordered by a pair of high, broad ridges.
Distribution.—Wordian—Changhsingian: Far Eastern Russia (South Primorye), northeastern Japan (Takakurayama in the South Kitakami Belt) and southwestern Japan (Kawahigashi in the Maizuru Belt and Tsunemori in the Akiyoshi Belt).
Figure 4.
Geographical and stratigraphical distributions of brachiopod species of the Takakurayama (T1) fauna, excluding five species uncertain.
Suborder Productidina Waagen, 1883
Superfamily Productelloidea Schuchert, 1929
Family Overtoniidae Muir-Wood and Cooper, 1960
Subfamily Plicatiferinae Muir-Wood and Cooper, 1960
Tribe Plicatiferini Muir-Wood and Cooper, 1960
Genus Rugoconcha Jin and Sun, 1981
Type species.—Plicatifera chaoi Grabau, 1936.
Rugoconcha sp.
Figure 5.2
Material.—One specimen, external mould of a dorsal valve. GSJ F18046.
Description.—Shell small size for genus, transversely subrectangular in outline, with greatest width slightly anterior to hinge; length about 16 mm, width about 18 mm in the single dorsal valve specimen (GSJ F18046). Dorsal valve gently convex, nearly flat on visceral disc, not geniculated; ears large, not clearly demarcated from visceral region; no fold. External surface of dorsal valve ornamented by numerous, strong, subangular and somewhat irregular concentric rugae on whole valve, numbering 3–4 rugae in 3 mm at about midlength; interspaces of rugae broad, with rounded bottom; no spines on valve entirely.
Remarks.—The material available is lacking the ventral valve, but is safely assigned to the genus Rugoconcha by its size, shape and external ornament of the dorsal valve. The Takakurayama specimen resembles the type species. Rugoconcha chaoi (Grabau, 1936, p. 171, pl. 6, fig. 9; pl. 8, fig. 2; pl. 14, figs. 21, 22), from the Maping Limestone of Guangxi, central-southern China and Guizhou, south-western China, but the Chinese species is somewhat larger in size.
Rugoconcha xainzaensis Yang and Fan (1983, p. 269, pl. 1. figs. 18. 19), from the upper Carboniferous Susuo Formation of Xainza, northern Xizang (Tibet), differs from the present species in its much smaller size.
Figure 5.
1, Chonetinella krotovi (Fredericks), ventral view of ventral valve, GSJ F18045; 2, Rugoconcha sp.; 2a, 2b, dorsal view of external latex cast and external mould of dorsal valve, GSJ F18046; 3, 4, Haydenella wenganensis (Huang); 3a, 3b, ventral view of external latex cast and internal mould of ventral valve, GSJ F18047; 4a, 4b, ventral view of internal mould of ventral valve and pseudopunctae on the surface of the specimen, GSJ F18048; 5–10, Transennatia gratiosa (Waagen); 5a, 5b, dorsal and lateral views of external mould of dorsal valve, GSJ F18051; 6, ventral view of external latex cast of ventral valve, GSJ F18052; 7, dorsal view of external mould of dorsal valve, GSJ F18063; 8a, 8b, 8c, dorsal, anterior and lateral views of external mould of dorsal valve, GSJ F18056; 9a, 9b, 9c, dorsal view of external latex cast of dorsal valve, and dorsal and lateral views of external mould of dorsal valve, GSJ F18058; 10, dorsal view of external mould of dorsal valve, GSJ F18064; 11–13, Lamnimaigus peregrinus (Fredericks); 11, ventral view of internal mould of ventral valve, GSJ F18076; 12a, 12b, 12c, 12d, ventral, anterior, posterior and lateral views of internal mould of ventral valve, GSJ F18067; 13, ventral view of internal mould of ventral valve, GSJ F18071. Scale bars represent 1 cm.
Family Productinidae Muir-Wood and Cooper, 1960
Subfamily Chonetellinae Licharew in Sarytcheva et al., 1960
Genus Haydenella Reed, 1944
Type species.—Productus kiangsiensis Kayser, 1883.
Haydenella wenganensis (Huang, 1932)
Figures 5.3, 5.4
Linoproductus kiangsiensis var. wenganensis Huang, 1932, p. 49, pl. 3, figs. 16–18.
Haydenella kiangsiensis wenganensis (Huang). Zhan, 1979, p. 81, pl. 5, figs. 1, 2.
Haydenella wenganensis (Huang). Liao, 1980, pl. 6, figs. 32, 33; Zeng et al., 1995, pl. 5, fig. 5.
Haydenella sp. Tazawa, 2008b, p. 24, fig. 3.20.
Material.—Four specimens: (1) external and internal moulds of a ventral valve, GSJ F18047; (2) internal moulds of three ventral valves, GSJ F18048—18050.
Description.—Shell small size for genus, transverse subelliptical in outline, with greatest width at hinge; length 12 mm. width 15 mm in the best preserved specimen (GSJ F18047). Ventral valve strongly and unevenly convex in lateral profile, most convex at umbonal region, slightly convex on anterior half of valve, not geniculated; strongly and unevenly convex in anterior profile, with nearly flat venter and steeply inclined lateral slopes; umbo small, tapering and scarcely projecting over hinge; ears small, slightly convex; no sulcus. External surface of ventral valve nearly smooth, faintly costellate; a row of spine bases at base of ears; numerous, very fine tubercles (pseudopunctae) scattered on decorticated surface of valve (Figure 5.4b). Internal structures of ventral valve obscure.
Remarks.—These specimens are referred to Haydenella wenganensis (Huang, 1932). originally described as Linoproductus kiangsiensis var. wenganensis Huang, 1932 from the upper Permian of Guizhou, southwestern China, in size, shape and external ornament of the ventral valves. The Takakurayama specimens resemble well the shells of H. wenganensis, described or figured by Zhan (1979), Liao (1980) and Zeng et al. (1995) from the Wuchiapingian—Changhsingian of Guangdong, Guizhou and Sichuan, respectively.
Haydenella minuta Sarytcheva (in Sarytcheva and Sokolskaya. 1965, p. 228. pl. 38. figs. 10. 14). from the Dzulfian and Induan stages of Armenia is similar in size, but the Armenian species differs from H. wenganensis in having stronger costae on the ventral valve.
The single ventral valve specimen, described by Shimizu (1961, p. 326, pl. 15, figs. 16, 17) as Linoproductus kiangsiensis (Kayser, 1883) from the upper Permian Maizuru Group of Kawahigashi, Maizuru Belt, south-western Japan, is also a small-sized Haydenella species, but it differs from H. wenganensis in having numerous fine costae on the ventral valve.
Distribution.—Wuchiapingian—Changhsingian: north-eastern Japan (Takakurayama in the South Kitakami Belt), central-southern China (Guangdong) and south-western China (Guizhou and Sichuan).
Superfamily Marginiferoidea Stehli, 1954
Family Marginiferidae Stehli, 1954
Subfamily Marginiferinae Stehli, 1954
Genus Transennatia Waterhouse, 1975
Type species.—Productus gratiosus Waagen, 1884.
Transennatia gratiosa (Waagen, 1884)
Figures 5.5–5.10
Productus gratiosus Waagen, 1884, p. 691, pl. 72, figs. 3–7; Diener, 1897, p. 23, pl. 3, figs. 3–7; Mansuy, 1913. p. 115, pl. 13. fig. 1; Colani, 1919, p. 10, pl. 1, fig. 2; Chao, 1927, p. 44, pl. 4, figs. 6– 10; Chi-Thuan, 1962, p. 491. pl. 2. figs. 5–7.
Productus (Dictyoclostus) gratiosus Waagen. Huang, 1933, p. 88, pl. 11, fig. 14; Hayasaka, 1960, p. 49, pl. 1, fig. 8.
Marginifera gratiosa (Waagen). Reed, 1944, p. 98, pl. 19, figs. 6, 7.
Dictyoclostus gratiosus (Waagen). Zhang and Jin, 1961, p. 411, pl. 4, figs. 12–18; Wang et al., 1964, p. 291, pl. 45, figs. 14–19; Leman, 1994, pl. 1, figs. 11–13.
Gratiosina gratiosa (Waagen). Grant, 1976, pl. 33, figs. 19–26; Licharew and Kotlyar, 1978, pl. 12, figs. 5. 6; pl. 20. fig. 1; Minato et al., 1979, pl. 61, figs. 11–13.
Asioproductus gratiosus (Waagen). Yang et al., 1977, p. 350, pl. 140, fig. 5; Feng and Jiang. 1978. p. 254, pl. 90. figs. 1, 2; Tong, 1978, p. 228. pl. 80, fig. 7; Lee et al., 1980, p. 373, pl. 164, fig. 14; pl. 166, figs. 5, 6.
Gratiosina sp., Minato et al., 1979, pl. 61, fig. 14.
Dictyoclostus minor Lee and Gu in Lee et al., 1980, p. 372, pl. 166, figs. 1–4.
Transennatia gratiosus (Waagen). Wang et al., 1982, p. 214, pl. 92, figs. 6–8; pl. 102, figs. 4–9; Liu et al., 1982, p. 185, pl. 132, fig. 9; Ding and Qi, 1983, p. 280, pl. 95, fig. 14; Zeng et al., 1995, pl. 5, figs. 14, 15.
Transennatia gratiosa (Waagen). Yang, 1984, p. 219, pl. 33, fig. 7; Jin, 1985, pl. 4, figs. 33, 34, 45, 46; Tazawa and Matsumoto, 1998, p. 6, pl. 1, figs. 4–8; Tazawa et al., 2000, p. 7, pl. 1, figs. 3–5; Tazawa, 2001, p. 289, figs. 6.1–6.7; Tazawa and Ibaraki, 2001, p. 7, pl. 1, figs. 1–3; Shen et al., 2002, p. 676, figs. 4.27–4.31; Tazawa, 2002, fig. 10.2; Chen et al., 2005. p. 354. figs. 10E–10H, 11; Tazawa, 2008b, p. 26, fig. 4.1; Tazawa, 2008c, p. 43, figs. 6.6, 6.7; Shen and Zhang, 2008, figs. 4.20–4.22; Shen and Clapham, 2009, p. 718, pl. 1, figs. 13–22; Shen and Shi. 2009. p. 157. figs. 3K–3O.
Material.—Sixteen specimens: (1) external mould of a ventral valve, GSJ F18052; (2) internal moulds of two ventral valves, GSJ F18053, 18054; (3) external moulds of thirteen dorsal valves, GSJ F18051, 18055–18066.
Description.—Shell small size for genus, transversely subquadrate in outline, with greatest width at hinge; length 13 mm, width about 20 mm in the largest dorsal valve specimen (GSJ F18066). Ventral valve strongly and unevenly convex in lateral profile, most convex at umbonal region, strongly geniculated and followed by long trail; umbo small, slightly incurved; ears small, pointed; sulcus narrow and deep; lateral slopes steep. Dorsal valve nearly flat on visceral disc, strongly geniculated and followed by long trail; fold invisible on visceral disc, but highly developed on trail. External surface of both valves reticulate on visceral disc and costate on trail; costae converging into ventral sulcus and dorsal fold anteriorly; numbering 7 costae in 5 mm at midlength of dorsal valve; spines or spine bases absent. Ventral interior with small, elongate, highly raised adductor scars and large, flabellate, longitudinally striated diductor scars in posterior portion of valve. Dorsal interior not observed.
Remarks.—These specimens are referred to Transennatia gratiosa (Waagen, 1884). originally described from the Wargal and Chhidru formations of the Salt Range, in their small size, strongly convex ventral valve and sharply reticulate ornament on the visceral disc of both valves, although the Takakurayama specimens are smaller in size than the type specimens from the Salt Range (Waagen, 1884, pl. 72. figs. 3–7).
Transennatia insculpta (Grant, 1976, p. 135. pl. 32. figs. 1–37; pl. 33, figs. 1–36), from the Ratburi Lime-stone of Ko Muk, southern Thailand, is also a small-sized Transennatia species, but it differs from T. gratiosa in its more transverse outline, more prominent ears and coarser costae on the ventral valve.
Distribution.—Wordian—Changhsingian: northwestern China (Shaanxi), northeastern China (Heilongjiang and Jilin), Russian Far East (South Primorye), northeastern Japan (Setamai, Kamiyasse, Kesennuma, Ogatsu and Takakurayama in the South Kitakami Belt), central Japan (Moribu and Oguradani in the Hida Gaien Belt), south-western Japan (Mizukoshi in Kyushu Island, western extension of the Hida Gaien Belt), eastern China (Zhejiang, Anhui and Jiangxi), central-southern China (Hubei, Hunan, Guangdong and Guangxi), southwestern China (Guizhou, Sichuan and Yunnan), Tibet (Xizang), Vietnam, Cambodia (Sisophon), Malaysia, Nepal (Kumaon Himalayas). Pakistan (Salt Range) and Greece (Hydra Island).
Family Paucispiniferidae Muir-Wood and Cooper, 1960
Subfamily Paucispiniferinae Muir-Wood and Cooper, 1960
Tribe Paucispiniferini Muir-Wood and Cooper, 1960
Genus Lamnimargus Waterhouse, 1975
Type species.—Marginifera himalayensis Diener, 1899.
Lamnimargus peregrinus (Fredericks, 1924b)
Figures 5.11–5.13
Paramarginiferci peregrina Fredericks, 1924b, p. 24, pl. 1, figs. 7, 8; Fredericks, 1925, p. 12, pl. 1, figs. 41–44.
Dictyoclostus zesiensis Lee and Gu, 1976, p. 256, pl. 167, figs. 5, 6; pl. 170, fig. 1.
Probolionia caucasica peregrina (Fredericks). Licharew and Kotlyar, 1978, p. 12, figs. 13, 14.
Paramarginifera? peregrina Fredericks. Duan and Li, 1985, p. 112, pl. 42, figs. 1–7; Lee et al. 1980. p. 356, pl. 166, figs. 18, 28.
Lamnimargus himalayensis (Diener). Kotlyar, 1989, pl. 23, fig. 9.
Lamnimargus peregrina (Fredericks). Wang and Zhang, 2003, p. 73, pl. 14, figs. 3, 8, 9; pl. 15. fig. 11; pl. 21, figs. 14–16, 22–24.
Lamnimargus peregrinus (Fredericks). Tazawa, 2008a, p. 7, figs. 3, 4; Tazawa, 2008b, p. 25, figs. 4.2–4.4; Tazawa in Tazawa and Miyake, 2011, p. 4, figs. 3.1–3.3; Tazawa, 2012. p. 21, figs. 4.4, 4.5.
Material.—Eleven specimens: (1) internal moulds of three ventral valves, with fragments of external moulds of the valves. GSJ F18067–18069; (2) an abraded ventral valve, GSJ F18070; (3) internal moulds of seven ventral valves, GSJ F18071–18077.
Description.—Shell medium size for genus; length 15 mm. width 18 mm in corpus of the largest specimen (GSJ F18067). Ventral valve strongly and unevenly convex in lateral profile, most convex at umbonal slope, bluntly geniculated and followed by long trail; sulcus narrow and deep, originating near umbo and extending to anterior margin; lateral slopes steep. External surface of ventral valve ornamented by numerous costae, numbering 3 in 2 mm at midlength of trail; a row of fine spines along hinge. Ventral interior with a pair of elongate, highly raised, smooth adductor scars and impressed, wide, oval and striated diductor scars; marginal ridge developed at anterior portion.
Remarks.—The material available is fragmentarily preserved internal moulds of ventral valves, lacking ears. However, the Takakurayama specimens are referred to Lamnimargus peregrinus (Fredericks, 1924b), from the Chandalaz Formation of the Vladivostok area, South Primorye, Russian Far East, by their size, shape, and external ornamentation of the ventral valve. The Takakurayama specimens resemble well the specimens of L. peregrinus described and figured by Tazawa (2008a, p. 7, figs. 3, 4) from the upper Toyoma Formation of Maeda, Ofunato area, South Kitakami Belt.
The type species, Lamnimargus himalayensis (Diener, 1899, p. 39, pl. 2, figs. 1–7; pl. 6, figs. 1, 2), from the Kuling Shales of the Punjab Himalayas, Kashmir, is distinguished from L. peregrinus by its much larger, prominent ears and coarser costellae on the ventral valve.
Distribution.—Wordian—Changhsingian: northern China (Inner Mongolia), northeastern China (Heilongjiang), Far Eastern Russia (South Primorye) and northeastern Japan (Ofunato, Kesennuma and Takakurayama in the South Kitakami Belt).
Family Costispiniferidae Muir-Wood and Cooper, 1960
Subfamily Costispiniferinae Muir-Wood and Cooper, 1960
Genus Echinauris Muir-Wood and Cooper, 1960
Type species.—Echinauris lateralis Muir-Wood and Cooper, 1960.
Echinauris sp.
Figure 6.1
Material—Two specimens: (1) external mould of a ventral valve, GSJ F18078; (2) external mould of a dorsal valve, GSJ F18079.
Description.—Shell small size for genus, transversely subcircular in outline, with greatest width at midlength; length 11 mm, width 13 mm in the ventral valve specimen (GSJ F18078). Ventral valve strongly and unevenly convex in lateral profile, maximum curvature in posterior third, not geniculated; anterior profile domed, with steep sides; umbonal region broadly swollen; ears small; sulcus wide and shallow in anterior slope. Dorsal valve evenly and moderately concave; ears small, flattened; fold absent. External surface of ventral valve ornamented by very fine growth lines and numerous stout spines, scattered over venter, numbering 2–3 in 3 mm width at about midlength of valve. External ornament of dorsal valve similar to the opposite valve. Internal structure of both ventral and dorsal valves not preserved.
Remarks.—This specimen is safely assigned to the genus Echinauris by its small size, subcircular outline, and numerous stout spines on the ventral valve. The Takakurayama specimen resembles Echinauris irregularis Cooper and Grant (1975, p. 1008. pl. 329. figs. 1– 24; pl. 330. figs. 1–43; pl. 331. figs. 1–34; pl. 332. figs. 25–32). from the upper Wolfcampian and Leonardian of western Texas, but differs from the latter in having no interrupted costae on the ventral valve.
The type species, Echinauris lateralis Muir-Wood and Cooper (1960, p. 222, pl. 68, figs. 1–13). from the Word Formation of the Glass Mountains, western Texas, is distinguished from the Takakurayama species by its larger and elongate oval shell.
Echinauris opuntia (Waagen, 1884, p. 707, pl. 79, figs. 1, 2), from the Wargal Formation of the Salt Range, is distinguished from the present species by its slightly elongate outline and less stout spines on the ventral valve.
Superfamily Linoproductoidea Stehli, 1954
Family Linoproductidae Stehli, 1954
Subfamily Anidanthinae Waterhouse, 1968b
Genus Megousia Muir-Wood and Cooper, 1960
Type species.—Megousia auriculata Muir-Wood and Cooper, 1960.
Megousia auriculata Muir-Wood and Cooper, 1960
Figure 6.2
Linoproductus waagenites Girty. King, 1931. p. 77, pl. 17, figs. 11–15 only.
Megousia auriculata Muir-Wood and Cooper, 1960, p. 310, pl. 113, figs. 1–11; Ferguson, 1969, pl. 1, figs. 5–11; Nakamura, 1972, p. 436, pl. 2, fig. 3; Cooper and Grant, 1975, p. 1192. pl. 450, figs. 1–48; pl. 451, figs. 1–49; pl. 452, figs. 19–28; pl. 453, figs. 13– 24; pl. 463, figs. 5–8; pl. 467. figs. 9–13; Tazawa, 2012. p. 25, figs. 3.8–3.10.
Megousia cf. auriculata Nakamura, 1972, p. 437, pl. 2, fig. 2.
Material.—Two specimens: (1) internal mould of a conjoined shell, with external mould of the ventral valve, GSJ F18080; (2) internal mould of a ventral valve, GSJ F18081.
Description.—Shell medium size for genus, transversely wider subcylindrical in outline, with greatest width at hinge; length 11 mm. width about 24 mm in the better preserved specimen (GSJ F18080). Ventral valve strongly convex in lateral profile; umbo narrowly swollen; ears large, prominent; lateral slopes steep. Dorsal valve moderately convex in lateral profile, strongly geniculated and followed by short trail; ears long, slender; fold broad and low on trail. External surface of dorsal valve ornamented by strong, irregular concentric lamellae and numerous fine costellae, numbering 8–9 in 5 mm at midlength; ears also ornamented with radial costellae. Ventral interior with a pair of small, triangular adductor scars and large, flabellate diductor scars. Dorsal interior with a small, short-shafted and trilobed cardinal process, and strong, short median septum.
Remarks.—The Takakurayama specimens can be referred to Megousia auriculata Muir-Wood and Cooper, 1960, originally described from the Word Formation of the Glass Mountains, western Texas, by their small, very transverse shell and in having radially ribbing, slender ears on the dorsal valve.
Megousia nakamurai Tazawa (1975, p. 635. pl. 3, figs. 5, 6). from the upper Toyoma Formation of Nabekoshiyama, Kesennuma area, South Kitakami Belt, is readily distinguished from M. auriculata by its less transverse outline, much broader ears and finer costellae on the dorsal valve.
Distribution.—Artinskian—Changhsingian: northeastern Japan (Kesennuma and Takakurayama in the South Kitakami Belt) and the United States (western Texas).
Figure 6.
1, Echinauris sp., ventral view of external latex cast of ventral valve, GSJ F18078; 2, Megousia auriculata Muir-Wood and Cooper; 2a, 2b, 2c, dorsal view of external mould of dorsal valve, and ventral and dorsal views of internal mould of conjoined shell, GSJ F18080; 3, Costatumulus tazawai Shen, Archbold, Shi and Chen; 3a, 3b, 3c, 3d, 3e, ventral views of external latex cast and internal mould of ventral valve, ventral and dorsal views of internal mould of conjoined shell, and dorsal view of external mould of dorsal valve, GSJ F18082; 4–6, Yakovlevia mammatiformis (Fredericks); 4, dorsal view of external mould of dorsal valve, GSJ F18088; 5, external latex cast of ventral valve, GSJ F18086; 6, ventral view of internal mould of ventral valve, GSJ F18087; 7, Hustedia ratburiensis Waterhouse and Piyasin, dorsal view of internal mould of dorsal valve, GSJ F18090; 8, Neospirifer sp.; 8a, 8b, dorsal views of external latex cast and internal mould of dorsal valve, GSJ F18091; 9, Gypospirifer kobiyamai Tazawa and Araki; 9a, 9b, dorsal views of external latex cast and internal mould of dorsal valve, GSJ F18092; 10, Alispiriferella sp.; 10a, 10b, ventral views of internal latex cast and internal mould of ventral valve, GSJ F18094; 11, 12, Pterospirifer alatus (von Schlotheim); 11a, 11b, 11c, ventral view of external latex cast of ventral valve, internal mould of ventral valve, and microornamentation of ventral valve, GSJ F18095; 12a, 12b, dorsal view of external latex cast and external mould of dorsal valve, GSJ F18096; 13, Dielasma sp., 13, ventral view of internal mould of ventral valve, GSJ F18097. Scale bars represent 1 cm.
Family Kansuellidae Mur-Wood and Cooper, 1960
Subfamily Auriculispininae Waterhouse, 1986
Tribe Auriculispinini Waterhouse, 1986
Genus Costatumulus Waterhouse, 1983
Type species.—Auriculispina tumida Waterhouse in Waterhouse et al., 1983.
Costatumulus tazawai Shen, Archbold, Shi and Chen, 2000
Figure 6.3
Costatumulus tazawai Shen, Archbold, Shi and Chen, 2000, p. 743, figs. 12.1–8, 11–14.
Costatumulus cf. tazawai Shen, Archbold, Shi and Chen. Tazawa, 2008b, p. 27, fig. 4.5.
Material.—Four specimens: (1) external moulds of a conjoined shell, with internal mould of the dorsal valve, GSJ F18082; (2) external moulds of three dorsal valves, GSJ F18083–18085.
Description.—Shell medium size for genus, suboval in outline; length more than 29 mm. width more than 22 mm in the largest specimen (GSJ F18082); length about 12 mm, width about 20 mm in the best preserved dorsal valve specimen. Ventral valve gently convex in lateral profile, with maximum convexity at umbonal slope; ears not preserved; no sulcus. Dorsal valve slightly concave in lateral profile, weakly geniculated and followed by short trail; no fold. External surface of ventral valve ornamented by strong, irregular concentric rugae, numerous, fine costellae and numerous, fine, recumbent spines on whole venter of valve; numbering 12–13 costellae in 5 mm at midlength. External surface of dorsal valve also ornamented by numerous strong rugae, fine costellae and several dimples, but no spines.
Remarks.—The specimens from Takakurayama are incompletely preserved and lacking most of the ears, but they can be referred to Costatumulus tazawai Shen, Archbold, Shi and Chen, 2000. from the middle part of the Selong Group of Selong, southern Xizang (Tibet), southwestern China, by their concavo-convex shell, with relatively coarse rugae and fine costellae on both ventral and dorsal valves. Costatumulus cf. tazawai, described by Tazawa (2008b), from the upper part of the Takakurayama Formation in the Takakurayama area, may be conspecific with the present species.
Costatumulus polliciformis (Waterhouse, 1978, p. 76. pl. 11, figs. 9–12). from the lower Senja Formation of northwest Nepal, differs from C. tazawai in having less strong rugae and coarser costellae on the ventral valve.
The type species. Costatumulus tumida (Waterhouse in Waterhouse et al., 1983. p. 133. pl. 3. figs. 2–4, 6–7). from the Tiverton Fonnation of northern Bowen Basin, eastern Australia, is readily distinguished from C. tazawai by its less strong rugae on the ventral valve.
Distribution.—Wuchiapingian: northeastern Japan (Takakurayama in the South Kitakami Belt) and Xizang (Tibet).
Family Yakovleviidae Waterhouse, 1975
Genus Yakovlevia Fredericks, 1925
Type species.—Yakovlevia kaluzinensis Fredericks, 1925.
Yakovlevia mammatiformis (Fredericks, 1926)
Figures 6.4–6.6
Productus mammatiformis Fredericks, 1926, p. 87, pl. 3, figs. 4–6.
Muirwoodia mammatiformis (Fredericks). Solomina, 1960, p. 65, pl. 12, figs. 7, 8; Kulikov, 1974, p. 89, pl. 3, fig. 6.
Yakovlevia mammatiformis (Fredericks). Kotlyar, 1961, text-figs. 7, 8; Ustritsky, 1963. p. 12, pl. 2, figs. 6–8; pl. 3, figs. 1–3; Mironova. 1964, p. 97, pl., fig. 14; Zavodowsky and Stepanov, 1970, p. 114, pl. 35. figs. 8–10; Ifanova, 1972, p. 119, pl. 6, figs. 15. 16; pl. 7, figs. 1. 2; Kalashnikov, 1983, p. 210, pl. 49, figs. 5, 6, 9; Manankov, 1991, p. 107, pl. 26, figs. 1–3; Kalashnikov, 1993, p. 61, pl. 16, figs. 1–4; Tazawa, 1999a, fig. 2.3; Tazawa, 1999b, p. 92, fig. 3.6; Klets et al., 2001, pl. 2, fig. 9; Klets, 2005, pl. 32, fig. 9; Tazawa, 2008b, p. 29, fig. 4.13; Tazawa, 2011. p. 175, fig. 4.8.
Linoproductus cf. mammatus (Keyserling). Yanagisawa, 1967, p. 88, pl. 2, fig. 7.
Yakovlevia sp. Tazawa and Niigata Pre-Tertiary Research Group, 1999, fig. 12 only.
Material.—Four specimens: (1) external mould of a ventral valve, GSJ F18086; (2) internal mould of a ventral valve, GSJ F18087; (3) external and internal moulds of a dorsal valve, GSJ F18088; (4) external mould of a dorsal valve, GSJ F18089.
Description.—Shell medium size for genus, transversely subrectangular in outline, with greatest width at hinge; length about 20 mm. width more than 36 mm in the largest ventral valve specimen (GSJ F18086). Ventral valve gently convex on visceral disc, strongly geniculated and followed by long trail; umbo small; sulcus narrow and moderately deep; lateral slopes steep. Dorsal
valve almost flat on visceral disc, strongly geniculated and followed by long trail. External surface of both valves ornamented by numerous fine costellae; numbering 10–11 costellae in 5 mm at midlength of ventral valve; 12–13 costellae in 5 mm at midlength of dorsal valve; a thick halteroid spine on lateral slope of ventral valve. Ventral interior with a pair of small, elongate adductor scars and large flabellate diductor scars, encircled by a ridge posterolaterally.
Remarks.—These specimens can be referred to Yakovlevia mammatiformis (Fredericks, 1926). from the lower Pennian (Artinskian) of the Pechora Basin, northern Russia, in size, shape and external ornament of the ventral valve.
Yakovlevia transversa (Cooper, 1957, p. 39. pl. 5, figs. 1–13). from the Coyote Butte Formation of central Oregon, resembles Y. mammatiformis in general appearance, but the former is distinguished from the latter by its smaller dimensions, more strongly developed fold commencing a little below the umbo, and coarser costellae on both the ventral and dorsal valves.
Yakovlevia mammata (Keyserling, 1846, p. 206, pl. 4, fig. 5), from the lower Pennian (possibly Sakmarian) of the Pechora Basin, differs from Y. mammatiformis in its smaller size, less transverse outline, and external ornament consisting of more numerous, finer costellae.
Distribution.—Sakmarian—Wuchiapingian: Russian Arctic and Far North (Novaya Zemlya, Pechora Basin, Pai Khoi, northern Urals, Verkhoyansk and Omolon Massif), southern Mongolia, northwestern China (Gansu), Russian Far East (South Primorye), northeastern Japan (Takakurayama in the South Kitakami Belt) and central Japan (Okutadami in the Joetsu Belt).
Order Athyridida Boucot, Johnson and Staton, 1964
Suborder Retziidina Boucot, Johnson and Staton, 1964
Superfamily Retzioidea Waagen, 1883
Family Neoretziidae Dagys, 1972
Subfamily Hustediinae Grunt, 1986
Genus Hustedia Hall and Clarke, 1893
Type species.—Terebratula mormoni Marcou, 1858.
Hustedia ratburiensis Waterhouse and Piyasin, 1970
Figure 6.7
Hustedia ratburiensis Waterhouse and Piyasin, 1970, p. 138, pl. 23, figs. 15–30; Grant, 1976, p. 241, pl. 66, figs. 1–69; pl. 67, figs. 51–58; Archbold, 1999, figs. 5E–5H; Yanagida and Nakornsri, 1999, p. 118, pl. 32, figs. 11–16; Tazawa, 2001, p. 299, fig. 8.6; Tazawa, 2008c, p. 53, figs. 8.2–8.6.
Hustedia thailandica Waterhouse and Piyasin, 1970, text-figs. 12, 13.
Hustedia nakornsrii Yanagida, 1970, p. 79, pl. 14, fig. 9.
Material.—One specimen, internal mould of a dorsal valve, GSJ F18090.
Remarks.—The single dorsal valve specimen from Takakurayama can be referred to Hustedia ratburiensis Waterhouse and Piyasin, 1970, from the Ratburi Limestone of Khao Phrik, southern Thailand, by its small size (length 7 mm, width 7 mm), subcircular outline, and nine strong costae on the dorsal valve.
Hustedia grandicosta (Davidson, 1862). redescribed by Waagen (1883, p. 491, pl. 34, figs. 6–12). from the Amb, Wargal and Chhidru formations of the Salt Range, differs from H. ratburiensis in having more numerous, fine costae on the dorsal valve.
Hustedia minuta Tazawa and Miyake (2011, p. 15, figs. 2.5, 2.6), from the upper Toyoma Formation of Maeda, in the Ofunato area, South Kitakami Belt, is distinguished from the present species by its smaller size and in having more numerous costae on the dorsal valve.
Distribution.—Kungurian—Wuchiapingian: northeastern Japan (Takakurayama in the South Kitakami Belt), central Japan (Moribu in the Hida Gaien Belt), southwestern Japan (Mizukoshi, central Kyushu) and southern Thailand (Khao Phrik, Khao Nong Ta On and Khao Hin King).
Order Spiriferida Waagen, 1883
Suborder Spiriferidina Waagen, 1883
Superfamily Spiriferoidea King, 1846
Family Trigonotretidae Schuchert, 1893
Subfamily Neospiriferinae Waterhouse, 1968a
Genus Neospirifer Fredericks, 1924a
Type species.—Spirifer fasciger Keyserling, 1846.
Neospirifer sp.
Figure. 6.8
Material.—One specimen, external and internal moulds of a dorsal valve. GSJ F18091.
Remarks.—This specimen is safely assigned to the genus Neospirifer by its transverse, medium-sized dorsal valve (length about 25 mm, width about 40 mm), with high, broad fold and ornamented by numerous fasciculate costae and very fine concentric growth lamellae. The Takakurayama species resembles the type species, Neospirifer fasciger (Keyserling, 1846), from the lower Permian of Timan, northern Russia in size and shape of the dorsal valve, but accurate comparison is difficult because of the former's ill preserved cardinal extremities. N. fasciger is characterized by its rounded cardinal extremities (Poletaev, 1997, pl. 4, figs. 2–7).
Genus Gypospirifer Cooper and Grant, 1976
Type species.—Gypospirifer nelsoni Cooper and Grant, 1976.
Gypospirifer kobiyamai Tazawa and Araki, 2013
Figure 6.9
Spirifer fasciger var. simplex Grabau. Kobiyama, 1956, fig. 4.
Neospirifer fasciger (Keyserling). Hayasaka, 1960, p. 42, pl. 2, figs. 1, 2 only; Yanagida, 1963, p. 71, pl. 8, figs. 3, 6; pl. 9, fig. 3 only; Koizumi, 1979, pl. 1, fig. 16 only.
Neospirifer aff. cameratus Morton, Yanagisawa, 1967, p. 91, pl. 2, fig. 11.
Neospirifer striato-paradoxus (Toula). Licharew and Kotlyar, 1978, pl. 18, fig. 1; Lee et al., 1980, p. 412, pl. 177, figs. 3, 6, 9.
Gypospirifer volatilis Duan and Li. Tazawa, 2001, p. 302, figs. 8.23– 8.26; Tazawa and Hasegawa, 2007, p. 7, figs. 4.8–4.12, 5.1, 5.2; Tazawa, 2008b, p. 39, fig. 6.17; Tazawa, 2008c, p. 54, figs. 9.3– 9.7.
Gypospirifer kobiyamai Tazawa and Araki, 2013, p. 7, figs. 2.3, 2.4.
Material.—Two specimens, external and internal moulds of two dorsal valves. GSJ F18092, 18093.
Remarks.—The specimens from Takakurayama are fragmentarily preserved, but they can be referred to Gypospirifer kobiyamai Tazawa and Araki, 2013, from the lower Kamiyasse Formation of Kamiyasse, South Kitakami Belt, northeastern Japan, by their large size (length more than 40 mm, width more than 65 mm in the larger specimen, GSJ F18093). narrow and high dorsal fold, and numerous fine, often bifurcated and weakly fasciculated costae (numbering 10–12 in 10 mm at mid-length) on the dorsal valve.
Gypospirifer volatilis Duan and Li (1985, p. 127, 207, pl. 48, figs. 1, 2; pl. 49, figs. 1, 2), from the Zhesi (Jisu) Formation of Zhesi, Inner Mongolia, northern China, differs from G. kobiyamai in having a much broader and higher fold on the dorsal valve.
Gypospirifer gryphus Cooper and Grant (1976, p. 2211, pl. 591, figs. 1–5). from the Graham Formation of western Texas, differs from G. kobiyamai in its more transverse outline.
Distribution.—Wordian—Wuchiapingian: northeastern China (Heilongjiang), Far Eastern Russia (South Primorye), northeastern Japan (Kesennuma and Takakurayama in the South Kitakami Belt), central Japan (Moribu in the Hida Gaien Belt) and southwestern Japan (Mizukoshi, central Kyushu, western extension of the Hida Gaien Belt).
Family Spiriferellidae Waterhouse, 1968a Genus Alispiriferella Waterhouse and Waddington, 1982
Type species.—Spirifer (Spiriferella) keilhavii (von Buch) var. ordinaria Einor in Licharew and Einor, 1939.
Alispiriferella sp.
Figure 6.10
Material.—One specimen, internal mould of a ventral valve, with external mould of the ventral interarea, GSJ F18094.
Remarks.—The material available is lacking external ornament of the ventral valve, but it resembles well the specimens of Alispiriferella lita (Fredericks, 1924b), described and figured by Tazawa (2008c, p. 55, figs. 9.8– 9.14) from the Mizukoshi Formation of Mizukoshi, central Kyushu, southwestern Japan. It is characterized by its large, transverse shell (length more than 30 mm, width more than 35 mm) and in having a pair of high dental plates and a deeply impressed heart-shaped muscle field in the ventral valve. The Takakurayama specimen may be an Alispiriferella species, although specific identification is difficult.
Superfamily Paeckelmanelloidea Ivanova, 1972
Family Strophopleuridae Carter, 1974
Subfamily Pterospiriferinae Waterhouse, 1975
Genus Pterospirifer Dunbar, 1955
Type species.—Spirifer alatus von Schlotheim, 1813.
Pterospirifer alatus (von Schlotheim, 1813)
Figures 6.11, 6.12
Spirifer alatus von Schlotheim, 1813, p. 87, pl. 16, figs. 1–7.
Pterospirifer alatus (von Schlotheim). Dunbar, 1955, p. 129, pl. 22, figs. 19–25; Ivanova, 1981, p. 33, pl. 1, figs. 1–3; pl. 2, fig. 1; text-figs. 1, 2b, 4a, 5, 10; Nakamura et al., 1992, pl. 5, figs. 1–3.
Material.—Two specimens: (1) external mould of a ventral valve. GSJ F18095; (2) external mould of a dorsal valve. GSJ F18096.
Remarks.—The Takakurayama specimens are fragmentarily preserved, but they can be referred to Pterospirifer alatus (von Schlotheim, 1813), from the lower Zechstein of Germany, in their large, transverse shells (length about 25 mm, width more than 57 mm in the better preserved dorsal valve specimen, GSJ F18096), with strongly alate cardinal extremities, broad, moderately high and smooth dorsal fold, numerous simple costae and microornament consisting of numerous very fine capillae and growth lamellae (Figure 6.11c) on both the ventral and dorsal valves.
Pterospirifer terechovi Zavodowsky (1968. p. 150. pl. 43. fig. 1). from the lower Permian of Kolyma, northeastern Russia, differs from P. alatus in its much larger size.
Distribution.—Capitanian—Wuchiapingian: eastern Greenland, Spitsbergen, Germany and northeastern Japan (Takakurayama in the South Kitakami Belt).
Order Terebratulida Waagen, 1883
Suborder Terebratulidina Waagen, 1883
Superfamily Dielasmatoidea Schuchert, 1913
Family Dielasmatidae Schuchert, 1913
Subfamily Dielasmatinae Schuchert, 1913
Genus Dielasma King, 1859
Type species.—Terebratulites elongains von Schlotheim, 1816.
Dielasma sp.
Figure 6.13
Material.—Two specimens, internal moulds of two ventral valves, GSJ F18097, 18098.
Remarks.—These specimens are safely assigned to the genus Dielasma by their medium size (length 17 mm, width 14 mm in the better preserved specimen, GSJ F18097), elongate, subpentagonal outline of the shell and gently convex dorsal valve with a shallow sulcus and a pair of strong dental plates. The Takakurayama species resembles well and may be identical with Dielasma sp., described by Tazawa (2008c, p. 57, figs. 9.15–9.18) from the upper part of the Mizukoshi Formation of Mizukoshi, central Kyushu, southwest Japan, in size and outline of the dorsal valve, but accurate comparison is difficult because of the poor preservation of the present material.
Acknowledgements
Sincere thanks are due to Kazuo Hashimoto (Iwaki City, Fukushima Prefecture) for his help in the field; and two reviewers, Shen Shuzhong (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing) and G. R. Shi (School of Life and Environmental Sciences, Deakin University, Melbourne) for their critical review of the manuscript by which this paper is greatly improved.
