Charophyte Gyrogonites from the Lower Cretaceous Kitadaniformationofthetetorigroupinthe Takinamigawa Area, Katsuyama City, Fukui Prefecture, Central Japan

Home , Akaiwa Formation, Kitadani Formation, Tetori Group

Paleontological Research, vol. 9, no. 2, pp. 203–213, June 30, 2005 6 by the Palaeontological Society of Japan

Charophyte gyrogonites from the Lower Cretaceous KitadaniFormationoftheTetoriGroupinthe Takinamigawa area, Katsuyama City, Fukui Prefecture, central Japan

KATSUHIRO KUBOTA

Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki, 305-8572, Japan (e-mail: [email protected])

Received December 22, 2004; Revised manuscript accepted April 21, 2005

Abstract. Well preserved charophyte gyrogonites are discovered from the Kitadani Formation of the Te- tori Group in the Takinamigawa area, Katsuyama City, Fukui Prefecture, central Japan. The Kitadani charoﬂora described in this paper is composed of ﬁve species of three genera, Clavator harrisii var. reyi (Grambast-Fessard), Mesochara harrisi (Ma¨dler), Mesochara stipitata (Wang), Mesochara sp., and Stel- latochara sp. C. harrisii var. reyi is a member of Clavatoraceae, which is a biostratigraphically well established charophyte group and is unique in Upper Jurassic to Cretaceous nonmarine deposits. The occurrences of C. harrisii var. reyi associated with other charophytes indicate that the charophyte-rich horizon of the Kitadani Formation is assigned to the Barremian, which well agrees with the age estimation established by the nonmarine molluscan assemblage.

Key words: charophyta, Early Cretaceous, Fukui Prefecture, Kitadani Formation, Takinamigawa area, Tetori Group

Introduction to fresh water. The body consists of a stem and fructifications. Since the fructification is calcified or Biostratigraphic correlation among Cretaceous preserved as an impression and has a rapid evo- nonmarine deposits in Japan has been studied by lutionary rate, it is suitable for an index fossil in non- many workers, who are dependent mainly on nonmarine deposits (Martin-Closas, 1996). Cumulative marine molluscan assemblages (Kobayashi and Su- knowledge on fossil charophytes reveals that they are zuki, 1936; Matsumoto et al., 1982; Tamura, 1990). available for biostratigraphic correlation (Grambast, Some of these nonmarine molluscs, which were found 1974; Schudack, 1987a, b; Riveline et al., 1996). from nonmarine strata interbeded with marine depos- In this study, well preserved charophyte gyrogonites its, have been used as index fossils (Isaji, 1993; Tashiro are discovered from the Kitadani Formation of the and Okuhira, 1993). However, the age determinations Tetori Group in the Takinamigawa area, Katsuyama by nonmarine molluscan assemblages in these studies City, Fukui Prefecture, central Japan (Figure 1). The are based upon limited materials. Recently, other fossils occurrence provides a new basis for the biostrati- from Cretaceous sediments, such as ostracods, spores, graphic correlation based on microfossils in Japan. and pollen, have been used for biostratigraphic cor- Hereinafter, this paper describes the charophyte gy- relation among nonmarine deposits in Japan (Haya- rogonites from the Kitadani Formation and dis- shi, 2001; Umetsu and Matsuoka, 2003). Occurrences cusses the age of this formation based on charophyte of charophytes have also been reported from several biostratigraphy. Cretaceous nonmarine deposits in Japan (Iwasaki and Tamura, 1990; Iwasaki, 1994; Komatsu et al., 2003; Geologic setting Isaji et al., 2005), but these charophytes from Japan have not been used for an age determination yet. The stratigraphy in the Takinamigawa area, Kat- Charophytes are green algae living in brackish suyama City, Fukui Prefecture, central Japan has been 204 Katsuhiro Kubota studied by Inai (1950), Maeda (1958, 1961b), Kawai composed of sandstone and mudstone in its lower part (1961), Tsukano (1969), Omura (1973), and Matsu- and sandstone and well rounded and poorly sorted kawa et al. (2003). This paper follows the stratigra- orthoquartzite pebbles, which are of smaller size and phy and correlation of Maeda (1958, 1961b) based on quantity than those of the Akaiwa Formation, in its personal data of the author. upper part. The lower part of the Kitadani Formation The Tetori Group, distributed in the Hida region, yields abundant vertebrates, nonmarine molluscs, os- Inner Zone of Southwest Japan, is divided into the tracods, and plants (Tamura, 1990; Isaji, 1993; Azuma Kuzuryu (mainly shallow marine strata), Itoshiro and Tomida, 1995; Cao, 1996; Yabe et al.,2003),while (nonmarine strata along with shallow marine beds), the upper part contains only a few plant fossils. Some and Akaiwa subgroups (mainly nonmarine strata), in sandstones of the lower part of this formation are ascending order (Maeda, 1961a). The Akaiwa Sub- green in color and were previously identified as tuffa- group is distributed in this study area and consists of ceous rocks (Maeda, 1958; Tsukano, 1969). Under the Akaiwa and Kitadani formations (Maeda, 1958, microscopic observation, the sandstone is composed 1961b). mainly of quartz grains and chlorite with a minor The Akaiwa Formation, which underlies the Kita- amount of tourmaline, zircon, garnet, and opaque dani Formation, is mainly characterized by fine- to minerals. medium-grained sandstone and mudstone in its lower The charophyte gyrogonites were collected from a part and coarse-grained sandstone and well rounded calcareous mudstone bed 30 cm in thickness, which is and poorly sorted orthoquartzite pebbles in its upper referred to the lower part of the Kitadani Formation, part. The lower part of this formation contains Myrene in the reach of the Sugiyamagawa River, a tributary sp. cf. M. tetoriensis, Myrene ? sp., and generically of the Takinamigawa River (Figure 1). The mudstone unidentified gastropods. The Kitadani Formation is bed is bioturbated and contains fish scales, nonmarine

Figure 1. Map showing the location of the study area. A. the Japanese Islands, B. Magnified map of area (B) surrounded by bold lines shown in fig. A, C. Geographic map of area (C) shown in fig. B. This is a part of 1 : 25,000 map of ‘‘Kitadani’’ published by the Geographical Survey Institute of Japan. Charophyte gyrogonites from the Lower Cretaceous Kitadani Formation of the Tetori Group 205

ophyte gyrogonites were obtained from only one horizon of the lower part of the Kitadani Formation (Figure 1). To extract charophyte gyrogonites, muddy rocks were shattered by a rock hammer. Well preserved gyrogonites were picked from the shattered rock powder and mounted on a stub under a stereo- scopic microscope for observation by scanning electron microscope (JEOL JSM-5500LV). The extract- ing method using sodium tetraphenylborate (Na[B(C6H5)4]) (Yasuda et al., 1985) was also exam- ined, but it damaged the surface of the gyrogonites. Systematic description of charophyta is based on the fructification. The terminology of charophyta is summarized by Horn af Rantzien (1956), whose terminology is widely accepted. This paper fundamen- tally uses the terminology of Horn af Rantzien (1956) partly modified by Schudack (1987b). The statistical evaluation of charophyta with histograms for eight parameters, which consist of five measured (based on photomicrograph by scanning electron microscope) and three computed values, is available for their identification. The abbreviations of each parameter stand for the following meanings: LPA: length of the polar axis, LED: largest equatorial diameter, ISI: isopolarity index (LPA/LED 100), AND: anisopolarity distance, which is from the apical pole to LED calculated along the polar axis, ANI: anisopolarity index (AND/LPA 100), NC: number of convolutions, ECD: equatorial cell diameter, and CDI: cell diameter index (ECD/LPA 1000). Figure 2. Lithologies of the Kitadani Formation in the Ta- kinamigawa area. 1. Photomicrograph of the calcareous mud- Systematic paleontology stone of the Kitadani Formation, under cross-polarized light. Note cross-sections of a charophyte gyrogonite at center, some molluscan fragments above, and a pair of ostracod valves below. Family Clavatoraceae Pia, 1927 Because of prominent recrystallization, their microstructures Subfamily Clavatoroidae (Grambast) emend. Martin- cannot be observed. Scale bar is 0.5 mm. 2. Photomicrograph of Closas, ex Schudack, 1993 the calcareous mudstone of the Kitadani Formation, under cross- polarized light. Note cross-sections of a charophyte gyrogonite at Genus Clavator (Reid and Groves) emend. Martin- center and a calcified root trace of which the inside is replaced with sparry calcite below. Scale bar is 0.5 mm. Closas, ex Schudack, 1993 molluscs, ostracods, plants, and calcified root traces Type species.—Clavator reidi (Groves) emend. besides charophyte gyrogonites, which are concen- Schudack, 1993. trated (Figures 2-1, 2-2). Most of the charophyte Remarks.—This taxon includes six traditional gyrogonites are well preserved without deformation members of Clavatoraceae, Clavator Reid and Groves, (Figure 3). The isolation and fragmentation of scales, 1916, Flabellochara Grambast, 1959, Clypeator Gram- molluscan valves, and plants show that these fossils bast, 1962, Triclypella Grambast, 1969, Lucernella are allochthonous. Grambast, 1968, and Septorella Grambast, 1962.

Materials and methods Clavator harrisii var. reyi (Grambast-Fessard) Martin-Closas, 1996 To collect charophytes, muddy rocks were sampled Figures 3.1–5 from approximately 100 horizons of the Akaiwa and Clavator sp. Neagu and Georgescu-Donos, 1973, p. 175–177, pl. 1, Kitadani formations in the Takinamigawa area. Char- ﬁgs. 1–3, text-ﬁg. 4. 206 Katsuhiro Kubota Charophyte gyrogonites from the Lower Cretaceous Kitadani Formation of the Tetori Group 207

Stellatochara reyi Grambast-Fessard, 1980, p. 42, 44, pl. 3, figs. 6–9. condition as a synapomorphy of this taxon. This paper Stellatochara nehdenensis Schudack, 1987a, p. 13–14, pl. 1, figs. also supports this specific placement of Martin-Closas 1–4. (1996) because partial remains or fragments of utricle Stellatochara aff. Stellatochara reyi Grambast-Fessard. Schudack, 1987b, p. 130–132, pl. 4, figs. 15–18, text-fig. 85. do not occur from the Kitadani Formation despite Clavatoroideae indet. Clavatoroide gyrogonite morphotype A K1 collecting more than 200 specimens. Schudack, 1989, p. 423, pl. 5, figs. 1–4; Choi, 1989, p. 33–34, pl. 1, figs. 4, 9–11, text-fig. 3. Family Characeae (Richard ex Agardh) emend. Martin-Clasas and Schudack, 1991 Repository.—University of Tsukuba (IGUT-KK- Genus Mesochara Grambast, 1962 0208 and 94 other specimens). Description.—Gyrogonite is large and oval-shaped, Type species.—Mesochara symmetrica (Peck) Gram- with truncated summit forming a beaked neck and bast, 1962 slightly projected and subtruncated base. Its size is 525–710 mm (mean is 622 mm) in length and 395– Mesochara harrisi (Ma¨dler) Shaikin, 1967 540 mm (mean is 465 mm) in width. The ratio of length Figures 3.6–9 to width is 1.14–1.53 (mean is 1.34). In lateral view, Tolypella harrisi Ma¨dler, 1952, p. 31–32, pl. B, figs. 31–35; Martin- spiral cells are slightly convex or flat, uniform in Closas and Grambast-Fessard, 1986, p. 49–50, pl. 10, figs. 9–12. width, and are bordered by shallow and narrow inter- Tolypella amoena Ma¨dler, 1952, p. 34–35, pl. B, figs. 43–49. cellular furrows. Nine to eleven ambiguous convolu- Tolypella minuta Ma¨dler, 1952, p. 35–36, pl. B, figs. 50–52. tions are observed and are 45–70 mm (mean is 60 mm) Mesochara amoena (Ma¨dler). Schudack, 1987b, p. 151–152, pl. 8, figs. 15–18, text-fig. 101; Schudack, 1989, p. 424, pl. 5, figs. 5–7. in width at its equator. At the apical center, five spiral Mesochara harrisi (Ma¨dler). Schudack, 1987b, p. 152–153, pl. 8, tips are bundled to form a peripheral ridge, and inside figs. 19–22; Schudack, 1990, p. 228, pl. 4, figs. 23–27. the depression is rosette-shaped. At the base, they ‘‘Tolypella’’ sp. Choi, 1989, p. 37, pl. 1, figs. 7–8. are extended downward, entangle, and also form a rosette-shaped surface. Repository.—University of Tsukuba (IGUT-KK- Variation.—LPA ¼ 525–710 (622), LED ¼ 395–540 0314 and 12 other specimens). (465), ISI ¼ 114–153 (134), AND ¼ 285–420 (342), Description.—The small gyrogonite is prolate ANI ¼ 48–62 (55), NC ¼ 9–11 (10), ECD ¼ 45–70 spheroidal- to subprolate-shaped with a rounded or (60), CDI ¼ 78–123 (98). The values in the parenthe- slightly pointed summit and an abruptly contracted ses indicate the average of each parameter. The his- base. Its length is 284–371 mm(meanis323mm), and tograms for the parameters are shown in Figure 4. its width is 235–297 mm (mean is 270 mm). The ratio of Discussion.—This species is referred to Clavator length to width is 1.09 to 1.35 (mean is 1.20). In lateral despite the lack of an utricle. Martin-Closas (1996) view, loosely concave spiral cells are wide and almost established C. harrisii var. reyi based on the phyloge- uniform in width and are separated by narrow and netic analysis of Clavatoraceae. Mesozoic charophytes sharp intercellular ridges. Eight to ten convolutions are classified into three families, Clavatoraceae, Char- are laterally visible. At the apex five spiral tips con- aceae, and Porocharaceae. The fructification of Clav- verge on its center without modification, while at the atoraceae is composed of a gyrogonite covered by a base a small pentagonal opening is formed. utricle, while those of Characeae and Porocharaceae Variation.—LPA ¼ 284–371 (323), LED ¼ 235–297 consist of only a gyrogonite. However, Martin-Closas (270), ISI ¼ 109–135 (120), AND ¼ 142–184 (155), (1996) suggested that most utricles of C. harrisii var. ANI ¼ 46–53 (48), NC ¼ 8–10 (9), ECD ¼ 32–42 reyi are not calcified, or that its gyrogonite is only en- (36), CDI ¼ 96–124 (113). The values in the paren- veloped by a thin calcified film, and regarded this theses indicate the average of each parameter.

U Figure 3. Charophyte gyrogonites collected from the calcareous mudstone of the Kitadani Formation. 1–5. Clavator harrisii var. reyi (Grambast-Fessard), 1: IGUT-KK-0208 in lateral view, 2: IGUT-KK-0208 in apical view, 3: IGUT-KK-0403 in lateral view, 4: IGUT-KK- 1111 in basal view, 5: IGUT-KK-1103 in apicolateral view, 6–9. Mesochara harrisi (Ma¨dler), 6: IGUT-KK-0314 in lateral view, 7: IGUT- KK-0703 in lateral view, 8: IGUT-KK-0703 in apical view, 9: IGUT-KK-1108 in basal view, 10–11. Mesochara stipitata (Wang), 10: IGUT- KK-1116 in lateral view, 11: IGUT-KK-1117 in lateral view, 12–13. Mesochara sp., 12: IGUT-KK-0315 in lateral view, 13: IGUT-KK-0613 in basal view, 14–16. Stellatochara sp., 14: IGUT-KK-0804 in lateral view, 15: IGUT-KK-1104 (inner mold) in lateral view, 16: IGUT-KK- 1104 (inner mold) in basal view. Scale bar is 200 mm. 208 Katsuhiro Kubota

Figure 4. Histograms showing the range of eight measured and computed values for 95 gyrogonites of Clavator harrisii var. reyi. For the abbreviations of each parameter see text.

Discussion.—Schudack (1990) indicated that T. length and 232–242 mm in width. The ratio of length to amoena, T. harrisi,andT. minuta, which were origi- width is 1.32–1.40. In lateral view, gently concave spi- nally described by Ma¨dler (1952), are all synonyms of ral cells are wide and shallow, while intercellular ridges M. harrisi in terms of a comparative study in Europe. are narrow and sharp. Ten or eleven convolutions The species from the Kitadani Formation has wide are observed, and its width is 26 mm at its equator. variation in its morphology and dimension. However, Whereas five spirals are strongly converged at the their statistical data support ambiguous division of apex, a small pentagonal pore opening surrounded by the species. Hence, this paper follows the opinion of five spiral tips is visible at the pointed base. Schudack (1990) and refers them to M. harrisi. Variation.—LPA ¼ 306–339, LED ¼ 232–242, ISI ¼ 132–140, AND ¼ 129–155, ANI ¼ 42–46, NC ¼ 10–11, ECD ¼ 26, CDI ¼ 76–84. The averages Mesochara stipitata (Wang) Wang, 1981 of each parameter are not given because there are Figures 3.10–11 only two specimens. Tolypella stipitata Wang, 1965, p. 482, pl. 1, figs. 19–42. Discussion.—Schudack (1987b) reported this spe- Mesochara stipitata (Wang) Wang, 1981, p. 320, pl. 1, figs. 8–13; cies from northern Spain. However, Schudack’s Hao et al., 1983, p. 140, pl. 30, figs. 1–4; Fu and Lu, 1997, p. 353– (1987b) specimen may be distinguished from the type 354,pl.1,figs.16–20;Penget al., 2003, p. 368, pl. 1, figs. 3–6. specimen in the following points; large ratio of length to width, wide convolution at its equator, and sharply Repository.—University of Tsukuba (IGUT-KK- concave spiral cells. Hence, this paper excluded it 1116 and IGUT-KK-1117). from the synonym list. Description.—The small gyrogonite is subprolate- to prolate-shaped with a slightly pointed apex and a Mesochara sp. distinctly protruding base. Its size is 306–339 mmin Figures 3.12–13 Charophyte gyrogonites from the Lower Cretaceous Kitadani Formation of the Tetori Group 209

Repository.—University of Tsukuba (IGUT-KK- not be observed in detail because of poor preservation 0315 and 8 other specimens). and limited specimens. In addition, some inner molds Description.—Small and spherical-shaped gyrogon- of gyrogonites can be obtained. At the base of the ite has widely rounded summit and base. Its size is mold, a relatively large pentagonal basal plate is bor- 252–319 mm (mean is 285 mm) in length and 245– dered by sharp and narrow ridge surrounded by five 303 mm (mean is 274 mm) in width. The ratio of length spiral tips. The flat and undivided plate is 75–90 mm to width is 1.03–1.05 (mean is 1.04). Wide spiral cells (mean is 80 mm) in diameter. have uniform width and shallow concavity and are Variation.—LPA ¼ 484–570 (538), LED ¼ 332–420 separated by narrow intercellular ridges. Eight or nine (375), ISI ¼ 135–149 (144), AND ¼ 181–255 (216), convolutions, which are 29–39 mm (mean is 35 mm) in ANI ¼ 35–45 (40), NC ¼ 8–10 (9), ECD ¼ 42–60 width at its equator, are laterally visible. At both ends, (52), CDI ¼ 80–110 (97). The values in the parenthe- five spiral tips are converged without modification. ses indicate the average of each parameter. The range Pentagonal opening is surrounded by their tips at the and average of parameters are calculated from six well base. preserved specimens. Variation.—LPA ¼ 252–319 (285), LED ¼ 245–303 Discussion.—This species may be like Stellatochara (274), ISI ¼ 103–105 (104), AND ¼ 129–158 (142), obovata (Peck), which was originally described by ANI ¼ 46–53 (50), NC ¼ 8–9 (8), ECD ¼ 29–39 (35), Peck (1937) as Aclistochara obovata Peck, in its CDI ¼ 103–142 (125). The values in the parentheses morphology and dimension. However, because the indicate the average of each parameter. materials from the Kitadani Formation are poorly Discussion.—This species is distinguished from M. preserved and limited in number, precise specific as- harrisi and M. stipitata in its spherical shape, showing signment is difficult. that ISI value is close to 100, and widely rounded summit and base. Based on the small-sized gyrogonite, concave spiral cells, unmodified spiral tips at ends, and Discussion the small pentagonal basal pore, this species is assigned to Mesochara. The charophytes described in this paper are widely used as one of the available groups of nonmarine Family Porocharaceae (Grambast) emend. Schudack, index fossils (Grambast, 1974; Schudack, 1987a, b; 1993 Riveline et al., 1996). Clavator harrisii var. reyi,a Infrafamily Stellatocharoideae Grambast, 1962 member of Clavatoraceae, possesses a precise age assignment and accounts for three-fourths of the Kita- Genus Stellatochara Horn af Rantzien, 1954 dani charoflora. An equivalent of C. harrisii var. reyi, originally reported as Clavator sp., has been found Type species.—Stellatochara sellingii Horn af Rant- from marls and reefal limestones in southern Do- zien, 1954 brogea, Romania (Neagu and Georgescu-Donos, 1973). These strata were assigned to the Barremian Stellatochara sp. based on a shallow marine molluscan assemblage Figures 3.14–16 (Neagu and Georgescu-Donos, 1973). Stellatochara nehdenensis Schudack, described from limnic karst Repository.—University of Tsukuba (IGUT-KK- filling near Nehden in Germany (Schudack, 1987a), 0804 and 13 other specimens). is a junior synonym of C. harrisii var. reyi. Schudack Description.—Medium-sized and ovoid gyrogonite (1987a) assigned charophyte-rich karst filling deposits has prominently projecting summit and truncated to the Barremian based on other clavatoracean char- base. It is 484–570 mm (mean is 538 mm) in length and ophytes, such as Atopochara trivolvis triquetra Gram- 332–420 mm (mean is 375 mm) in width. The ratio of bast and Clypeator jiuquanensis (Wang). These two length to width is 1.35–1.49 (mean is 1.44). In lateral species are used as Barremian markers (Neagu and view, the gyrogonite has loosely convex spiral ridges, Georgescu-Donos, 1973; Grambast, 1974; Martin- which are separated by shallow and wide furrows. Closas, 1996). The Barremian Nehden charoflora was Eight to ten convolutions, which are 42–60 mm(mean comparable to a charoflora which contained an equiv- is 52 mm) in width at the equator of the gyrogonite, alent of C. harrisii var. reyi from the Nagdong For- are laterally observed. At the apex, a strongly pro- mation, Sindong Group, Gyeongsang Basin, Korea truding neck is formed by five spirals which tightly (Choi, 1989). The age of the Nagdong Formation is entangle and leave a small circular pore. The base can well concordant with that assigned by palynomorphs 210 Katsuhiro Kubota

Figure 5. Age ranges of charophytes and nonmarine molluscs from the Kitadani Formation. The age range of Nippononaia ryosekiana is based on Isaji (1993) and Matsukawa et al. (1997), while that of Trigonioides (Wakinoa) tetoriensis is based on Tashiro and Okuhira (1993). The absolute ages are based on Gradstein et al. (2004).

(Choi, 1985; Yi et al., 1994). In addition, Grambast- Mesochara harrisi, reported as Tolypella harrisi Fessard (1980) reported Stellatochara reyi from the from the Maestrat region in northeastern Spain, co- Barremian or lower Aptian strata in Portugal, and occurs with A. trivolvis triquetra of Barremian age Schudack (1987b, 1989) also recorded an equivalent of (Martin-Closas and Grambast-Fessard, 1986). M. har- C. harrisii var. reyi from the Barremian beds on the risi has also been found in Kimmeridgian strata in Iberian Peninsula. These data suggest that C. harrisii Tho¨ ren, Germany (Ma¨dler, 1952) and in Kimmer- var. reyi can be interpreted to be a Barremian to early idgian and Berriasian strata in Spain (Schudack, Aptian marker despite the uncertain age assignment 1987b). These occurrences indicate that the age range of the lower Aptian strata (Neagu and Georgescu- of M. harrisi is Kimmeridgian to Barremian (Figure Donos, 1973; Grambast-Fessard, 1980; Schudack, 5). 1987a, b, 1989) (Figure 5). Mesochara stipitata has been reported from the Charophyte gyrogonites from the Lower Cretaceous Kitadani Formation of the Tetori Group 211

Bayingebi and Suhongtu formations in the Inggen- However, two age ranges of nonmarine molluscs from Ejin Qi Basin, Inner Mongolia Autonomous Region the Kitadani Formation do not overlap each other. (Peng et al., 2003). These formations also yield two The result suggests that the age of this formation can clavatoracean charophytes, C. jiuquanensis and Fla- not be assigned based on the concurrent range of bellochara hebeiensis Lu, Zhang, and Zhao. Based on nonmarine molluscs. the well established biostratigraphy of clavatoracean If the most expanded age range of nonmarine charophytes (Grambast, 1974; Martin-Closas, 1996), molluscs (an interval from the appearance of T. (W.) they are assigned to the Barremian (Peng et al., 2003). tetoriensis to the disappearance of N. ryosekiana)is This species also occurs in Berriasian to Barremian used, on the other hand, it indicates that this forma- strata in China (Chen, 1983) and in the Barremian tion is assigned a late Hauterivian to late Aptian age Xiagou Formation of the Huahai Basin, Gansu Prov- (Figure 5; as summarized in Tsubamoto et al., 2004). ince in China (Fu and Lu, 1997). Therefore, the age The charophyte-rich horizon is stratigraphically posi- range of M. stipitata is Berriasian to Barremian tioned just below the outcrop from which N. ryo- (Figure 5). sekiana and T. (W.) tetoriensis are reported by Isaji Based on the concurrent range of charophyte fos- (1993). As mentioned above, the concurrent range of sils, the charophyte-rich horizon of the Kitadani For- charophyte fossils indicates that the lower part of the mation is assigned to the Barremian (Figure 5). The KitadaniFormationisreferredtoaBarremianage, age range of the charoflora described in this paper which falls in with the most expanded age range of does not indicate the whole period when the Kitadani nonmarine molluscs. Formation was deposited, but refers to the age of a This study demonstrates that the charoflora is a po- small portion of the lower Kitadani Formation. In ad- tentially useful tool for the age assignments of non- dition, because the underlying Akaiwa Formation and marine deposits in the Tetori Group, which always the upper part of the Kitadani Formation contain few have been problematic, and that combining evidence available index fossils, the ages of the lower and upper from nonmarine molluscs with charoflora may give us limits of the Kitadani Formation are poorly defined. better age estimations. However, the charoflora is re- In previous studies on the age assignment of ported from only one horizon for this study, so that the Lower Cretaceous deposits in Japan, the strati- the age estimation is limited to a small portion of graphic positions of nonmarine deposits with respect the Kitadani Formation. Further intensive studies will to ammonite-bearing marine deposits and common provide us additional occurrences of the charoflora occurrences of nonmarine molluscs in different areas from other horizons of the Kitadani Formation and have been used (Matsumoto et al., 1982; Isaji, 1993; other formations of the Tetori Group in the future, Tashiro and Okuhira, 1993). Isaji (1993) reported which will eventually help us elucidate the geologic Nippononaia ryosekiana, associated with Plicatounio ages and stratigraphic correlations of the Kitadani (s.s.) kobayashii, Pseudohyria matsumotoi, Nagdongia Formation with Cretaceous nonmarine strata in other soni, Nippononaia aff. tetoriensis, Trigonioides (Waki- regions of the world. noa) tetoriensis,andViviparus sp., from the Kitadani Formation in the Takinamigawa area. N. ryosekiana is also described from the lower part of the Sebayashi Acknowledgments Formation of the Sanchu Graben in the Kanto Moun- tains, interbedded with the marine deposits bearing I give my thanks to Hiroko Okazaki, Shinji ammonites (Hayami and Ichikawa, 1965). Based on Isaji (Natural History Museum and Institute, Chiba), the occurrence of ammonites, the age range of N. and Yoshitsugu Kobayashi (Hokkaido University ryosekiana is late Barremian to late Aptian (Isaji, Museum) for reading an earlier version of the manu- 1993; Matsukawa et al., 1997) (Figure 5). T. (W.) script and providing valuable suggestions, and Michael tetoriensis has been found in the uppermost part of the Schudack (Freie Universita¨t, Berlin) and Yasuhide Tatsukawa Formation of the Monobegawa Group as Iwasaki (Professor Emeritus, Kumamoto University) well as in the Kitadani Formation (Tashiro and Oku- for their reviews of the manuscript. I also would like hira, 1993). The basal part of the Hanoura Formation, to thank Kazunori Miyata (Fukui Prefectural Dino- which conformably overlies the Tatsukawa Formation, saur Museum) for help in collecting the charophyte yields late Hauterivian to early Barremian ammonites fossils, Minoru Tamura (Professor Emeritus, Kuma- (Shimizu, 1931). Therefore, Tashiro and Okuhira moto University) for identification of molluscs from (1993) showed that the age range of T. (W.) tetoriensis the Akaiwa Formation, Shin-ichi Sano (Fukui Prefec- is late Hauterivian to early Barremian (Figure 5). tural Dinosaur Museum) and Yohei Arakawa (Hiro- 212 Katsuhiro Kubota shima University) for providing suggestions and use- Hao, Y., Ruan, P., Zhou, X., Song, Q., Yang, G., Cheng, S. ful information, and Shigeo Yasuoka (landowner) for and Wei, Z., 1983: Middle Jurassic-Tertiary deposits giving permission to collect the charophyte-bearing and ostracod-Charophyta fossil assemblages of Xining and Minhe basins. 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