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 estab- lished charophyte group and is unique in Upper Jurassic to Cretaceous nonmarine deposits. The occur- rences 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 non- marine 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 ho- rizon of the lower part of the Kitadani Formation (Figure 1). To extract charophyte gyrogonites, muddy rocks were shattered by a rock hammer. Well pre- served gyrogonites were picked from the shattered rock powder and mounted on a stub under a stereo- scopic microscope for observation by scanning elec- tron 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 ter- minology 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 micro- scope) and three computed values, is available for their identification. The abbreviations of each param- eter stand for the following meanings: LPA: length of the polar axis, LED: largest equatorial diameter, ISI: isopolarity index (LPA/LED Â 100), AND: anisopo- larity distance, which is from the apical pole to LED calculated along the polar axis, ANI: anisopolarity in- dex (AND/LPA Â 100), NC: number of convolutions, ECD: equatorial cell diameter, and CDI: cell diameter index (ECD/LPA Â 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