Mesodictyon japonicum, a new fossil diatom species from the Miocene marine sediments distributed in the Niigata area, central Japan

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1 Diatom 30: December 2014 Mesodictyon japonicum sp. nov. 1 Mesodictyon japonicum, a new fossil diatom species from the Miocene marine sediments distributed in the Niigata area, central Japan Yukio Yanagisawa 1 and Hiroyuki Tanaka 2 1 Institute of Geology and Geoinformation, Geological Survey of Japan, Advanced Industrial Science and Technology, AIST Tsukuba Central 7, Higashi, Tsukuba, Ibaraki , Japan * Correspondence author. y.yanagisawa@aist.go.jp 2 Maebashi Diatom Institute, 57 3 Kawamagari, Maebashi, Gunma , Japan Abstract A new Mesodictyon species is described from the upper Miocene marine sediments of the Nomura Formation, distributed in the Tsugawa area of Aga Town, Niigata Prefecture, Japan. The new taxon exhibits the main characteristics of the genus, with cribra inside the areolae, no valve face fultoportulae and a single rimoportula on the mantle, but has mantle fultoportulae with three satellite pores and areolae with small external projections similar to the basal parts of volae, which distinguishes it from any other Mesodictyon species reported to date. The species is described herein as Mesodictyon japonicum sp. nov., and is illustrated using both LM and SEM photographs. The species has a restricted stratigraphic range in the late Miocene; it first appeared at around 8.6 Ma, became common at 8.2 Ma and disappeared at around 5.5 Ma. Although it has been found in marine sediments, the species is presumed to be a freshwater diatom because it exhibits a similar occurrence pattern to that of the accompanying Aulacoseira species. Key index words: diatom, fossil, freshwater, Japan, Mesodictyon, Miocene Introduction Received 23 June 2014 Accepted 26 August 2014 In Japan, Neogene open marine deposits in the onland sections have been well known to contain considerable amounts of non-marine planktonic diatom fossils such as Aulacoseira, Cyclotella and Stephanodiscus, which comprise over 50% of the total diatom assemblages in some cases (e.g. Koizumi 1968). These non-marine diatoms probably have been transported from inland lakes or streams as silt-sized sediments through rivers into marine environments. During the past decade, the taxonomy of fossil freshwater diatoms has been greatly advanced through the steady accumulation of knowledge on morphology revealed by scanning electron microscopy along with successive descriptions of a number of new species (e.g. Tanaka 2007, 2014). This progress was achieved by examining well preserved diatoms in lacustrine sediments deposited in ancient lakes. However, little attention has been paid to the taxonomy of fossil freshwater diatoms preserved in open marine sediments. In the course of marine diatom biostratigraphic study of the Neogene sequences in the Niigata sedimentary basin, Yanagisawa et al. (2003a, 2003b, 2010) reported that Miocene marine sediments contained freshwater diatom assemblages mainly composed of Aulacoseira spp. and Cyclotella spp. They also found and illustrated an unnamed centric diatom taxon which would be designated to the genus Mesodictyon. Exact taxonomic position of this taxon, however, remains uncertain because of the lack of information on the microstructure of the valve. We have, therefore, carried out SEM observations on this taxon in the Miocene Nomura Formation in the Tsugawa area, Niigata Prefecture. As a result, we have revealed that this taxon has the diagnostic characteristics of the genus Mesodictyon, but has different characteristics from any other Mesodictyon taxa known to date. This leads us to propose it as a new species, Mesodictyon japonicum. Materials and methods For taxonomic examination, we used the samples taken by Yanagisawa et al. (2003a) from the Nomura Formation exposed along Shinazawagawa stream in the Tsugawa area of Aga Town, Niigata Prefecture, central Japan (Fig. 1). The upper Miocene Nomura Formation is composed of marine diatomaceous mudstone and sandy mudstone intercalating numerous thin tephra beds (Fig. 2, Yanagisawa et al. 2003a). The new Mesodictyon species occurs in the upper part of the formation. It is usually rare to very rare in occurrence and mostly damaged, but it accounts for 21% of the total assemblage in sample Snd-69 (original number Tsug-149), from which we designated the type material (Fig. 2), even though this material is also in a poor state of preservation. The samples were boiled in a 30% H 2 O 2 solution to

2 2 Yukio Yanagisawa and Hiroyuki Tanaka Fig. 1. Location of sampling site, Tsugawa area, Aga Town, Niigata Prefecture, Japan. separate the sediment particles from the diatom valves and to eliminate organic material, and then washed several times with distilled water. The cleaned material was mounted in Pleurax or Styrax. The holotype slide was made as a single specimen slide using a method by Tanaka (2007). LM observations were made using a Nikon Apophot microscope with a Nikon plan-apochromat 100 oil immersion objective lens (NA 1.4). SEM observations were made using a Hitachi S-4000 field emission microscope. Measurements of striae and areolae were carried out using method of anonymous (1975) and terminology based on Ross et al. (1979). Stratigraphic range of this new species is established based on previous diatom biostratigraphic studies on Neogene sediments in the Niigata sedimentary basin. Four areas in the basin were chosen for this study (Fig. 1): the Tsugawa area (Yanagisawa et al. 2003a, 2010, Hiranaka et al. 2007), the Tainai area (Yanagisawa et al. 2003b, Hiranaka et al. 2004), the Kamo area (Yanagisawa & Kudo 2011) and the Sado Island (Yanagisawa et al. 2009). Marine diatom biostratigraphic zonation used in this paper is that proposed by Akiba (1986), and modified by Yanagisawa & Akiba (1998) and Watanabe & Yanagisawa (2005). Ages of marker diatom biohorizons are calibrated by correlation to a geomagnetic time scale of Gradstein et al. (2012). Results Mesodictyon japonicum Yanagisawa & H.Tanaka sp. nov. (Figs 3 23) Valves circular, diameter μm, with convex or concave valve face and radiating single rows of areolae on the valve face, 5 10 (mostly 6 9) in 10 μm at the valve face/mantle junction and areolae in row 8 12 in 10 μm. Near the margin, shorter rows of areolae appear randomly between the longer rows. Usually, no areolae on the center of the valve face. In smaller valves, few areolae on mantle or on a horizontally positioned row between mantle and rim. No signs of spines detectable. Cribra within areolae and external openings of areolae with small projections. Sometimes internal openings of areolae are elliptical. No valve face fultoportulae. Mantle fultoportulae with three satellite pores usually located every, to every 2 or 3 end of stria, and in rare cases, at the end of interstria. Single sessile rimoportula located at the end of a stria on mantle. Holotype: MPC Micropaleontology Collection, National Museum of Nature and Science, Japan (Figs 3, 4). Isotype: TA3556, Maebashi Diatom Institute, 57 3 Kawamagari, Maebashi City, Gunma, Japan. Type locality: A road-side cliff at Shinazawagawa, Tsugawa area in Aga Town, Niigata Prefecture, Japan N, E. Detailed locality is shown in fig. 3 of Yanagisawa et al. (2003a). Type material: Snd-69 (original number Tsug-149), collected from mudstone of the Nomura Formation just below the tephra bed Sn535 (fig. 4 of Yanagisawa et al., 2003a). Age of type material: Late Miocene. The material is placed between diatom biohorizons D65 (7.9 Ma) and D70 (7.7 Ma) in the diatom zone NPD6B (Thalassionema schraderi Zone), and dated at ca. 7.9 Ma in age (Fig. 2). Etymology: The specific epithet refers to Japan. Synonymy: Mesodictyon sp., Yanagisawa et al. 2003a, pl. 2, figs Mesodictyon sp., Yanagisawa et al. 2004, pl. 1, fig. 18. Mesodictyon? sp., Yanagisawa et al. 2010, pl. 1, fig. 11. Morphology LM observations show that the density of striae or areolae are variable, and there are usually no areolae on the center of the valve face (Figs 3 10). SEM observations show evidence of external areolae openings with small

3 Mesodictyon japonicum sp. nov. 3 projections, most of which are similar to basal parts of volae (Figs 14, 15) and cribra inside the areolae (Figs 16, 23). Mantle areolae are finer than those of valve face (Figs 12, 14), and smaller valves have only one areolae row running around the circumference between the lowest mantle and rim (Fig. 15). SEM internal views show that some internal areolae openings are elliptical (Fig. 17), and in rare cases, irregularly-shaped. Interareolae rows are raised (Fig. 21), mantle is relatively low, usually covered by a siliceous membrane extending from margin (Figs 18 19, 22), and mantle fultoportulae with three satellite pores and single rimoportula are usually located at the end of an areolae row on mantle (Figs 18 19, 22). Fig. 2. Stratigraphic distribution of Mesodictyon japonicum in the Tsugawa area, Niigata Prefecture. Composite stratigraphic section of Shinazawagawa, Fukazawagawa and Ubadogawa routes is based on Yanagisawa et al. (2003a, 2010) and Hiranaka et al. (2007). Diatom zonation follows Akiba (1986), Yanagisawa & Akiba (1998) and Watanabe & Yanagisawa (2005). Ages of diatom biohorizons are estimated on the basis of the geomagnetic time scale of Gradstein et al. (2012). Stratigraphic range Stratigraphic occurrences of M. japonicum in the marine sediments of the Tsugawa, Tainai, Kamo and Sado areas are illustrated in Figs 2, 24, 25 and 26, respectively, along with the occurrences of freshwater diatom genera Aulacoseira, Cyclotella, Pliocaenicus and Stephanodiscus. The open marine sediments in these areas contains abundant marine diatoms which serve as useful biostratigraphic markers to establish detailed diatom biochronology. Figure 2 shows the stratigraphic occurrence of M. japonicum in the marine Nomura and Tokonami formations exposed along the Shinazawagawa, Fukazawagawa and Ubadogawa sections in the Tsugawa area (Yanagisawa et al. 2003a, 2010, Hiranaka et al. 2007). The first occurrence of M. japonicum is recognized in the Nomura Formation, between the biohorizon D65 (8.7 Ma) and D66 (8.5 Ma) in the lowermost of diatom zone NPD6B, and is dated at ca. 8.6 Ma. The species is extremely rare up to the middle part of the zone at about 8.2 Ma, and then it increases in abundance and consistently occurs upward to the top of the Nomura Formation. In the overlying Tokonami Formation, M japonicum is common to rare in abundance. In the Jinaigawa section of the Tainai area, M. japonicum was found in the Uchisugawa Formation which is composed of marine mudstone and sandy mudstone (Fig. 24; Yanagisawa et al. 2003b, this study). The species has the first occurrence at ca. 8.1 Ma and becomes common at 8.0 Ma. It occurs in small abundance throughout the diatom zones NPD6B, 7A and 7Ba, and disappears temporarily at the top of the zone NPD7Ba. Mesodictyon japonicum is absent in the lower interval of the overlying zone NPD7Bb, but it appears again in the upper part of zone NPD7Bb sporadically in very small abundances. These discontinuous and isolated occurrences of this species may be due to reworking. Yanagisawa & Kudo (2011) reported Mesodictyon japonicum as Mesodictyon sp. from numerous sections in the Kamo area. In this paper, the stratigraphic range of

4 4 Yukio Yanagisawa and Hiroyuki Tanaka Figs Mesodictyon japonicum. LM. Figs 3 4. Holotype, MPC-26543, housed in National Museum of Nature and Science, Japan. Same valve at different focal planes. Figs Four different sized valves. Figs 5 6, Same valves at different focal planes. Fig. 7. Broken valve (same valve shown in SEM, Figs 21 23). the species in the Kakumagawa section is shown in Fig. 25. The species occurs continuously in the lower part of the Minami-imogawa Formation, but it is sporadic and rare in the upper part of the formation and the overlying Kakuma Formation. In the Sado area, M. japonicum is found in the upper part of the Nakayama Formation which is composed mainly of diatomaceous mudstone (Fig. 26; Yanagisawa et al. 2009, this study). Sporadic occurrence of this species is recognized in the interval ranging from 8.4 Ma to 5.6 Ma in age. Yanagisawa et al. (2004) also reported and illustrated the species from the Taga Formation in the Hitachi area, Ibaraki Prefecture (Fig. 1) in the zone NPD7B between the biohorizons D68 (7.9 Ma) and D70 (7.7 Ma). Stratigraphic data mentioned above indicate that M. japonicum has a restricted stratigraphic range in the late Miocene (Fig. 27). It appeared in the lowermost NPD6B at around 8.6 Ma, and became common at 8.2 Ma and disappeared at around 5.5 Ma near the Miocene/Pliocene boundary. The first appearance of this species is slightly diachronous; it appears earlier in the Tsugawa area than the other areas. This species is also found in the lower Pliocene sediments, but its sporadic and isolated occurrence can be interpreted as reworking from older horizons. Discussion One of the diagnostic characteristics of this species is cribra inside of areolae (medial cribra). There are two genera with medial cribra among freshwater fossil diatoms. Theriot & Bradbury (1987) proposed the genus Mesodictyon Theriot & Bradbury based on this charac-

5 Mesodictyon japonicum sp. nov. 5 Figs Mesodictyon japonicum. SEM, external views. Fig. 11. Whole view of concave valve. Fig. 12. Oblique view of Fig. 11. Fig. 13. Oblique view of convex valve with bands. Fig. 14. Enlarged view of valve margin showing small areolae rows of mantle. Fig. 15. Valve margin showing openings of areolae with small projections, mantle areolae row located horizontally between mantle and rim, and an opening of mantle fultoportula (arrow). Fig. 16. Detailed view of areolae showing cribra inside (arrowhead). Fig. 11, scale bar 10 μm; Figs 12 13, scale bars 5 μm; Figs 14 15, scale bars 2 μm; Fig. 16, scale bar 1 μm. teristic, along with no valve face fultoportulae, mantle fultoportulae with two satellite pores and a rimoportula on mantle. Khursevich et al. (2004) proposed the genus Mesodictyopsis Khursevich, Iwashita, Kociolek & Fedenya also based on medial cribra of areolae along with valve face fultoportulae, mantle fultoportulae with three satellite pores and rimoportula(e) on valve face. The new species has cribra inside of areolae, no valve face fultoportulae, mantle fultoportulae with three satellite pores and a single rimoportula located on mantle, characteristics between Mesodictyon and Mesodictyopsis. The new species reported here does have mantle fultoportulae with three satellite pores, a characteristic of Mesodictyopsis. Though it may be true, weighting of taxonomic evidence is a dangerous procedure, to be avoided wherever possible as mentioned by Round et al. (1990), we consider the characteristics of the presence or absence of valve face fultoportulae important, and the presence of a rimoportula on mantle as even more important than others, in particular its location, and therefore, we be-

6 6 Yukio Yanagisawa and Hiroyuki Tanaka Figs Mesodictyon japonicum. SEM, internal views. Fig. 17. Oblique whole view of convex valve showing internal openings of areolae, some of which are elliptical. Fig. 18. Enlarged view of part of Fig. 17 showing mantle fultoportula with three satellite pores (large arrow) and rimoportula (arrowhead) on mantle slightly covered by siliceous membrane (small arrow). Fig. 19. Valve margin, mantle fultoportula with three satellite pores (arrow) and rimoportula on mantle (arrowhead). Fig. 20. Oblique view of whole convex valve. Fig. 21. Oblique view of broken concave valve showing raised interstriae (same valve as Fig. 7). Fig. 22. Enlarged view of marginal valve of Fig. 21 showing mantle fultoportula with three satellite pores (arrow) and rimoportula on mantle (arrowhead). Fig. 23. Cross section of areolae row (arrow of Fig. 21) showing cribra inside the areolae (arrow).

7 Mesodictyon japonicum sp. nov. 7 Fig. 25. Stratigraphic distribution of Mesodictyon japonicum in the Kakumagawa section of Kamo area. This figure is based on fig. F and table F in Yanagisawa & Kudo (2011). Fig. 24. Stratigraphic distribution of Mesodictyon japonicum in the Jinaigawa section of Tainai area compiled from Yanagisawa et al. (2003b) and this study. lieve the new taxon belongs to Mesodictyon. In the genus Mesodictyon, M. nativus Khursevich & Fedenya and M. sp. (Fourtanir et al. 1993) have similar shapes to the new species in LM. They have, however, mantle fultoportulae with two satellite pores, as do all the other previously known Mesodictyon species, as opposed to the three satellite pores of the new species. Though we could not actually observe the volae due to the poor state of preservation, we can assume that the volae originally existed because of the remaining small projections that seem to form their basal parts. This characteristic distinguishes the new taxon from all the other Mesodictyon species which have areolae with external foramina. Thus, we propose the new taxon as M. japonicum, a new species in the genus Mesodictyon, making it the second Mesodictyon species after Mesodictyon yanagisawae H.Tanaka (Houk et al. 2014) to be found in Japan. Mesodictyon yanagisawae was described from the lacustrine siltstone of the upper Miocene Shirasawa

8 8 Yukio Yanagisawa and Hiroyuki Tanaka Fig. 27. Stratigraphic range of Mesodictyon japonicum. The stratigraphic ranges of the genera Mesodictyon, Aulacoseira, Cyclotella, Pliocaenicus and Stephanodiscus in the Niigata sedimentary basin are also shown. Diatom zonation follows Akiba (1986), Yanagisawa & Akiba (1998) and Watanabe & Yanagisawa (2005). Ages of diatom biohorizons are calculated by correlation to the geomagnetic time scale of Gradstein et al. (2012). Fig. 26. Stratigraphic distribution of Mesodictyon japonicum in Sado Island. Composite stratigraphic section of the Miocene to lower Pliocene sequence around Nakayama Pass area in Sado Island is based on Yanagisawa et al. (2009) and this study. NK: Nakayama prefectural road section, WK: Wakamiya agricultural road section, SN: Shiinoki forestry road section, TB-I, II and III: Shichiba River sections I, II and III, SW: Hanyugawa section. Location of each stratigraphic section is shown in Yanagisawa et al. (2009) and Sakamoto T. (1992). Formation distributed near Sendai, Miyagi Prefecture (Fig. 1). M. yanagisawae is similar to M. japonicum in LM image, but is distinguished from the latter in having non-penetrating dots at the center of valve surface and two satellite pores of mantle fultoportulae. Light microscopic image of Mesodictyon japonicum is superficially similar to that of Actinocyclus ingens Rattray, one of the most common marine diatoms in the middle to upper Miocene marine sediments. It is possible that the new species may have been misidentified as A. ingens in previous studies in some cases. Mesodictyon japonicum is described and found in open marine sediments which contain abundant marine diatoms. Therefore, we cannot rule out the possibility that M. japonicum is a marine diatom. It is, however, more plausible that M. japonicum is a freshwater diatom

9 Mesodictyon japonicum sp. nov. 9 because the occurrence pattern of M. japonicum is closely similar to that of freshwater Aulacoseira species as are seen in the sections studied. In the Nomura Formation in the Tsugawa area (Fig. 2), M. japonicum has an abrupt increase in abundance at 185 meter (at around 8.2 Ma). This first abundance peak just coincides with an increase of Aulacoseira spp. The second, third and fourth increase of this species are also coincident with the abundance peaks of Aulacosira spp. The coincidence of occurrence peaks of M. japonicum and Aulacoseira spp. is also recognized in the upper zone NPD6B of the Uchisugawa Formation in the Tainai area (Fig. 24). In the Minami-imogawa and Kakuma Formations in the Kamo area, the same occurrence pattern is identified, although it is slightly vague (Fig. 25). These synchronous occurrence of M. japonicum and Aulacoseira strongly suggests that M. japonicum is a freshwater diatoms. Acknowledgments We wish to thank to Dr. T. 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