Jour. Geol. Soc. Japan, Vol. 110, No. 12, p. 758－764, December 2004

Lingularia sp.（Brachiopoda）from Middle bedded chert in Shikoku, Japan

Abstract

Rie S. Hori ＊ and A lingulid Lingularia sp. is described from Middle Tri- Hamish J. Campbell ＊＊ assic（Anisian）bedded chert in the Northern Chichibu Terrane, Shikoku, Japan. The fossil occurrence is unusual because of its asso- Received January 14, 2004 ciation within deep-sea sediments. Lingulids are well known from Early Accepted October 25, 2004 and Middle Triassic successions and their apparent abundance may ＊ Department of Earth Sciences, Faculty of Sci- relate to the recovery following the /Triassic（P/T）extinction ence, Ehime University, Matsuyama 790-8577, event. This find in Japan tends to support the view that lingulids Japan favoured dysoxic conditions and became widespread in vacant niches ＊＊ Institute of Geological and Nuclear Sciences, of the ocean after the P/T boundary anoxic event as“opportunistic PO Box 30-368, Lower Hutt, New Zealand taxa”.

Key words: bedded chert, dysoxic condition, Lingularia, Permian/Triassic boundary, Middle Triassic

stone of Imoto 1984 Fig.1 . The Toishi-type siliceous Introduction （ ）（ ） ‘ ’ shale is observed between sequences of the Upper Permi- Bedded chert sequences are distributed widely in Meso- an and Lower Triassic bedded chert, and is occasionally zoic accretionary complexes in Japan. Cherts are general- associated with carbonaceous black shale, pyrite nodules, ly considered to be ancient deep-sea sediments with and dolostones. This remarkable lithology is interpreted extremely low sedimentation rates and long accumulation to indicate development of extreme oxygen depleted times（e.g. Matsuda and Isozaki, 1991）. They commonly （dysoxic to anoxic）oceanic environmental conditions on preserve rich assemblages of microfossils, such as radio- the deep ocean floor resulting from the end-Permian larians, sponge spicules, conodonts and fish teeth. These extinction event（e.g. Isozaki, 1996, 1997; Kakuwa, 1996）. microfossils permit determination of depositional age of the original sediment and constrain tectonic histories of chert-bearing Mesozoic complexes（e.g. Matsuda et al., 1980; Matsuoka, 1984; Otsuka, 1985; Wakita, 1985）. How- ever, macrofossils from these bedded cherts are extreme- ly rare with only one previous record by Iijima et al. （1988）. These workers described Brachyoxylon（fossil wood）within the Ashio Terrane, exposed in the upper- most part of the Adoyama Chert, Kuzu area of central Japan. The specimen was collected from a horizon that is 20 cm below the base of the Middle Maki Sand- stone. Iijima et al.（1988）speculated that the fragment of wood was derived from an adjacent landmass associated with submarine slumping. Bedded chert sequences from Mesozoic accretionary complexes in the Chichibu and Tamba－Mino－Ashio Ter- ranes range in age from Late to Jurassic. Within them, the Permian/Triassic（P/T）boundary is rec- Fig.1. Lithostratigraphic column of the Triassic bedded chert ognized by the presence of a distinctive clay-rich litholo- sequence indicating the stratigraphic level of the bed with Lingu- gy referred to as the‘Toishi-type’ siliceous shale/clay- laria sp. Time scale after Kozur（2003）. CThe Geological Society of Japan 2004 758 Jour. Geol. Soc. Japan, 110（ 12 ） Lingularia from Triassic chert in Shikoku 759

Fig.2. Geological map of Shikoku Island and an index map of the Kumosoyama area. The Lingularia sp. bearing bedded chert locality is indicated. The geological map is modified from Yamakita（1986）.

Isozaki（1997）has argued that the facies change from the Kumosoyama, near Kamiyama Town, Tokushima Prefec- ‘Toishi-type’siliceous shale（claystone）to Middle Trias- ture, Shikoku, Japan（Fig.2）. The specimen was collected sic bedded chert deposition indicates the slow environ- from a chert horizon within a Middle Triassic bedded mental recovery from anoxic to normal oxic conditions, chert sequence in the Tenjinmaru Formation. The fossil following the P/T boundary event. During the Middle Tri- locality is within a roadside outcrop along Route 193, assic（Anisian）recovery period, a rhythmic bedded Kumoso Road, on the northwest flank of Mt. Kumosoya- sequence of alternating chert and relatively thick shale ma, Shikoku（Fig.2）. characteristically occur just above the‘Toishi-type’ In the Kumosoyama area, the Jurassic accretionary siliceous shale. This sequence differs from normal or com- complex of the Northern Chichibu Terrane is widely dis- mon regularly-bedded chert by having centimetre-thick tributed and has been mapped in terms of two formations: shale beds. the Tenjinmaru and Takashiroyama Formations（Yamaki- We report here on the discovery of a single brachiopod ta, 1986）. The Tenjinmaru Formation consists of bedded specimen, Lingularia sp., from an Anisian bedded chert chert-siliceous rocks and sandstone-mudstone units. It in Shikoku（Fig.1）. This is a most unusual find and is the forms relatively coherent sequences. The Takashiroyama first record of a brachiopod fossil from Triassic bedded Formation is comprised of basic volcanic rocks with asso- chert in Japan. However, occurrences of Lingulacea, gen- ciated lenses or blocks of chert and limestone. Geological erally referred to as“Lingula”, are not unusual among sketch maps and microfossils localities from this area are Lower to Middle Triassic sequences globally（e.g. Rodland also available in Isozaki（1981, 1987）and Suyari et al. and Bottjer, 2001）. Lingula has been interpreted as an （1982）. opportunistic‘disaster’taxon that flourished in marine At the fossil locality（Fig.2）, typical‘Toishi-type’ environments following the P/T extinction event. The find siliceous shale is observed, consisting of a sequence of recorded herein is relevant to these phenomena in terms black and grey siliceous shale beds that overlies a Permi- of the deep-sea sedimentary record. an bedded chert sequence. Up sequence, these shale beds are overlain by a Middle Triassic regular-bedded chert Sample Material facies. The‘Toishi-type’siliceous shale contains Early The brachiopod was discovered in an exposure on Mt. Triassic conodonts such as Neospathodus cf. dieneri and 760 Hori, R. S. and Campbell, H. J. 2004―12

slightly more pale in colour with an interior-internal aspect, and the other is darker coloured with a more exte- rior-external aspect. The fossils represents a single spec- imen of one valve of an inarticulate brachiopod identified here as Lingularia sp.（Fig.4）. The shell is not entire: the mid to upper left hand portion（5×1 mm）of the commis- sure is missing, as if it were torn off（possibly predated）, and the edge of the shell is irregular in shape.（See details in the systematic description below）.

Discussion

Several possible scenarios explaining the presence of Lingularia within the lower Middle Triassic bedded chert sequence of the Northern Chichibu Terrane are consid- ered below: 1）Lingularia may have been relatively abundant in Fig.3. Photomicrograph of a thin section of the Lingularia-bearing oceanic environments during early Middle Triassic shale-chert couplet. time, when deep-sea floors were dysoxic for at least T ＝ Triassocampe sp.（Radiolaria）; upper dark area ＝ shale; ten million years following the P/T boundary anoxic lower pale area ＝ chert. Scale bar ＝ 200 μm. event. 2）The Lingularia specimen may be regarded as an iso- Neohindeodella cf. nevadaensis（Yamakita, 1986）. Mid- lated one-off redeposited‘float’fossil derived from a dle Triassic radiolarian fossils such as Triassocampe spp. source area up-slope within the chert/shale deposi- occurr from associated bedded cherts with the siliceous tional environment. mudstone（Suyari et al, 1982）. In the present study, we 3）The great regression associated with the P/T boundary have also recognized primitive types of Triassocampe may have provided a habitat（e.g. marginal sea） and fragments of conodonts（? juvenile forms of Neogon- favourable to Lingularia in the area proximal to the dollella）from a chert bed closely associated with the sedimentary basin where the bedded cherts accumu- shale bed from which the brachiopod fossil was collected. lated. According to Kozur and Mostler（1994）, primitive Tri- 4）Lingularia may have been a natural component of the assocampe first appear at the boundary between the Pel- deep ocean fauna in the Triassic time. sonian（middle Anisian）and Illyrian（late Anisian）. The last scenario is the most unlikely because Lingu- Therefore, the bedded chert horizon from which the bra- laria are uncommon in these rocks; they have not been chiopod fossil was collected is considered to be of early reported previously from Triassic chert sequences, or Middle Triassic age. This age determination is consistent indeed chert sequences in general. At the end of Permian with previous reports based on radiolarian studies（e.g. time, sea level dropped ca. 200 m. It also dropped during Suyari et al., 1982）. middle Anisian time, but much less so and certainly not The brachiopod-bearing rock is a regularly-bedded enough to dramatically alter Lingularia habitat area chert facies consisting of alternating grey chert beds（2-3 （Erwin, 1993.）. Therefore, the third scenario is not con- cm thick）and yellowish grey shale beds（1-2 cm thick）. sidered plausible. Petrographically, the shale comprises mainly spherical The occurrence of Lingularia sp. from bedded chert is radiolarian tests, sponge spicules, a small number of con- unusual because it is preserved within a deep-sea facies odonts, and clay minerals, such as illite and white mica. that almost certainly accumulated below CCD. However, The chert comprises rich radiolarian tests, sponge occurrences of lingulids traditionally assigned to the spicules, rare conodonts, clay minerals and microcrys- genus“Lingula”, are not rare from Early to Middle Tri- talline quartz（Fig.3）. Stylolites are occasionally devel- assic marine sedimentary sequences（e.g. Rodland and oped within the chert Bottjer, 2001）. Accordingly, the first scenario is the most The fossil was discovered within a fragmental piece of reasonable explanation. chert measuring ca. 45×20×15 mm. Because the rock There are numerous records of abundant Lingularia has split sub-parallel to the surface of the fossil shell, the （ usually recorded as‘ Lingula’）from widespread brachiopod is preserved in two pieces of rock. One is sequences of Early Triassic. These occurrences have been Jour. Geol. Soc. Japan, 110（ 12 ） Lingularia from Triassic chert in Shikoku 761

Fig.4. Lingularia sp. from Middle Triassic bedded chert in the Kumosoyama area, Northern Chichibu Terrane, Shikoku, Japan:（a）exterior of ventral valve, ESEU Coll. MB0001a,（b）interior of the ventral valve, ESEU Coll. MB0001b. Scale bar ＝ 1 mm.

recognised from China, Spitsbergen, North America, Rus- Accordingly, Lingularia was probably well adapted to sia and also New Zealand. Recently, Lingularia has been survive low oxygen levels in the aftermath of the P/T cri- recognised in shale horizons within Middle Triassic sis（e.g., Hallam and Wignall, 1997）. （upper Anisian - lower Ladinian）bedded chert sequences Geochemically the Lingularia-bearing bedded chert in exposed on Waiheke Island in the Hauraki Gulf, North Japan indicates slightly anoxic（dysoxic）conditions on Island, New Zealand（S. Yamakita, pers. com.）. Similar the basis of moderate enrichment of U, S, V, As and Mo specimens of Lingularia-like inarticulate elements（Hori et al., 2000; Maeda et al., 2003）. The spec- have also been reported from upper Lower Triassic imens from Sikhote-Alin, Russia, were obtained from （Spathian）to lower Middle Triassic（lower Anisian）shale chert sequences at a very similar stratigraphic horizon beds associated with bedded chert sequences exposed on and probably with a very similar redox level（dysoxic）. In Sikhote-Alin, Russia（Bragin, 1991）. This record is from addition, a large number of Lingularia specimens, similar rocks within a Mesozoic accretionary complex that is to the present specimen, were obtained from framboidal inferred to be a northern extension of the Tamba-Mino- pyrite-rich strata of the Norian Kamonosho Formation of Ashio Terranes in Japan（Kojima, 1989; Bragin, 1991）. the Triassic Mine Group（Yanagida et al., 1995）. No other Furthermore, this stratigraphic horizon is similar to that fossils have been observed but framboidal pyrite is com- in Japan within the Tenjinmaru Formation. mon suggesting that the strata formed under oxygen These records and observations reveal a global distrib- depleted conditions. ution of Lingularia during Early to Middle Triassic time. Consequently, the genus Lingularia has been inter- The variety of occurrence and lithofacies from which Lin- preted as an opportunistic‘disaster’taxon that flour- gularia have been obtained may be broadly related to the ished in dysoxic marine environments following the P/T recovery period following the P/T boundary extinction biotic crisis（Rodland and Bottjer, 2001）. The present find event and to the nature of‘oceanic damage’such as the is relevant to these environmental interpretations as well oceanic anoxic event（OAE）or‘Superanoxia’of Isozaki as knowledge of early Middle Triassic deep-sea sedimen- （1997）. tation. The Lingularia-bearing chert described herein is from Although all facts support the first scenario as dis- within chert that is just above black shale of the‘Toishi- cussed above, we can not exclude the second possibility. type’siliceous shale. The latter is suggestive of a long A redeposited‘float’mechanism can be used to satisfac- anoxic period associated with post-P/T boundary event torily explain taphonomy of most fossils. Therefore, the conditions, yet it underlies a sequence of normally bedded unusual presence of Lingularia within Triassic bedded chert suggestive of normal oxic conditions. Generally chert is probably due to a mix of suitable circumstances speaking, Lingularia is long-ranging and appears to have relating to chance redeposition processes on the deep sea favoured dysoxic environments in the late Paleozoic. floor, and relative abundance of opportunistic lingulid 762 Hori, R. S. and Campbell, H. J. 2004―12

brachiopods as a function of widespread prevailing dysox- teristic of phosphatic shell preservation. The shell is 8.5 ic conditions. mm long and 4.5 mm wide. It has well-developed growth lines. The outermost rim is thickened and is notably black Systematic description in colour. The surface of the shell has a mesh-like pattern （H. J. Campbell） or structure. There is a light asymmetry of the hinge line Phylum Brachiopoda to either side of the beak but this is due to minor defor- Order Lingulida Waagen, 1885 mation as a function of compaction. The fossil has dis- Superfamily Lingulacea Menke, 1828 tinctive protuberances just beneath the beak that corre- Family Lingulidae Menke, 1828 spond to muscle scars. Remarks. -According to Holmer and Popov（2000: p.36- Remarks. -Overall, the preservation of the sample is 38）, the genus Lingularia belongs to Family Lingulidae entirely compatible with an inarticulate（phosphatic）bra- Menke, 1828, and is described as follows:“Shell elongate chiopod. The valve is probably ventral rather than dorsal oval with subparallel margins; ventral pseudointerarea based on the more elongate nature of the umbonal region. small, with broad triangular pedicle groove and vestigial The specimen described here as Lingularia sp. resem- propareas; umbonal scar heart-shaped, bisected by bles the type species of this genus Lingularia similis impression of pedicle nerve; dorsal visceral area extend- （Bienat and Emig, 1993）in terms of gross morphology, ing anteriorly beyond mid-length, with short median ridge but lack of detail due to preservational effects precludes a bisecting anterior lateral muscle scars, vascula lateralia more precise assignation. Accordingly, a certain generic in both valves convergent, but generally somewhat short- determination is assigned. er in ventral valve.” This specimen is of comparable size to those reported from Sikhote-Alin, Far East Russia as‘Lingula sp.’by Genus Lingularia Biernat and Emig, 1993 Bragin（1991）, which are about 8 mm long and 5 mm Type species. -Lingularia similis Biernat and Emig, wide, but the Russian material is more square in anterior 1993 outline. Remarks. -We accept the thinking of Biernat and Emig The specimen described herein closely resembles Lin- （1993）in restricting usage of the genus Lingula and gularia aff. lindströmi reported by Yanagita et al.（1995） accordingly have embraced their concept of the genus from the Triassic（Norian）Kamonosho Formation of the Lingularia for the Mesozoic lingulides. Biernat and Emig Mine Group, southwest Japan, but the Kamonosho speci- （1993）have established to our satisfaction that there are men has a relatively larger shell. significant morphological differences between these gen- Another specimen of Triassic Lingularia sp. has also era. Notably, they concluded that Lingularia has longer been recorded by Murata（1973）from the upper part of lohophoral cavities, shorter ventral canals, better-devel- the Osawa Formation（Olenekian; Lower Triassic）of the oped posterior adductor muscles and less acute umbones Inai Group, southern Kitakami Massif, northeast Japan. than Lingula. The Kitakami specimen is similar, but slightly older in age The type species of the genus is Lingularia similes, to our material. It also differs in that it has a low described from the upper Early Jurassic（Toarcian-Aalen- width/length ratio and a more triangular posterior outline. ian）Brentskardhaugen beds in Wimanfjellet, Spitsbergen Repository. -ESEU Coll. MB0001a and 1b in Department （Biernat and Emig, 1993）. The genus Lingularia is wide- of Earth Sciences, Ehime University, Japan. ly ranging from the Carboniferous to , and pos- Age. -Anisian（early Middle Triassic） sibly into the Tertiary. It is known from Triassic Acknowledgements sequences in Europe, Spitsbergen, Russia, North America, New Zealand, Pakistan and Japan（Lundgren, 1883; Bit- We would like to thank T. Okamoto and Y. Nara for their tner, 1899; Wittenberg, 1910; Newell and Kummel 1942; advice on identification of this brachiopod; S. Yamakita Dagys, 1965; Rowell, 1970; Broglio Loriga et al., 1980; for unpublished information relating to conodont and bra- Dagys and Kurushin, 1985; Campbell, 1987; Biernat and chiopod faunas that have been recently discovered on Emig, 1993）. Waiheke Island, New Zealand; K. Sashida for comments on conodont fossils from this material; N. Bragin for Lingularia sp. detailed information of Lingularia（recorded as Lingula Figures 1a , 1b sp.）from Shikhote-Alin; J. Tazawa and Y. Isozaki for crit- Description. -The shell is elliptical in shape and is pre- ical reviewing of this manuscript, and also D. MacFarlan served in shades of black to pale yellow-brown, charac- for his critical comment on Lingularia sp. Jour. Geol. Soc. Japan, 110（ 12 ） Lingularia from Triassic chert in Shikoku 763

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用語対比 Nakatsu: 中津 Kumoso-zuido: 雲早隧道 Takashiroyama Formation: 高城山層 Mt. Kumosoyama: 雲早山 Tenjinmaru Formation: 天神丸層

（要 旨）

Hori, R. and Hamish J. Campbell., 2004, Lingularia sp.（Brachiopoda）from Middle Triassic bedded chert in Shikoku, Japan. Jour. Geol. Soc. Japan, 110, 758－764. （堀 利栄・ヘーミッシュ・キャンベル，2004，四国の三畳系層状チャートから産出した Lingularia sp.（腕足類）．地質雑，110, 758－764.） 四国東部秩父累帯中の三畳系中部（Lower Anisian）層状チャートより Lingularia sp. （腕足類）の化石が産出した．産出層準は、砥石型珪質粘土岩から頁岩とチャートの等厚互 層への漸移部であり、堆積環境が還元状態から回復する過程の貧酸素状態を示す部分であ る．本標本と類似する Lingularia sp.は、ロシア極東シホテアリン山地の下部～中部三畳 系（Spathian-Lower Anisian）層状チャート層の頁岩部やニュージーランド付加体中の層状 チャートに伴う中期三畳紀の半遠洋性堆積岩からも見つかっている．この発見は、ペルム 紀／三畳紀境界イベントに伴う OAE（海洋貧酸素事変）からの回復過程でいわゆる“Lin- gula”（本研究での Lingularia も含む）が大繁栄したという見解を支持し、かつその影響 が、古太平洋の深海域まで及んだ可能性を示唆している.