geo 470 Xu - Brandon University

Geol. Mag. 137 (6), 2000, pp. 623–650. Printed in the United Kingdom © 2000 Cambridge University Press

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Late Ordovician to earliest Silurian graptolite and brachiopod biozonation from the Yangtze region, South China, with a global correlation CHEN XU*§, RONG JIAYU*, CHARLES E. MITCHELL†, DAVID A. T. HARPER‡, FAN JUNXUAN*, ZHAN RENBIN*, ZHANG YUANDONG*, LI RONGYU* & WANG YI* *Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, 210008, the People’s Republic of China †Department of Geology, State University of New York at Buffalo, Buffalo, NY 14260, USA ‡Geological Museum, University of Copenhagen, Oster-Volgade 5-7, DK-1350, Copenhagen, Denmark

(Received 2 March 2000; accepted 15 August 2000)

Abstract – Late Ordovician to earliest Silurian is an important geological period marked by large geological and biological events. However, the strata and fossils of this interval are not complete in many parts of the world. Based on studies of 43 sites in South China, in particular the continuous sections on the Yangtze platform, we recognize a complete succession including seven graptolite zones and two shelly faunas. In ascending order, the graptolite zones are the Dicellograptus complanatus, Dicellograptus complexus, Paraorthograptus pacificus (including Lower Subzone, Tangyagraptus typicus Subzone and Diceratograptus mirus Subzone), Normalograptus extraordinarius–Normalograptus ojsuensis, Normalograptus persculptus, Akidograptus ascensus and Parakidograptus acuminatus zones. The shelly faunas are the Foliomena–Nankinolithus and Hirnantia faunas, which may be correlated with D. complanatus Zone and N. extraordinarius–N. ojsuensis to part of N. persculptus zones respectively. The biozonation through this interval from the Yangtze region can be correlated with that of other parts of the world such as Dob’s Linn in Scotland, Spain and Portugal, Thuringia–Saxonia– Bavaria, Bohemia, Poland, Kazakhstan, Kolyma, Malaya Peninsula, Yukon, Canadian Arctic Islands, Nevada, Argentina, Niger and Victoria, Australia. The Hirnantian Substage, which has been proposed by us recently, includes the N. extraordinarius–N. ojsuensis Zone, Hirnantia fauna and N. persculptus Zone. The base of the Hirnantian Substage is marked by the First Appearance Data (FADs) of N. extraordinarius and N. ojsuensis, which have been determined to be synchronous on a global scale.

1. Introduction Latest Ordovician to earliest Silurian strata and fossils, mainly the strata of the Wufeng Formation, Kuanyinchiao Formation, and the lowest Lungmachi Formation, are widely distributed on the Yangtze platform (Fig. 1). Graptolites and brachiopods occur commonly in these rocks throughout the region. Thirty-seven sections from the Yangtze region (Locs 1–4, 6–38 in Fig. 1) have been measured and collected by the authors and other Chinese colleagues in the past 30 years (Mu et al. 1993). One section at Bajiaokou, Ziyang (Loc. 5) was recorded from the South Qinling mobile belt, along the north slope of the Yangtze platform. Five (Locs 39–43) are from the Xiangwan basin, the successor of the Zhujiang preAshgill basin. Localities 39 to 41, along the former Jiangnan belt, occur along the slope of the Xiangwan basin. Cathaysian Land experienced uplift beginning in the Ashgill and merged with Dianqian Land §Author for correspondence: [email protected]

(Central Guizhou Land). Moreover, Cathaysian Land became more prominent northwestwards during the late Ashgill. Thus, the Yangtze platform was surrounded by Cathaysian Land, Dianqian Land and the Chengdu uplift, forming a semi-enclosed bay that opened northwards during late Ashgill time (Chen, Xiao & Chen, 1987). Lee & Chao (1924) and Grabau (1924) were the first to collect and identify the latest Ordovician to earliest Silurian fossils when they erected the Lungma Shale from the Yangtze Gorges. Sun (1931) identified the graptolites collected by Hsieh and Liu from Yuyangguan, Wufeng County, southern Hubei, as Ashgill in age and then named the strata the Wufeng Formation. Sun (1933) subsequently described the Wufeng graptolites. Despite this work, the Ashgill age of the Wufeng Formation was still not commonly accepted by palaeontologists in China. It was Mu (1954), when he re-studied the Wufeng graptolite fauna and correlated the Wufeng Formation with other related strata within South China, who firmly established the age of the formation, which was then used as

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Figure 1. Distribution of the latest Ordovician to earliest Silurian strata with the reference sections and localities from the Yangtze platform and its neighbouring areas. 1 – Cuijiagou, Ningqiang (Chen in Mu et al. 1993), 2 – Qiaoting, Nanjiang (Liu, Chen & Zhang, 1964), 3 – Fucheng, Nanzheng (Chen, Xu & Cheng, 1994), 4 – Gaojiawan, Xixiang (Yu, Fang & Zhang, 1988), 5 – Bajiaokou, Ziyang (Fu & Song, 1986), 6 – Yangjiaba, Chengkou (Ge in Zhu et al. 1977), 7 – Xintan, Zigui (Mu, 1954), 8 – Jianyangping, Xinshan (the present paper), 9 – Maliangping, Baokang (Ge in Mu et al. 1993), 10 – Tangya, Yichang (Mu et al. 1993), 11 – Wangjiawan, Yichang, (Mu et al. 1984, the present paper), 12 – Fenxiang, Yichang (Mu et al. 1984), 13 – Huanghuachang, Yichang (Mu et al. 1984), 14 – Shibangou, Hanyuan (Feng, Yu & Fang, 1991), 15 – Qiaodingshan, Hanyuan (Mu et al. 1993, Feng, Yu & Fang, 1991), 16 – Maokoshan, Hanyuan (Feng, Yu & Fang, 1991), 17 – Binlingxiang, Hongya (Mu et al. 1993), 18 – Weiyuan (Mu and Chen field excursion in 1965, unpub. data), 19 – Shuanghe, Changning (Mu et al. 1978), 20 – Yanzikou, Bijie (Rong in Zhang et al. 1964), 21 – Donggongsi, Zunyi (Chen in Zhang et al. 1964), 22 – Honghuayuan, Tongzi (Chen in Zhang et al. 1964, the present paper), 23 – Hanjiadian, Tongzi (Chen in Zhang et al. 1964), 24 – Kuanyingqiao, Qijiang (Chen in Zhang et al. 1964, Jin et al. 1982), 25 – Yanwanggou, Huayingshan (Mu, 1954), 26 – Ganxi, Yanhe (Mu et al. 1993), 27 – Ludiping, Songtao (Rong in Zhang et al. 1964, the present paper), 28 – Huangban, Songtao (Mu et al. 1993, the present paper), 29 – Datianba, Xiushan (Ge Zhi-zhou et al. in Mu et al. 1993), 30 – Wentang, Dayong (Chen and Rong field excursion in 1990, unpub. data), 31 – Nishi, Shimeng (Ge Zhi-zhou et al. in Mu et al. 1993), 32 – Yuyangguan, Wufeng (Ge in Mu et al. 1993), 33 – Huaqiao, Changyang (Ge in Mu et al. 1993), 34 – Wulipai, Linxiang (Mu et al. 1993), 35 – Beigongli, Jingxian (Li, 1984), 36 – Sinianpan, Hexian (Zhang Quan-zhong et al. 1966), 37 – Tangshan, Nanjing (Mu, Pan & Yu, 1955), 38 – Lunshan, Jurong (Chen et al. 1988), 39 – Helixi, Ninggou (Li, 1984), 40 – Xinkailing, Wuning (Yu & Fang, 1986), 41 – Dafuping, Anhua (Liu & Fu, 1984), 42 – Shengping, Xingan (Chen et al. 1981), 43 – Guting, Chongyi (Chen, Xiao & Chen, 1987).

a mapping unit. Following upon the acceptance of an Ashgill age for the Wufeng Formation, a related problem, the age of the Hirnantia–Dalmanitina-bearing Kuanyinchiao Formation, arose during the 1960s. This problem was not solved until Rong (1979) placed the Kuanyinchiao Formation with a global correlation of the Hirnantia fauna. Mu (1954) initially subdivided the Wufeng graptolites into two zones, the Pleurograptus lui Zone in the lower part and the Dicellograptus szechuanensis Zone in the upper. The graptolite biozonation was significantly improved when Mu and co-workers defined five graptolite zones from the Wufeng Formation across the Yangtze region at the 1965 Convention of the Palaeontological Society of China, Nanjing. The five zones were the Pleurograptus lui (W1), Dicellograptus

szechuanensis (W2), Tangyagraptus typicus (W3), Diceratograptus mirus (W4) and Paraorthograptus uniformis (W5) zones, in ascending order. Unfortunately, this zonation was not published until 1974. Mu (1974) finally published the five Wufeng graptolite zones with an additional one, the Diplograptus bohemicus Zone (W6), at the top of the Wufeng Formation. The Pleurograptus lui Zone was renamed the Amplexograptus disjunctus yangtzensis/Pleurograptus lui Zone by Mu (1980) and then the Amplexograptus disjunctus yangtzensis Zone by Mu et al. (1993). Chen & Zhang (1995) and Chen et al. (1995) abandoned the Amplexograptus disjunctus yangtzensis Zone since Riva (1987) demonstrated that the zonal fossil was a junior synonym of Amplexograptus latus (Elles & Wood). Chen & Zhang (1995) and Chen et al. (1995)

Ordovician–Silurian biozonation of Yangtze region, China proposed that the D. szechuanensis Zone should be replaced by Dicellograptus complexus Zone since the former is obviously a junior synonym of the latter. They also abandoned the P. uniformis Zone. They specified two species, N. bohemicus (Marek) (based on the Chinese specimens) and N. extraordinarius (Sobolevskaya), as zonal index forms. They accepted the Normalograptus persculptus Zone, which was until then considered as the lowest Silurian biozone, as the highest Ordovician graptolite zone. During 1996 to 1998, four reference sections, the Wangjiawan (Loc. 11 of Fig. 1) and Fenxiang (Loc. 12), Yichang County, western Hubei Province, Honghuayuan (Loc. 22), Tongzi County, northern Guizhou Province and Ludiping (Loc. 27), Songtao County, northeastern Guizhou Province, were remeasured and re-collected layer by layer through the Wufeng Formation to the base of Lungmachi Formation by the present authors. We propose that the Tangyagraptus typicus Zone and Diceratograptus mirus Zone should be two subzones of the Paraorthograptus pacificus Zone. The First Appearance Datum (FAD) of P. pacificus (Ruedemann) is lower than that of Tangyagraptus typicus Mu but higher than that of D. complexus Davies. Thus, the lower part of the P. pacificus Zone is temporarily referred to as the Lower Subzone. P. pacificus (Ruedemann) is a more appropriate zonal fossil because of its wide palaeogeographic distribution and common occurrence within the interval. The occurrence of Paraorthograptus uniformis Mu & Li in the interval with N. extraordinarius and N. ojsuensis from the Wangjiawan and Fenxiang sections indicates that Mu’s Paraorthograptus uniformis Zone should be a part of the Normalograptus extraordinarius–N. ojsuensis Zone. Koren (in Apollonov, Bandaletov & Nikitin, 1980) has considered that Normalograptus bohemicus (Marek) (=Glyptograptus bohemicus Marek, 1954) is a subjective junior synonym of Normalograptus persculptus s.s. (=Glyptograptus? persculptus forma B, Koren, 1980). Sˇtorch & Loydell (1996) recently demonstrated that Normalograptus bohemicus (Marek) is conspecific with Normalograptus persculptus (Elles & Wood) based on their re-study of the type specimens of the former and additional specimens collected from both the Czech Republic and Wales. Thus, the conception of N. persculptus (Elles & Wood) has been slightly broadened. We recognize that the form which occurs in the Yangtze region and has been referred to as N. persculptus is indeed N. persculptus (Elles & Wood) sensu stricto. However, Fan (Fan Jun-xuan, unpub. Masters thesis, Nanjing Institute of Geology & Palaeontology, 1998) recently concluded that the species which Mu and his successors (1993) as well as Wang et al. (1987) called ‘Glyptograptus bohemicus’ or ‘Diplograptus bohemicus’ from below the Kuanyinchiao Formation is conspecific with Normalograptus ojsuensis (Koren & Mikhailova) based on his morphometric study of

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these two forms. Thus, the Chinese pre-Hirnantia specimens identified to ‘Glyptograptus bohemicus’ or ‘Diplograptus bohemicus’ by Chinese authors should be assigned to Normalograptus ojsuensis (Koren & Mikhailova) rather than Normalograptus persculptus (Elles & Wood). N. extraordinarius (Sobolevskaya) now is known to be a widely distributed species not only in Siberia, Kazakhstan, Europe and Australia, but also in the Yangtze region of China. Thus, we prefer to use the N. extraordinarius–N. ojsuensis Zone for the interval in China between the Diceratograptus mirus Subzone (below) and the N. persculptus Zone (above), which was formerly called the N. bohemicus–N. extraordinarius Zone in Chen et al. (1995). Rong (1984a) suggested that the base of Kuanyinchiao Formation (Hirnantia-bearing beds) is diachronous across the Yangtze region and may correspond to the base of the Diceratograptus mirus Subzone in northeastern Guizhou. The top of the Kuanyinchiao Formation was regarded to be always below the base of the N. persculptus Zone. In the Yangtze Gorge area, however, he noted later (in Chen et al. 1995) that the top of Hirnantia-bearing beds may reach into the N. persculptus Zone. The brachiopod fauna of the strata below the N. extraordinarius–N. ojsuensis Zone is a very low diversity fauna characterized by Manosia. The Manosia assemblage is particularly abundant in the Diceratograptus mirus Subzone and extends to the N. extraordinarius–N. ojsuensis Zone (see Fig. 2). At the Honghuayuan section (Loc. 22 of Fig. 1), some elements of the Hirnantia fauna such as Eostropheodonta and Hindella may extend into the higher beds, possibly corresponding to A. ascensus Zone where the real Hirnantia is absent. 2. Biozonation The Ashgill graptolites of the Yangtze region (Wufeng Formation) are highly diverse. The region was a semienclosed platform in this time interval, and lay at a low latitude within the warm water Pacific realm. Consideration of the high individual abundance of graptolites as well as the high species diversity in the Wufeng Formation, in the light of recent palaeoecological models (e.g. Finney & Berry, 1998), leads us to suggest that the Yangtze region may have experienced high and stable organic productivity. Chen & Zhang (1995) have reviewed the Wufeng graptolites described by Mu et al. (1993). They concluded that the Wufeng graptoloid fauna includes 30 genera and about 80 species rather than the 27 genera and 176 species recognized by Mu et al. (1993). However, the diversity of the Wufeng graptolite fauna is still very high and more than half of the late Ashgill fauna are endemic species. In the present study, the Ashgill to earliest Llandovery biozones from the Chientsaokou Formation to the base of the Lungmachi Formation are based mainly on four reference sections: the Wangjiawan, Fenxiang, Honghuayuan and

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Figure 2. Ashgill to earliest Llandovery biozonation of South China employed by Mu An-zhi and colleagues, compared to that of the present study.

Ludiping sections. In the present systematic collections from these four sections, only four monospecific genera, Neurograptus, Nymphograptus, Orthoretiograptus and Sinoretiograptus described by Mu et al. (1993) have not yet been re-collected since they are all based on very rare specimens. The ranges of the graptolites and shelly fossil groups, mainly brachiopods, are demonstrated on four range charts (Figs 3–6). We also demonstrate some important forms of the related graptolite fauna in Figure 7 and the shelly fauna in Figure 8. Finally, we list the important species for which we differ significantly in our interpretations from those of Mu et al. (1993) in Table 1. The following abbreviations indicate different biozones: Dc, Dicellograptus complexus Zone; L–Tt, Lower Subzone to Tangyagraptus typicus Subzone of the Paraorthograptus pacificus Zone; Dm, Diceratograptus mirus Subzone of the P. pacificus Zone; Neo, Normalograptus extraordinarius–N. ojsuensis Zone; Np, N. persculptus Zone. 2.a. Foliomena–Nankinolithus Zone

The Foliomena–Nankinolithus Zone is widely recognized from the Chientsaokou Formation and corresponding units (lower Ashgill) on the Yangtze platform. The brachiopods from this horizon belong to the Foliomena fauna and trilobites to the Nankinolithus fauna. The Foliomena fauna was recorded from rocks of Caradoc to mid-Ashgill age in many localities from South China, North China,

Avalonia, Baltica, Laurentia, Kazakhstan, Southern Europe, Sibumasu and others (Cocks & Rong, 1988; Rong & Zhan, 1995; Rong, Zhan & Harper, 1999). It has been commonly recovered from the lower Ashgill rocks in South China. Rong et al. (1994) described a Kassinella– Christiania association in the early Ashgill Foliomena fauna of northern and northeastern Guizhou. A single specimen of Foliomena collected there is associated with Orbiculoidea, Paterula, Glyptorthis, Dedzetina, Leptestiina, Sericoidea, Kassinella, Eoplectodonta and Christiania in the Chientsaokou Formation at Donggongsi, Zunyi, northern Guizhou. In this region the Foliomena fauna is associated with a low diversity trilobite assemblage that includes Nankinolithus. This association has been recovered from the southernmost areas of the Upper Yangtze epicontinental sea, and occurred in relatively near-shore, shallower environments adjacent to Dianqian Land during the early Ashgill. However, the Foliomena fauna from Tangtou and Huangnekang formations (early Ashgill) of southern Jiangsu, southern Anhui and western Zhejiang, east China, is more diverse, and includes the brachiopods Philhedra, Dedzetina, Kozlowskites, Leptestiina, Christiania, Foliomena, Cyclospira and others (Cocks & Rong, 1988). It is associated with a more diverse trilobite fauna (Lu & Zhou, 1981). In addition to Nankinolithus, there occur Lonchodomas, Ovalocephalus, Shumardia, Corrugatagnostus, Nileus, Cyclopyge, Microparia, Dionide, Miaopopsis and 27

Ordovician–Silurian biozonation of Yangtze region, China

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Figure 3. Lithological column and the vertical distribution of graptolite and shelly fossils from Wangjiawan, Yichang, western Hubei (Loc. 11).

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Table 1. Revision of the Ashgillian graptolites described by Mu et al. (1993) Graptolite zonation (present paper) Revised species (present study) Dicellograptus graciliramosus Yin & Mu D. ornatus E.& W. D. complexus Davies D. anceps (Nicholson) Glyptograptus gracilis (Ge)

Normalograptus ojsuensis (Koren & Mikhailova)

N. angustus (Perner) N. sp. N.? spicatus (Ge) N. extraordinarius (Sobolevskaya) N.? sp. Amplexograptus latus Elles & Wood

A. sp. nov. Pseudoclimacograptus chiai (Mu)

Climacograptus? hastatus (T.S. Hall)

Climacograptus? hastatus vesicicaulis(Ge) Appendispinograptus. longispinus hvalross (Ross & Berry)

A. hubeiensis (Ge) A. supernus (Elles & Wood)

A. supernus sinicus (Ge) A. venustus (Hsu)

Species described by Mu et al. (1993) D. graciliramosus Yin & Mu Leptograptus transformis Chen D. ornatus E.& W. D. excavatus Mu D. szechuanensis Mu D. anceps (Nicholson) D. brevis Mu & Chen G. gracilis Ge G. mirus Mu & Lin (pars) Diplograptus acutus Lin D. carcharus Lin Diplograptus bohemicus (Marek) D. vicatus Lin D. wangcangensis Mu & Li Diplogr. vicatus Lin (pars) Glyptogr. sp. Glyptogr. salignus Lin C. miserabilis E.& W. C. pygmaeus Ruedemann Climacogr. angustatus Ekström C. celsus Ekström C. spicatus Ge G. salignus Lin Diplograptus orientalis Mu et al. C. raricaudatus Ross & Berry C. corneus Ge A. regularis Mu & Lin A. disjunctus yangtzensis Mu & Lin (pars) A. inuiti (Cox) A. suni (Mu) A. gansuensis yichangicus Mu & Lin (pars) Amplexogr. hubeiensis Mu & Lin Pseudoclimacogr. yilingensis Mu & Lin Pseudocli. arcanus Lin C. hastatus T.S. Hall C. hastatus finis Ge C. hastatus tentaculatus Ge C. hastatus tumidulus Ge C. cf. tridentatus Lapworth C. acutus Ge C. abnormispinus Ge C. yingpanensis Ge Climacograptus vesicicaulis Ge C. tubuliferus Lapworth Climacograptus bellulus Mu & Zhang (pars.) C. bicornis (Hall), Ge 1993 C. longispinus T.S. Hall (pars.) C. xintanensis Ge C. cf. hvalross Ross & Berry C. textilis Ge C. textilis yichangensis Ge Climacograptus hubeiensis Ge C. hanyuangensis Ge Climacograptus supernus Elles & Wood C. bellulus Mu & Zhang (pars.) C. minor Ge C. notabilis Ge C. superus longus Ge Climacograptus sinicus Ge C. diplacanthus Bulman, Ge (1993) Climacograptus venustus Hsu C. venustus simplex Ge

Dc

L–Tt

Dm

+ + + + + + +

+

+

+ + +

+

Neo

Np

+ + + + + + + + + + + + + + +

+ + + + +

+ +

+ +

+ +

+

+ + + + +

+

+

cf.

+ + +

+

+

+ +

+ + + + + + + + + + +

+ + + + + + + +

+ + +

+ +

+

+

+ + + + + + + + + + + +

+ +

+

+

+

+

+

+ + + + + + +

+

+

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Ordovician–Silurian biozonation of Yangtze region, China Table 1. (Cont.) Graptolite zonation (present paper) Revised species (present study)

A. leptothecalis (Mu & Ge) A.? fibratus Ge Anticostia fastigata (Davies) A. uniformis (Mu & Lin) Orthograptus maximus Mu

Orthograptus sp. aff. O. maximus Mu Rectograptus obesus Li R. abbreviatus E. & W.

R. uniformis (Mu & Lee) R. socialis (Lapworth) R. songtaoensis Li Paraorthograptus pacificus (Ruedemann)

P. brevispinus Mu & Li P. longispinus Mu & Li Nymphograptus velatus Elles & Wood Pararetiograptus sinensis Mu Paraplegmatograptus uniformis Mu P. connectus Mu Yinograptus disjunctus Yin & Mu

Yangzigraptus yangziensis Mu

Species described by Mu et al. (1993) C. aequus Ge C. mirus Ge & Jiao Climacograptus leptothecalis Mu & Ge Climacograptus fibratus Ge Orthograptus sextans Li Diplograptus palaris Lin D. ostreatus Lin Glyptograptus uniformis Mu & Lin Orthograptus maximus Mu O. rigidus Lee Glyptograptus triangulatus Mu & Lin Glyptograptus lobosus Lin Orthograptus xinanensis Li O. augescens Li O. yangziensis Li Rectograptus truncatus obesus Li Rectograptus abbreviatus E. & W. R. abbreviatus holoensis Li Rectograptus ensiformis Li R. carnei T.S. Hall R. gracilis sinicus Li Orthograptus tsunyiensis (Mu) R. uniformis (Mu & Lee) R. truncatus (Lapworth) R. socialis (Lapworth) R. pauperatus (E. & W.) R. songtaoensis Li R. intermedius (E. & W.) Paraorthograptus pacificus (Ruedemann) P. angustus Mu & Li P. typicus Mu P. occidentatus (Ruedemann) P. longispinus Mu & Li P. nanchuanensis Li P. simplex Li Nymphograptus sichuanensis Mu Pararetiograptus sinensis Mu P. regularis Mu P. turgidus Mu Paraplegmatograptus uniformis Mu P. gracilis Mu P. formosus Mu P. connectus Mu P. hubeiensis Mu P. sextans Mu Yinograptus disjunctus Yin & Mu Y. robustus Mu Y. gracilispinus Mu Y. brevispinus Mu Yangzigraptus yangziensis Mu Y. minutus Mu

Dc

L–Tt

+ + +

+ + +

+ + + + + + ? +

+

+ + + +

Dm

Neo

+ +

+

+

+

+

+

+

+ +

+

+

+

+

cf.

cf.

+ + + + ? +

+ +

+

+

+

+

+

+

+ + +

+ +

+

+

+

+ +

Np

+ + + + +

+ + ? + ? + + + ?

+ + + + + + + + + + + + + + +

+ + + + + + +

?

Biozone abbreviations: Dc, Dicellograptus complexus Zone; L–Tt, Lower Subzone to Tangyagraptus typicus Subzone of the Paraorthograptus pacificus Zone; Dm, Diceratograptus mirus Subzone of the P. pacificus Zone; Neo, Normalograptus extraordinarius–N. ojsuensis Zone; Np, N. persculptus Zone.

other genera represented by a total of 41 species. Although Foliomena, Christiania and Nankinolithus are also present in the Chientsaokou Formation, the trilobites and brachiopods in the Tangtou Formation are more diverse than those in the Chientsaokou Formation. Moreover, only a single specimen of Foliomena and no Cyclospira have been recovered from

the latter, whereas these two genera are common in the Tangtou Formation. It should be pointed out that the brachiopods in the Tangtou Formation represent a typical Foliomena fauna inhabiting a relatively deeper water regime (Sheehan, 1973). Two species of graptolites, Dicellograptus cf. johnstrupi (Hadding) and Rectograptus pauperatus (Elles &

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Figure 4. Lithological column and the vertical distribution of graptolite and shelly fossils from Fenxiang, Yichang, western Hubei (Loc. 12).

Wood), have been recorded from the Chientsaokou Formation at Jiadanwan, near Donggongsi of Zunyi (Loc. 21 of Fig. 1) (see Zhang et al. 1964). The former was first reported and described by Hadding (1912) from the Zone of Pleurograptus linearis of Bornholm,

Denmark. The latter was described by Elles & Wood (1907) from the Dicranograptus clingani Zone to Pleurograptus linearis Zone at Dob’s Linn, Scotland. Mu et al. (1993) have described R. pauperatus also from the Dicellograptus complexus Zone of the

Ordovician–Silurian biozonation of Yangtze region, China

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Figure 5. Lithological column and the vertical distribution of graptolite and shelly fossils from Honghuayuan, Tongzi, northern Guizhou (Loc. 22).

Wufeng Formation. However, we have not recognized this species in the Wufeng Formation of the present four reference sections.

At the Honghuayuan and Ludiping sections (Locs 22 and 27 of Figs 1, 5 and 6), we found Dicellograptus complanatus Lapworth at the base of the Wufeng

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Figure 6. Lithological column and the vertical distribution of graptolite and shelly fossils from Ludiping, Songtao, northeastern Guizhou (Loc. 27).

Ordovician–Silurian biozonation of Yangtze region, China Formation. This is the first time that D. complanatus has been reported from the Yangtze region. The graptolite fauna from these two sections may indicate the upper D. complanatus Zone. The fauna consists of 12 genera and 21 species, including several wellknown species such as Amplexograptus latus (Elles & Wood), Anticostia fastigata (Davies) and Appendispinograptus supernus (Elles & Wood). Among the members of this assemblage, only the cosmopolitan Appendispinograptus longispinus (T.S. Hall), is limited in the D. complanatus Zone. All the species of Dicellograptus, Normalograptus, Anticostia, Rectograptus, Orthograptus, Amplexograptus, Pseudoclimacograptus and Pararetiograptus, as well as other Appendispinograptus species, extend to the succeeding Dicellograptus complexus Zone (Figs 5, 6). At the Ludiping section, the FAD of D. complexus Davies is at bed AFA 382, 1.15 m above the LAD (last appearance datum) of D. complanatus Lapworth. This lowest Wufeng Formation fauna indicates that the base of the Wufeng Formation at this section lies slightly below the top of the D. complanatus Zone. It is, however, the lowest base of the Wufeng Formation within the Yangtze region. At most localities in the Yangtze region, the base of the Wufeng Formation is roughly coincident with the base of the D. complexus Zone. The Chientsaokou Formation, characterized by the shelly Foliomena–Nankinolithus fauna with a few graptolites, is early Ashgill in age. The occurrence of the D. complanatus fauna at the base of the Wufeng Formation from Honghuayuan and Ludiping is in agreement with the biostratigraphic implications of this underlying fauna and likewise places the base of the Wufeng Formation in the lower Ashgill. The Chientsaokou Formation is characterized by mudstone and shale and is distributed around Dianqian Land. It appears to have been deposited in near-shore, low energy, mudstone facies or a mixed facies environment. The top of the unit also may be diachronous, from within the upper D. complanatus Zone (as at Honghuayuan and Ludiping) into the lowermost D. complexus Zone (Donggongsi, Zunyi, Loc. 21 of Fig. 1). 2.b. Dicellograptus complexus Zone

The D. complexus Zone, which is reported from all 43 Wufeng Formation localities, is the most widely distributed of the Ordovician biozones in South China. This biozone not only covers the whole Yangtze platform but also the whole Xiangwan basin as well as the slope belt in between. The base of the zone coincides with the FAD of the eponymous form based on the Honghuayuan and Ludiping sections (Figs 3, 5), where the boundary between the D. complexus Zone and the underlying D. complanatus Zone is within a continuous graptolite sequence. In the succeeding level to the base of the D. complexus Zone, more endemic forms occur, such as Dicellograptus sp. aff. D. com-

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planatus Lapworth, D. ornatus Elles & Wood, Paraplegmatograptus connectus Mu, Rectograptus songtaoensis Li, R. uniformis Mu & Li, Appendispinograptus? fibratus (Ge) and Pararetiograptus sinensis Mu. The species D. sp. aff. D. complanatus Lapworth described by Chen (as D. cf. complanatus in Mu et al. 1993) is similar to D. complanatus Lapworth in general characters, but the stipes of the former are narrower and more uniform in width. It might be a geographical subspecies of D. complanatus. However, D. sp. aff. D. complanatus ranges into higher strata than D. complanatus. In the middle to upper part of the D. complexus Zone, Dicellograptus anceps (Nicholson), D. minor Toghill, Leptograptus annectans amplectens Ruedemann, Rectograptus abbreviatus (Elles & Wood) and Climacograptus? hastatus (T.S. Hall) occur with two endemic forms, such as Dicellograptus graciliramosus Yin & Mu and Yinograptus disjunctus (Yin & Mu). Other widely distributed species in this interval include Leptograptus macer Elles & Wood, Dicellograptus complanatus arkansasensis Ruedemann, Amplexograptus latus (Elles & Wood), Normalograptus angustus (Perner), Rectograptus socialis (Lapworth), Appendispinograptus longispinus hvalross (Ross & Berry), A. supernus (Elles & Wood), A. hubeiensis (Ge) and Anticostia fastigata (Davies). Many of these species continue into this zone from the underlying D. complanatus Zone. The diversity of the D. complexus Zone is obviously greater than that of the lower D. complanatus Zone. There are 17 genera and a total of 41 species in the D. complexus fauna (not the genera Neurograptus, Nymphograptus, Orthoretiograptus and Sinoretiograptus, Mu et al. 1993). About half of the species (22 species) are endemic. Ten genera, including Neurograptus, Nymphograptus, Orthoretiograptus, Sinoretiograptus, Yangzigraptus, Yinograptus, Paraplegmatograptus, Arachniograptus, Pleurograptus and Paraorthograptus, originate in this zone. This pattern indicates that a burst of speciation in graptoloids, especially in reteograptids and archiretiolitids, began in this time interval. The Ashgill graptoloid radiation took place during the D. complexus Zone and Paraorthograptus pacificus Zone interval, which was followed by the climax episode of the late Ashgill mass extinction (Chen & Rong, 1991). 2.c. Paraorthograptus pacificus Zone

The base of the P. pacificus Zone coincides with the FADs of Paraorthograptus pacificus (Ruedemann), P. brevispinus Mu & Li, Paraplegmatograptus uniformis Mu, Leptograptus planus Chen and Dicellograptus tumidus Chen. The P. pacificus Zone may be subdivided into three parts: the Lower Subzone, the Tangyagraptus typicus Subzone and the Diceratograptus mirus Subzone. The middle and upper parts correspond to the Tangyagraptus typicus and

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Figure 7. For legend see facing page.

CHEN XU AND OTHERS

Ordovician–Silurian biozonation of Yangtze region, China Diceratograptus mirus zones, respectively, of Mu et al. (1984, 1993). 2.c.1. Lower Subzone

In a similar fauna to that of the D. complexus Zone, 17 genera and 42 species have been recognized from this interval. Among them, 19 species are cosmopolitan or intercontinental. The diversity of the fauna is almost the same as that of the underlying D. complexus Zone, and the fauna consists mostly of the same species as in the D. complexus Zone with the addition of seven new species. In addition to the five species that appear at the base of the zone as mentioned above, two further species, Pararetiograptus parvus Mu and Pseudoreteograptus nanus Mu, originate within the subzone. This subzone and the underlying strata have not yet been restudied from the Wangjiawan section because of poor exposures. 2.c.2. Tangyagraptus typicus Subzone

The base of the subzone is marked by the FADs of Tangyagraptus typicus Mu, T. remotus Mu & Chen, T. flexilis Mu & Chen and T.? grandis Mu & Chen. A remarkable faunal change occurs at this level. It involves not only the occurrence of the Tangyagraptus species but also the appearance of many new species of Dicellograptus, Normalograptus, Paraorthograptus, Paraplegmatograptus, Yinograptus and Sunigraptus. The most important of these additional new species

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are Dicellograptus mirabilis Mu & Chen, Normalograptus normalis (Lapworth), N. tatianae (Keller), Paraorthograptus longispinus Mu & Li and Yinograptus dubius Mu. The tangyagraptid species group, characterized by the presence of specialized secondary thecal cladia, is derived from Dicellograptus. The entire stratigraphic range of Tangyagraptus is limited to the T. typicus Subzone. Several new normalograptids, such as N. normalis (Lapworth), first appear within the subzone and become abundant higher up the succession in lower Silurian strata. There are four species of Normalograptus in this subzone, two of which originate near the top of the unit. The diversity of the T. typicus Subzone reaches a late Ordovician peak with a total of 18 genera and 53 species. Among them, only 23 species are cosmopolitan or intercontinentally distributed. The Manosia inarticulate brachiopod assemblage occurs near the top of the subzone and flourishes in the succeeding Diceratograptus mirus Subzone. 2.c.3. Diceratograptus mirus Subzone

This Subzone occupies only a very thin stratigraphic interval. At the Ludiping section where the Wufeng Formation reaches its greatest thickness (17.53 m), the D. mirus Subzone is only 0.4 m in thickness. The base of D. mirus Subzone is marked by the FAD of Diceratograptus mirus, which is another specialized branch from Dicellograptus. Normalograptids, such as

Figure 7. Important Ashgillian to earliest Llandovery graptolites from the Yangtze region. (a–b) Paraplegmatograptus uniformis Mu, 1978, T. typicus Subzone of P. pacificus Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei, AFA84. (a) main part of the rhabdosome; (b) proximal end of the rhabdosome. (c, m) Parakidograptus acuminatus (Nicholson, 1867), P. acuminatus Zone, Lungmachi Formation, Wangjiawan, Yichang, Hubei, AFA107, AFA108. (d) Paraorthograptus pacificus (Ruedemann, 1947), T. typicus Subzone of P. pacificus Zone, Wufeng Formation, Honghuayuan, Tongzi, Guizhou, AFA287. (e, ab) Normalograptus extraordinarius (Sobolevskaya, 1974), N. extraordinarius–N. ojsuensis Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei and Honghuayuan, Tongzi, Guizhou, AFA97, AFA290. (f) Appendispinograptus venustus (Hsu, 1959), T. typicus Subzone of P. pacificus Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei, AFA86. (g, h, i, aa) Akidograptus ascensus Davies, 1929, A. ascensus Zone, Lungmachi Formation, Wangjiawan, Yichang, Hubei, AFA105-106, AFA106, AFA106, AFA108. (j) Diceratograptus mirus Mu, 1963, D. mirus Subzone of P. pacificus Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei, AFA95. (k) Appendispinograptus supernus (Elles & Wood, 1906), D. mirus Subzone of P. pacificus Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei, AFA95. (l) Normalograptus sp. aff. N. persculptus (Elles & Wood, 1907), N. persculptus Zone, Base of the Lungmachi Formation, Wangjiawan, Yichang, Hubei, AFA101b. (n) Yinograptus disjunctus (Yin & Mu, 1945), D. complexus Zone, Wufeng Formation, Honghuayuan, Tongzi, Guizhou, AFA276. (o) Normalograptus madernii (Koren & Mikhailova, 1980 in Apollonov, Bandaletov & Nikitin, 1980), N. persculptus Zone, Base of the Lungmachi Formation, Wangjiawan, Yichang, Hubei, AFA101b. (p) Dicellograptus complexus Davies, 1929, D. complexus Zone, Wufeng Formation, Honghuayuan, Tongzi, Guizhou, AFA277. (q) Normalograptus persculptus (Elles & Wood, 1907), N. persculptus Zone, Lungmachi Formation, Wangjiawan, Yichang, Hubei, AFA102. (r) Tangyagraptus typicus Mu, T. typicus Subzone of P. pacificus Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei, AFA89. (s) Dicellograptus complanatus (Lapworth), D. complanatus Zone, Wufeng Formation, Ludiping, Songtao, Guizhou, AFA379. (t) Dicellograptus anceps (Nicholson), D. complexus Zone, Wufeng Formation, Ludiping, Songtao, Guizhou, AFA384. (u) Normalograptus ojsuensis (Koren & Mikhailova, 1980 in Apollonov, Bandaletov & Nikitin, 1980), N. extraordinarius–N. ojsuensis Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei, AFA99. (v) Pararetiograptus sinensis Mu, 1974, Lower Subzone of P. pacificus Zone, Wufeng Formation, Ludiping, Songtao, Guizhou, AFA395. (w) Anticostia fastigata (Davies, 1929), T. typicus Subzone of P. pacificus Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei, AFA89. (x, y) Orthograptus maximus Mu, 1945 (=Glyptograptus posterus Koren & Tzai, 1980 in Apollonov, Bandaletov & Nikitin, 1980), T. typicus Subzone of P. pacificus Zone, Wufeng Formation, Wangjiawan, Yichang, Hubei, AFA89. (z) Dicellograptus ornatus Elles & Wood, T. typicus Subzone of P. pacificus Zone, Wufeng Formation, Honghuayuan, Tongzi, Guizhou, AFA287. All the figures are × 6.5.

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Figure 8. For legend see facing page.

CHEN XU AND OTHERS

Ordovician–Silurian biozonation of Yangtze region, China N. sp. aff. N. angustus (Perner) and N. imperfectus (Legrand), first appear in this subzone. There are a total of 17 genera and 38 species in this unit with 18 cosmopolitan or intercontinentally distributed species. However, in the shallow water areas such as Honghuayuan and Ludiping, the diversity of late D. mirus fauna decreased substantially compared with that of the early D. mirus fauna. Despite this overall diversity decline, it is noteworthy that the species diversity of Normalograptus increased from four species in T. typicus Subzone to five species in the present subzone. If this pattern of diversity change can be taken at face value (the subzone is markedly thinner than neighbouring units, so that some differences in diversity may reflect unequal sampling intervals or sampling opportunities or both), then normalograptids had begun to increase their representation in the latest Ordovician graptolite faunas of this region in the upper parts of the P. pacificus Zone, that is, prior to the onset of mass extinction. 2.d. Normalograptus extraordinarius–N. ojsuensis Zone

The base of N. extraordinarius–N. ojsuensis Zone is marked by the FADs of N. extraordinarius (Sobolevskaya) and N. ojsuensis (Koren & Mikhailova). The appearance of Paraorthograptus uniformis Mu & Li suggests that Mu’s P. uniformis Zone may correspond to a part of the present zone. Neodiplograptus shanchongensis (Li) first appears in the same level as P. uniformis Mu & Li. The N. extraordinarius–N. ojsuensis Zone fauna contains 11 genera and 31 species in the deeper water facies from Yichang (Locs 10–13 of Fig. 1). Among them, 17 species are cosmopolitan or intercontinentally distributed. The N. extraordinarius–N. ojsuensis fauna is dominated by Normalograptus species. The nine Normalograptus species (seven of which are intercontinentally distributed) present in this interval comprise 29 % of the fauna as well as the great preponderance of

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newly evolved species in the assemblage. In the relatively more shallow water facies such as at the Honghuayuan section near Tongzi, the N. extraordinarius–N. ojsuensis Zone includes only the two eponymous forms associated with N. angustus (Perner), N. normalis (Lapworth), as well as a new subspecies of N. extraordinarius (Sobolevskaya). Thus, normalograptids make up the entire graptolite fauna in the shallower facies of this zone. The episode of mass extinction reached its peak in the deeper area of central Yangtze (Yichang region) within the interval of this zone. 2.e. Hirnantia fauna

The well-known Hirnantia fauna is a short-lived (Hirnantian), widely distributed, relatively shallow and cool water brachiopod assemblage (Havlícˇ ek, 1976; Brenchley, 1984; Sheehan, 1988; Rong & Harper, 1988; Sheehan & Coorough, 1990, 1996; Owen, Harper & Rong, 1991; Brenchley et al. 1994; Brenchley, Carden & Marshall, 1995; Harper & Rong, 1995; Rong, Zhan & Harper, 1999, and others). It is known from the Hirnantian rocks of South China, Sibumasu, Tibet, Kazakhstan, Baltica, Avalonia, Southern Europe, Northern Africa, South America, Laurentia and other palaeoplates. It is characterized by the association of typical, distinctive constituents such as Hirnantia, Dalmanella, Kinnella, Paromalomena, Eostropheodonta, Plectothyrella and Hindella (=Cryptothyrella). In addition, Cliftonia, Leptaena (=Leptaenopoma) and Fardenia or Coolinia (=Chilidiopsis) are sometimes common. We found the majority of these taxa associated with Dalmanitina in the Kuanyinchiao Formation at Ludiping, Songtao, northeastern Guizhou, which represents a near-shore, shallower water and lower diversity manifestation of the Hirnantia fauna that is typical in the Yangtze region. However, the Hirnantia fauna of the Kuanyinchiao Formation in the Yichang area, western Hubei and Changning, southern Sichuan, comprises

Figure 8. Representative Ashgillian shelly fossils from the Yangtze region. (a) Skenidioides hubeiensis Zeng, brachial internal mould, Miaopo Formation (early Caradoc), Huanghuachang, Yichang, western Hubei. (b–d) Cyclospira sp., ventral, anterior and dorsal views, Pagoda Formation (mid-late Caradoc), Liangshan, Nanzheng, southern Shaanxi. (e) Leptaena trifidum (Marek & Havlícˇ ek, 1967), brachial internal mould, Kuanyinchiao Formation (Hirnantian), Qicaoba, Yichang, western Hubei. (f) Foliomena sp., ventral exterior, locality and horizon as in (b). (g) Nankinolithus wanyuanensis Chen & Jian, cranidium, Linhsiang Formation (early Ashgill), Hanyuan, central Sichuan (following Lu Yenhao, 1975, pl. 38, fig. 12). (h) Dysprosorthis sinensis Rong, 1984b, brachial internal mould, Kuanyinchiao Formation (Hirnantian), Tangya, Yichang, western Hubei. (i, r) Hirnantia sagittifera (M’Coy), pedicle and brachial internal moulds, Kuanyinchiao Formation (Hirnantian), Tangya (i) and Huanghuachang (r), Yichang, western Hubei. (j) Hindella crassa incipiens (Williams), pedicle internal mould, locality and horizon as (r). (k,m) Kinnella kielanae (Temple), brachial and pedicle internal moulds, locality and horizon as (r). (l) Draborthis caelebs Marek & Havlícˇ ek, 1967, brachial internal mould, locality and horizon as (i). (n) Amorphognathus ordovicicus Branson & Mehl, oral view, × 100, top of the Wufeng Formation (Normalograptus extraordinarius–N. ojsuensis Zone), Wangjiawan, Yichang, western Hubei. (o) Dalmanella testudinaria (Dalman), brachial internal mould, Kuanyinchiao Formation (Hirnantian), Yanzikou, Bijie, northwestern Guizhou. (p) Paromalomena polonica (Temple), brachial internal mould, locality and horizon as (i). (q, t, v) Dalmanitina sp., cranidium, hypostome and pygidium, Kuanyinchiao Formation (Hirnantian), Donggongsi, Zunyi, northern Guizhou. (s) Mirorthis mira Zeng, brachial internal mould, locality and horizon as (r). (u) Eostropheodonta parvicostellata (Rong, 1984b), three pedicle internal moulds, Kuanyinchiao Formation (Hirnantian), Fenxiang, Yichang, western Hubei. All scale bars are 2 mm with the exception of the bar for n (0.2 mm).

638 more taxa and lived in deeper water environmental conditions. In addition to the genera mentioned above, Philhedra, Acanthocrania, Pseudopholidops (very abundant), Toxorthis, Onniella, Draborthis, Dysprosorthis, Mirorthis, Triplesia, Aegiromena, Sphenotreta, Dorytreta and others are also present, along with Dalmanitina, Platycoryphe and Leonaspis (Rong, 1984b). The assemblages of Yichang and Changning are similar to that of the Kosov bed of Bohemia (Marek & Havlícˇ ek, 1967). Hirnantia sagittifera at Honghuayuan, Tongzi, northern Guizhou, has been recovered from the lower part of the Kuanyinchiao Formation (Hirnantia bed: AFA295–305) (Fig. 5) and is associated with Dalmanella testudinaria, Kinnella? sp., Cliftonia? sp. nov., Leptaena trifidum, Paromalomena polonica, Eostropheodonta parvicostellata, Plectothyrella sp., Hindella crassa incipiens and a few others. In the upper part of the unit (shelly bed: AFA306–311c) no H. sagittifera specimens have been found, but Dalmanella testudinaria, Leptaena trifidum, Paromalomena polonica, Eostropheodonta parvicostellata, Plectothyrella sp. and Hindella crassa incipiens extend up to the upper part in which Eospirifer sp., Nalivkinia sp., Hyattidina? sp., Brevilamnulella? sp., Pseudopholidops sp., Palaeoglossa? sp., Trucizetina? sp. and a few others occur. It is interesting to note that some of the last-named genera also occur from the level of AFA305 (Fig. 5) with H. sagittifera. Thus, the brachiopod assemblage in the upper part of the Kuanyinchiao Formation at Honghuayuan is mixed, containing some typical elements of the Hirnantia fauna, along with some distinctive taxa, such as Nalivkinia and Eospirifer, which have not been found in the typical Hirnantia fauna. Stratigraphically, the Hirnantia sagittifera-bearing beds (AFA295–305) may be correlated with the upper part of the Normalograptus extraordinarius–N. ojsuensis Zone and part of the N. persculptus Zone, whereas the overlying shelly beds (AFA306– 311c) may correspond to part of the N. persculptus, Akidograptus ascensus and part of Parakidograptus acuminatus Zone. Therefore, Honghuayuan may be one of the well-exposed sections across the Ordovician– Silurian boundary in a continuous sequence of shelly facies with graptolite controls. It should be pointed out that the Hirnantia fauna (the lower and upper boundaries of the Hirnantia beds) is diachronous within the Yangtze region. The timing of the first appearance of the Hirnantia fauna differs markedly among the studied sections. At relatively shallow water sites such as Ludiping, Songtao County, northeastern Guizhou, the Hirnantia fauna (AFA414–416) occurs immediately above the Diceratograptus mirus Subzone (AFA413) (see Fig. 6), indicating that the Hirnantia fauna appears at the base of the N. extraordinarius–N. ojsuensis Zone. As far as we know, this is one of the earliest occurrences of the Hirnantia fauna in South China (Rong et al. 1999).

CHEN XU AND OTHERS

Based on Mu et al. (1979), Rong (1984a) correlated the base of the Kuanyinchiao Bed at Ganxi, Yanhe County, northeastern Guizhou, with the Diceratograptus mirus Subzone (=upper Paraorthograptus pacificus Zone). However, having checked the graptolite lists (see Mu et al. 1979) and their ranges, this conclusion should be further confirmed. There are two horizons (ACC97a, lower and ACC97b, upper) at the top of the Wufeng Formation at Ganxi. ACC97a contains Dicellograptus ornatus Elles & Wood, Yinograptus sp., Appendispinograputs venustus (Hsu), Rectograptus abbreviatus (Elles & Wood) and others, and was correlated with Tangyagraptus typicus Subzone (=middle Paraorthograptus pacificus Zone) (Mu et al. 1979). It is followed in this paper. ACC97b yields Yinograptus sp., Amplexograptus latus (Elles & Wood) [=A. suni (Mu)], Paraplegmatograptus sp., Pararetiograptus sp. and others, and was correlated with Tangyagraptus typicus Zone by Mu and others (1979) (see Rong, 1984a). However, those taxa in the bed of ACC97b (such as Yinograptus and Amplexograptus latus) can extend into higher horizons. It is likely, therefore, that the assignment of the ACC97b to the Diceratograptus mirus Subzone (=upper Paraorthograptus pacificus Zone) cannot be excluded. If this is correct, the Kuanyinchiao Bed at Ganxi may be correlated with the horizon younger than the Diceratograptus mirus Subzone (that is, younger than Paraorthograptus pacificus Zone). Further investigation on this section is needed. At Honghuayuan, Tongzi County, north of Dianqian Land, Hirnantia and its associates (AFA295–305) appear a little higher stratigraphically than the base of the N. extraordinarius–N. ojsuensis Zone (AFA290) (see Fig. 5). At deeper water sites towards the central part of the Yangtze platform, such as those in the Yichang area (Wangjiawan and Fenxiang) (see Figs 3 and 4), the Hirnantia fauna (AFA 100–101, AFA144–145) is known from a higher level above the base of the N. extraordinarius–N. ojsuensis Zone (AFA 97, AFA 141). There the shelly fauna is stratigraphically between the N. extraordinarius–N. ojsuensis Zone and the N. persculptus Zone. It cannot be excluded that the Hirnantia fauna may extend to the Normalograptus persculptus Zone. The Honghuayuan section further indicates that many typical constituents of the Hirnantia fauna such as Dalmanella, Paromalomena, Eostropheodonta, Plectothyrella and Hindella, extend to Akidograptus ascensus Zone or even higher. It is clear that occurrences of the Hirnantia fauna coincide with the sea-level drop associated with latest Ordovician glaciation. During this regression, the appropriate habitat for Hirnantia fauna developed first in the shallow water regime, whereas the conditions were established somewhat later, at the time of maximum regression, along the deeper water regime in the Yichang and other areas of the Yangtze platform.

Ordovician–Silurian biozonation of Yangtze region, China 2.f. Normalograptus persculptus Zone

The base of the N. persculptus Zone is marked by the appearance of Normalograptus persculptus (Elles & Wood), N. avitus (Davies), N. wangjiawanensis (Mu & Lin), N. tortithecatus (Hsu), Pseudorthograptus angustifolius (Chen & Lin), Glyptograptus laciniosus Churkin & Carter, G. lungmaensis Sun, etc. The N. persculptus Zone fauna contains only biserial graptoloids, predominately normalograptids. This fauna includes only 9 genera and 29 species. Thus, the diversity at the generic level continued its dramatic decline. Among the 29 species, 17 are cosmopolitan or intercontinentally distributed. Five of these species, including Normalograptus extraordinarius (Sobolevskaya), Paraorthograptus brevispinus (Mu & Li), Climacograptus? hastatus (T. S. Hall), Amplexograptus sp. aff. A. latus (Elles & Wood) and Paraplegmatograptus sp. (P. connectus group), are only supported by one or two specimens. These are all survivors from the D. mirus Zone extinction climax event but all expired in the N. persculptus episode. This last episode was identified by Chen & Rong (1991) as the epilogue episode. The samples from the D. mirus Subzone to N. persculptus Zone have been continuously and densely collected through the shallow water to deeper water facies belts in the Yangtze region. It seems unlikely that the disappearances of some taxa are the result of a Signor-Lipps effect. Mu (1988) correlated the European Akidograptus ascensus Zone with the ‘Glyptograptus persculptus’ Zone of China. This correlation depends on data from before 1988. Based on the present detailed collections from the four reference sections mentioned above, the Normalograptus persculptus Zone and Akidograptus ascensus Zone are easily distinguished.

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such as Normalograptus praemedius (Waern), N. rectangularis (M’Coy) and Pseudorthograptus illustris (Koren & Mikhailova) followed at slightly higher levels by Hirsutograptus sinitzini (Chaletskaya), etc. The P. acuminatus Zone fauna contains a total of 11 genera and 39 species. Among them, 5 genera and 21 species are new. Twenty-two species are cosmopolitan or intercontinentally distributed. Thus, a substantial faunal turnover occurs at this level. Half of the genera present in the P. acuminatus Zone are newly evolved in this interval. 3. Correlation A global correlation of the Ashgill to earliest Llandovery is based mainly on the graptolite sequences as well as the shelly Hirnantia fauna. Elements of the Hirnantia fauna occur in many sections that we have examined from the Yangtze platform (Locs 2–4, 7–15, 18–24, 26, 28, 29, 35, 37 and 40 of Fig. 1). A complete graptolite sequence without shelly Hirnantia fauna has been known from He Xian (Loc. 36 of Fig. 1; Zhang et al. 1966) and Jurong (Loc. 38 of Fig. 1; Chen et al. 1988) in the Lower Yangtze region. The palaeogeographic setting of the Yangtze region (particularly its nature as a semi-enclosed subtropical platform) fostered development of the most diverse and continuously preserved Ashgill graptolite fauna in the world. Despite high levels of endemism, the fauna includes a very good representation of the widely distributed species that were present during the latest Ordovician. These widely distributed elements of the Wufeng Formation fauna permit global correlation of this unit. 3.a. Dob’s Linn, southern Scotland

2.g. Akidograptus ascensus Zone

The base of the Zone coincides with the FADs of Akidograptus ascensus Davies. Neodiplograptus bicaudatus Chen & Lin and Normalograptus lubricus (Chen & Lin) are also newcomers and occur at a slightly higher level. There are a total of 6 genera and 23 species in this Zone. Among them, 13 species occur elsewhere. Thus, more than half of the Lungmachi A. ascensus Zone fauna have an intercontinental distribution. The A. ascensus Zone is widely distributed through the Yangtze platform. It appears to reach its maximum thickness at Ludiping, where it is 8.22 m thick. At Yichang, in the central part of the Yangtze basin, for instance, it is only 0.18 m thick. 2.h. Parakidograptus acuminatus Zone

The base of the P. acuminatus Zone is marked by the appearance of P. acuminatus (Nicholson), as well as many globally or intercontinentally distributed species

The Dob’s Linn section was approved as the Global Stratotype Section and Point (GSSP) of the Ordovician– Silurian boundary (Holland, 1984; Williams, 1988). The Hartfell Shale and Birkhill Shale are the lithostratigraphic units that include the uppermost Ordovician and lowermost Silurian strata. However, continuous graptolitic shale within this interval is present only from the Dicranograptus clingani Zone to lower Pleurograptus linearis Zone, and from the upper Normalograptus persculptus Zone to higher Llandovery graptolite zones. Between these two continuous graptolitic shales, there are 28 m of mostly pale grey, barren mudstone or shale (Williams, 1982b). Two graptolitic bands bear a Dicellograptus complanatus fauna in the D. complanatus Zone (Williams, 1982b). Five graptolitic bands comprise the D. anceps Zone including Bands A and B in the D. complexus Subzone, and Bands C to E in the P. pacificus Subzone. Toghill (1970) described the graptolites from the complanatus Band, including D. complanatus Lapworth, Rectograptus socialis (Lapworth) and

640 Normalograptus miserabilis (Elles & Wood). These species are also present at the Ludiping section, Songtao, northeastern Guizhou. Toghill also recorded rare examples of Plegmatograptus nebula Elles & Wood from the complanatus Band. However, it has not yet been found from the D. complanatus Zone of the Yangtze region. P. nebula Elles & Wood occurs mainly within the D. clingani and P. linearis zones (Williams, 1982a). Plegmatograptus became extinct by the middle Ashgill (D. complexus Zone) where it was replaced by its descendent Paraplegmatograptus. The Bands A and B contain a D. complexus–D. anceps fauna with a total of 9 genera and 12 species. All of them occur in the D. complexus Zone of the Yangtze region as well, except Normalograptus normalis Lapworth. N. normalis Lapworth appears a bit higher in the Yangtze region (in the Diceratograptus mirus Subzone), and is most abundant in the Parakidograptus acuminatus Zone. Williams (1982b) proposed Dicellograptus anceps (Nicholson), which occurs in both bands A and B, as the zonal species for this interval. Based on the continuous sections from the Yangtze region, D. anceps occurs from the middle D. complexus Zone into the lower part of Tangyagraptus typicus Subzone. Thus, the presence of D. anceps and absence of P. pacificus (Ruedemann) from Bands A and B at Dob’s Linn indicate that these two bands may correlate to an interval above the middle D. complexus Zone and below the base of the P. pacificus Zone in the Yangtze region. The occurrence of D. anceps is later than that of D. complexus in the Yangtze region. This implies that the base of the ‘D. complexus Subzone’ at Dob’s Linn may not be higher than that of D. anceps as suggested by Williams (1982b, fig. 2; see Fig. 9 herein). Dicellograptus anceps may extend into the N. extraordinarius Zone based on its occurrence in the Dob’s Linn section. Williams (1982b) reported the species Plegmatograptus? craticulus Williams and Orthoretiograptus denticulatus Mu (this species was first cited by Wang et al. (1977) from Mu et al.’s manuscript) from the D. anceps Zone at Dob’s Linn. Based on his illustrated specimens, P.? craticulus Williams may be a junior synonym of Paraplegmatograptus uniformis Mu, which was first cited by Wang et al. (1977) from Mu et al.’s manuscript. Orthoretiograptus denticulatus specimens illustrated by Williams (1982b, p. 43, pl. 4, figs 13–15, text-figs 12a–d) may not be Orthoretiograptus, but rather Pararetiograptus sinensis Mu 1974. These two species are well-known components of the D. complexus and P. pacificus zone faunas in the Yangtze region. The Bands C and D from the Dob’s Linn section contain a P. pacificus fauna that Williams (1982b) referred to as the P. pacificus Subzone of his D. anceps Zone (Fig. 9). Twelve genera and 16 species have been recorded from these two bands. However, except for Paraorthograptus pacificus (Ruedemann),

CHEN XU AND OTHERS

Plegmatograptus? nebula lautus Koren & Tzaj and Nymphograptus velatus Elles & Wood, most of them continue from the lower bands. P. nebula lautus Koren & Tzaj was described based on incomplete specimens without proximal ends. Thus, we are not sure whether it is a plegmatograptid. Nymphograptus velatus Elles & Wood (D. anceps Zone) is the only species which has not yet been recorded in the Yangtze region within this interval. Thus, the whole fauna is comparable with that of the P. pacificus Zone fauna from the Yangtze region. The specimens described by Williams (1982b) as Dicellograptus ornatus Elles & Wood from Band E differ somewhat from one another and do not agree exactly with the form of D. ornatus sensu stricto. His specimens illustrated in plate 1, figure 6 and text-fig. 6f, 6g represent Dicellograptus mirabilis Mu & Chen, which occurs in the Tangyagraptus typicus Subzone and Diceratograptus mirus Subzone in the Yangtze region. Williams’ (1982b) specimen illustrated in plate 1, figure 8 is a specimen of Dicellograptus turgidus Mu, which occurs commonly in the Tangyagraptus typicus Subzone of P. pacificus Zone in Yangtze. Thus, it appears that these two species extend to a higher horizon, into the N. extraordinarius Zone at Dob’s Linn. The most important member of the Band E fauna is Normalograptus extraordinarius (Sobolevskaya), which indicates the existence of N. extraordinarius–N. ojsuensis Zone at this level. Williams (1983) stated that the D. anceps–N. extraordinarius zonal boundary lies within the pale, unfossiliferous mudstone that occurs a short distance above the D. anceps bands. If the FAD of N. extraordinarius (Sobolevskaya) coincides with the base of the named zone, then the base of the N. extraordinarius–N. ojsuensis Zone should lie below Band E at Dob’s Linn. The FAD of N. cf. persculptus (Elles & Wood) (=N. parvulus (H. Lapworth; see Zalasiewicz & Tunnicliff, 1994)) is about 0.7 m above the base of the Birkhill shale at the Linn Branch section (Williams, 1983). Melchin (pers. comm., 1998) has re-examined the graptolite specimens of the Linn Branch section and confirms that no typical specimens of Normalograptus persculptus (Elles & Wood) occur there. Since there is an unfossiliferous interval between the extraordinarius Band and the Birkhill graptolite succession, the exact location of the base of the N. persculptus Zone at Linn Branch remains uncertain. However, the N. persculptus Zone from Linn Branch may partly correlate to the N. persculptus Zone in Yichang. The N. persculptus fauna from Linn Branch includes N. parvulus (H. Lapworth) (=G. cf. persculptus of Williams, 1983), N. avitus (Davies), N. normalis (Lapworth), N. miserabilis (Elles & Wood) (=N. angustus (Perner)), N. medius (Törnquist) and N. pseudovenustus (Legrand) (=Glyptograptus? venustus cf. venustus Legrand: Williams, 1983). N. normalis occurs from the D. mirus Subzone and extends to the Rhuddanian strata near

Ordovician–Silurian biozonation of Yangtze region, China

641

Figure 9. Correlation of the Ashgill to earliest Llandovery biozones between Yangtze and different continents. * was reported by Melchin, McCracken & Oliff (1991) as N. bohemicus Zone.

Yichang. N. angustus (Perner) is a long-lived species, which occurs from the D. complanatus Zone to the Rhuddanian strata in the Yangtze region. N. pseudovenustus (Legrand) occurs in the N. persculptusbearing beds from Djerance region of Algeria (Legrand, 1986). N. medius first appears in the P. acuminatus Zone of the Yangtze platform sections and is not present at lower levels. As Melchin, Koren & Williams (1998) pointed out, the world-wide occurrences of A. ascensus and P. acuminatus have been dealt with in different ways. In many parts of the world, including Kazakhstan, Kolyma, Bohemia, Poland and Thuringia, as well as the present study area in the Yangtze region of China, a distinct interval with abundant A. ascensus occurs below the first appearance of P. acuminatus, and is often recognized as a separate A. ascensus Zone below the P. acuminatus Zone (Apollonov, Bandaletov &

Nikitin, 1980; Koren et al. 1979; Sˇtorch & Loydell, 1996; Schauer, 1971; Teller, 1969; Chen & Lin, 1978). Melchin, Koren & Williams (1998) also noted that the specimens of P. acuminatus s.l. that occur in the lower part of the combined P. acuminatus–A. ascensus Zone (that is, with the early A. ascensus specimens at the base of the Silurian System) at Dob’s Linn differ from typical specimens of P. acuminatus s.s. They referred these early representatives to the subspecies P. acuminatus praematurus (Davies). This suggests that the FADs of A. ascensus and P. acuminatus acuminatus may not coincide even at Dob’s Linn. Whatever the case at Dob’s Linn, it is clear that in many regions occurrences of A. ascensus and P. acuminatus acuminatus reliably define two distinct intervals, and it is the lower of these two that most nearly corresponds to the Silurian GSSP at Dob’s Linn. A fuller understanding of these species ranges, and such a secure correlation

642 of the base of the Silurian System, will emerge only by the evaluation of these species and their ranges relative to a carefully and consistently defined set of other species ranges. It is our hope that the data presented here from the Yangtze platform will contribute to this effort. 3.b. Spain and Portugal

The glaciomarine sediments of Hirnantian age and the earliest Llandovery graptolite shale are widely distributed in the West Asturian–Leonese Zone, Iberian Cordillera, Central Iberian Zone and Ossa-Morena Zone with low diversity shelly and graptolite faunas (Gutiérrez-Marco, Robardet & Piçarra, 1998). A typical Hirnantian fauna including Hirnantia sagittifera and Plectothyrella crassicosta chauveli occurs from the lower part of the ‘Criadero Quartzite’ in the southern Central Iberian Zone, Spain. The lowest Llandovery graptolites have been documented as ascensus–acuminatus Zone from the four tectonic belts as mentioned above. Ashgillian conodonts of the Amorphognathus ordovicicus Zone have been also documented from Truchas syncline of the north Central Iberian Zone. Although to conduct a precise correlation between the Iberian Peninsula and the Yangtze region is premature, the Hirnantian to earliest Llandovery fossil sequence is similar between these two regions. 3.c. Thuringia–Saxonia–N. Bavaria, Germany

Two different facies, the Thuringian facies (east) and Bavarian facies (west) are recognized from this region through Ordovician to Silurian (Jaeger, 1988). Although the lithofacies of these two areas are different, the graptolite sequences are similar. Normalograptus persculptus (Elles & Wood) (=Diplograptus bohemicus (Marek), Jaeger, 1977) was described by Jaeger (1977) from the uppermost Döbra Sandstone (Bavarian facies belt). The Akidograptus ascensus Zone was recognized as the lowest biozone from the Thuringian facies belt (Ronneburg and Oelsnitz) by Schauer (1971) and from the Bavarian facies belt by Stein (in Jaeger, 1988). The overlying strata are assigned to the P. acuminatus and C. vesiculosus zones, which are readily correlated with those zones in the Yangtze region (Fig. 9). 3.d. Bohemia

Glyptograptus bohemicus Marek was the first graptolite described from the upper Kosov Formation by Marek (1954). Since then, both the Czech and Chinese workers have used it as a zonal fossil for the latest Ordovician biozone (Mu, 1974; Sˇtorch, 1988). The type specimens as well as additional materials from the Czech Republic and Wales have been re-studied by Sˇtorch & Loydell (1996). They concluded that

CHEN XU AND OTHERS

‘Glyptograptus bohemicus (Marek)’ is a junior subjective synonym of Normalograptus persculptus (Elles & Wood). This revision is accepted here. From the redescription of N. persculptus (Elles & Wood) given by Sˇtorch & Loydell (1996), we note that the definition of N. persculptus (Elles & Wood) has been slightly broadened. As noted in Section 1, Fan (Fan Jun-xuan, unpub. Masters thesis, Nanjing Institute of Geology & Palaeontology, 1998) recently has made a morphometric study of these taxa and their relations to the species recorded from the Yangtze region. He concluded that the ‘Glyptograptus bohemicus’ materials described by Chinese authors in the past agree very well with Normalograptus ojsuensis (Koren & Mikhailova). Thus, we refer the ‘Glyptograptus bohemicus’ materials described by Mu and his colleagues to Normalograptus ojsuensis (Koren & Mikhailova). N. persculptus (Elles & Wood) occurs at the top of the Kosov Formation with only a few other graptolites, such as Normalograptus sp. aff. N. miserabilis (Elles & Wood) and N. normalis (Sˇtorch, 1988). The occurrence of the persculptus graptolite fauna indicates that the Hirnantia fauna in Bohemia may be persculptus Zone in age. However, it is difficult to correlate the N. persculptus Zone of Bohemia with other regions in the world (Sˇtorch & Loydell, 1996). We are inclined to the view that N. persculptus Zone of Bohemia may roughly correspond to the same zone in the Yangtze region, that is, to a level somewhere within the N. persculptus Zone, however, its exact location relative to the base of this zone cannot be determined (Fig. 9). The Akidograptus ascensus Zone has also been employed by Sˇtorch & Loydell (1996). The base of the zone lies 70 cm below the base of the Silurian black shales, and yields Akidograptus ascensus, Normalograptus angustus, Neodiplograptus lanceolatus Sˇtorch & Serpagli, as well as the highest identifiable ‘Glyptograptus bohemicus’ (that is, N. persculptus s.l.). The whole A. ascensus Zone fauna may include more species, such as Neodiplograptus modestus, Neodiplograptus sp. aff. N. parvulus (Lapworth), N. parajanus (Sˇtorch), N. elongatus (Churkin & Carter), Cystograptus ancestralis Sˇtorch, Normalograptus sp. aff. N. praemedius (Waern) and N. trifilis (Manck). The P. acuminatus Zone fauna has been recorded by Sˇtorch (1988). It is marked by the incoming of three species, Parakidograptus acuminatus, Neodiplograptus diminutus apographon (Sˇtorch) and Normalograptus longifilis (Manck). The first two species have also been recorded from the P. acuminatus Zone of the Yangtze region. 3.e. Poland

A typical Hirnantia fauna, containing Hirnantia sagittifera, Eostropheodonta, Dalmanella testudinaria and trilobites Mucronaspis mucronata etc., occurs in the uppermost Ashgill silty beds from the Holy Cross

Ordovician–Silurian biozonation of Yangtze region, China Mountains, southeastern Poland (Temple, 1965; Teller, 1969). ‘Ashgillian’ to early Llandovery fossils and strata have been recorded from the Lebork borehole, northern Poland, Peribaltic depression and Podiasie depression, eastern Poland, as well as the Carpathian and Holy Cross Mountains, southeastern Poland. However, from the fossils and strata through the Ordovician–Silurian boundary interval, the Lebork 1G-1 borehole sequence in northern Poland is probably the most important (Tomczyk & Tomczykowa, 1976). In ascending order, the sequence is Mucronaspis mucronata, Normalograptus persculptus, Akidograptus ascensus and Parakidograptus acuminatus zones, etc. Thus, it is correlative with those of the Yangtze region. 3.f. Southern Kazakhstan

Middle Ashgill to earliest Llandovery graptolite-bearing strata with Hirnantia–Dalmanitina beds are well developed in southern Kazakhstan (Apollonov, Bandaletov & Nikitin, 1980). The lower Subzone of the Appendispinograptus supernus Zone, the Appendispinograptus longispinus Subzone, includes A. supernus, A. longispinus hvalross and Rectograptus amplexicaulis (Hall). These species are all common forms from the Dicellograptus complanatus Zone to the Lower Subzone of the Paraorthograptus pacificus Zone in the Yangtze region. Without a set of more characteristic species in the A. longispinus Subzone in southern Kazakhstan, precise correlation between southern Kazakhstan and the Yangtze region may not be possible at this level. The upper Subzone of the A. supernus Zone in Kazakhstan, the Paraorthograptus pacificus Subzone, includes Paraorthograptus pacificus, Appendispinograptus supernus, A. longispinus hvalross, Dicellograptus ornatus, D. minor, Normalograptus tatianae, N. cf. normalis, Rectograptus amplexicaulis, Orthograptus maximus Mu (=Glyptograptus posterus Koren & Tzaj) and Amplexograptus latus (Elles & Wood) (=A. stukalinae Mikhailova). Most of these species are also common in Paraorthograptus pacificus Zone fauna in Yangtze. The FAD of N. ojsuensis is located near the LAD of Paraorthograptus pacificus (Ruedemann) and a little lower than the FAD of Normalograptus extraordinarius (Sobolevskaya) in southern Kazakhstan (Apollonov, Bandaletov & Nikitin, 1980). In the Yangtze region sections, the FADs of N. ojsuensis and N. extraordinarius coincide, and both appear immediately following the extinction of Diceratograptus mirus in Yichang area (deeper water area). Among the sections in this area (such as at Wangjiawan), Paraorthograptus pacificus and many of the associated species continue into the interval of the N. extraordinarius–N. ojsuensis Zone whereas among the southern, more near-shore sections (e.g. Honghuayuan), the P. pacificus assemblage disappears with D. mirus. This consistent pattern

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of occurrence relative to the highly distinctive and short-lived D. mirus, suggests that the simultaneous origin of N. ojsuensis and N. extraordinarius in our sections reflects the true timing of their evolutionary origins. Thus, the recorded FAD of N. extraordinarius (Sobolevskaya) from southern Kazakhstan may be a little later than that in the Yangtze region. The Hirnantia–Dalmanitina-bearing beds in southern Kazakhstan include two graptolite species, N. ojsuensis (Koren & Mikhailova) and N. extraordinarius (Sobolevskaya) (=Glyptograptus persculptus forma A). Thus, the occurrence of the Hirnantia–Dalmanitina fauna in southern Kazakhstan may be comparable to the majority of localities in the Yangtze region. Koren (in Apollonov, Bandaletov & Nikitin, 1980) had already recognized that ‘Glyptograptus persculptus forma B’ (=N. persculptus s.s.) was synonymous with ‘Glyptograptus bohemicus Marek’. Mu & Lin (1984) and Chen (1984) also suggested that ‘Glyptograptus persculptus forma B’ from southern Kazakhstan is a junior synonym of Glyptograptus bohemicus Marek. However, recent investigations by Sˇtorch & Loydell (1996) demonstrate that ‘G. persculptus forma B’ is synonymous with N. persculptus (Elles & Wood). Thus, the ‘G. persculptus forma A’ and ‘G. persculptus forma B’ from Kazakhstan, which occur separately, may represent the N. extraordinarius–N. ojsuensis Zone and N. persculptus Zone, respectively, as recognized in the Yangtze region and elsewhere. The N. persculptus Zone from southern Kazakhstan includes the eponymous form as well as N. angustus (Perner), N. normalis (Lapworth) and ‘Pseudoclimacograptus’ sp. Among them only the last, which is an early representative of Metaclimacograptus, has not yet been recognized in the Yangtze region. The Hirnantian shelly fauna from southern Kazakhstan is associated with the N. persculptus graptolite fauna (field number 287) at north Krylo section II (Apollonov, Bandaletov & Nikitin, 1980). Along the Zhideli river, there occurs a limestone bed (field number 264) yielding a typical Hirnantia fauna (Apollonov, Bandaletov & Nikitin, 1980) which is underlain by pacificus Zone (263) and overlain by mudstone (267) with some graptolites. The latter is in turn overlain by A. ascensus–P. cf. acuminatus-bearing beds (269, 270). It suggests that the brachiopod fauna may be corresponding to the extraordinarius Zone and part of the persculptus Zone. Koren et al. (1979) did not recognize a separate Akidograptus ascensus Zone in southern Kazakhstan. However, 2.5–3 km northwest of the Ojsu section, there is a graptolite fauna (F-290) above the Hirnantia beds (Apollonov, Bandaletov & Nikitin, 1980). This graptolite fauna includes Akidograptus cf. ascensus Davies, Normalograptus angustus (Perner), N. sp. ex gr. N. normalis, Neodiplograptus modestus primus Mikhailova and Normalograptus cf. madernii (Koren & Mikhailova). This assemblage appears to be a typical

644 Akidograptus ascensus Zone fauna. The A. ascensus fauna also occurs at the Dulban section (F-280 of Apollonov, Bandaletov & Nikitin, 1980, p. 10). Bed F280 yields Akidograptus sp., Normalograptus angustus, N. acceptus, N. sp. ex gr. N. normalis and N. madernii. Both of these two graptolite-bearing beds with Akidograptus ascensus or Akidograptus sp. lie stratigraphically below occurrences of Parakidograptus acuminatus Zone fauna (Apollonov, Bandaletov & Nikitin, 1980). These observations suggest the presence in southern Kazakhstan of a separate A. ascensus Zone below the P. acuminatus Zone, which is comparable to the situation in the Yangtze region. The Parakidograptus acuminatus Zone in southern Kazakhstan (in the sense used herein) includes P. acuminatus, Normalograptus acceptus, N. sp. aff. N. angustus, Akidograptus ascensus cultus Mikhailova, Metaclimacograptus fidus Koren & Mikhailova and M. pictus Koren & Mikhailova. The first three species are common in the P. acuminatus Zone of the Yangtze region. Metaclimacograptus is also present in the P. acuminatus Zone of the study area, but is represented by different species. 3.g. Kolyma, northeastern Siberia

The Appendispinograptus supernus Zone in this area includes Dicellograptus complanatus, numerous subspecies of A. longispinus, Paraorthograptus pacificus and Rectograptus amplexicaulis, etc. (Koren et al. 1979). The A. longispinus Subzone of Kolyma contains mainly D. complanatus and species of the R. amplexicaulis group. The P. pacificus Subzone includes P. pacificus, R. amplexicaulis, A. longispinus hvalross, A. supernus, Climacograptus? hastatus, Normalograptus cf. angustus and D. complanatus, etc. Additional species occur in the upper part of the Subzone. These newcomers include P. pacificus affinis, Normalograptus tatianae and Nymphograptus velatus. However, Koren et al. (1983) reported that N. ojsuensis, N. angustus, N. normalis, P. pacificus and other new species occur near the top of the P. pacificus Subzone. Since the graptolite diversity from Kolyma is relatively low compared to that of the Yangtze region, it is difficult to make a precise correlation with these subzones. The A. supernus Zone of Kolyma is roughly correlative with the D. complanatus Zone to P. pacificus Zone interval in the Yangtze region (Fig. 9). Mirny Creek, Kolyma River is the type locality of Normalograptus extraordinarius (Sobolevskaya). The N. extraordinarius–N. ojsuensis Zone at this locality contains mainly N. extraordinarius, N. ojsuensis, N. angustus, N. normalis and a few others (Oradovskaya & Sobolevskaya, 1979). The FADs of N. ojsuensis (108-2/1.3-5) and N. extraordinarius (107-1/1.2) are from successive collections separated by 2 m in the Mirny Creek section. Compared with the 89 m thickness of the N. extraordinarius and N. persculptus

CHEN XU AND OTHERS

zones in this section, the 2 m separation may not be significant. However, from the Ina River section, Normalograptus ojsuensis occurs at the base of the N. extraordinarius Zone (Koren et al. 1983). This occurrence further supports the inference that the FAD of N. ojsuensis is coincident with that of N. extraordinarius in Kolyma region. The upper Tirekhtyakh horizon was further studied as two parts, the lower N. extraordinarius Zone and the upper N. persculptus Zone by Koren, Oradovskaya & Sobolevskaya (1988). The N. persculptus Zone includes N. persculptus, N. angustus, N. normalis, N. mirnyensis (Obut & Sobolevskaya) (closely related to N. angustus) and N. torosus Koren & Sobolevskaya. Koren, Oradovskaya & Sobolevskaya (1988) reported that the most complete and well-known section of the Tirekhtyakh horizon and the overlying Chalmak horizon is exposed along Mirny Creek. The systemic boundary between the Ordovician and Silurian systems is drawn at the base of unit 73, which is 1.5 m thick and coincides with the base of the Maut Formation. The index species Akidograptus ascensus occurs at the base of unit 74, 1.5 m above the boundary, and Parakidograptus acuminatus occurs in the lower part of the unit 75, 11 m above the boundary. Their report (1988) clearly described two separated graptolite horizons that may correspond to the Akidograptus ascensus and Parakidograptus acuminatus zones in the Yangtze region. It was reported that a Hirnantia fauna occurs from the Q Horizon of the Omulev Mountains, Kolyma (Oradovskaya in Koren et al. 1983). Rong & Harper (1988, p. 395) concluded that the Kolyma brachiopod fauna has no species common to the Hirnantia fauna and is better assigned to the Lower Edgewood fauna. 3.h. Malaya Peninsula

Uppermost Ordovician to lower Silurian graptolite and shelly sequences have been reported from the Langkawi Islands, northwestern Malaya (Jones, 1973). The Normalograptus persculptus Zone spans 1.1 m thick platy black siltstones with N. persculptus (Elles & Wood), Neodiplograptus modestus (Lapworth) and Glyptograptus temalaensis Jones. The top of the Zone is marked by a thick bed of gritty siltstone carrying shelly fossils amongst which Dalmanitina malayensis is prominent. There are 13.8 m thick cherty beds without fossils above the D. malayensis bed. The Cystograptus vesiculosus Zone and younger graptolite zones have been recognized above the cherty beds. Rong (1979) correlated the N. persculptus, D. malayensis as well as the unfossiliferous cherty beds to the N. persculptus Zone to P. acuminatus Zone interval. If the identification of N. persculptus from the Langkawi Islands is correct (it has not yet been described), the shelly beds with the endemic Dalmanitina malayensis should be correlated to the upper N. persculptus Zone or higher

Ordovician–Silurian biozonation of Yangtze region, China level. Thus, the Dalmanitina malayensis beds may be a little higher than that of the Hirnantia-bearing beds from Yangtze region. In southern Thailand, close to the border with Malaysia, a Hirnantia fauna containing Hirnantia sagittifera, Aegiromena planissima, Paromalomena sp. etc., as well as trilobite Mucronatus mucronata, has been recognized (Cocks & Fortey, 1997). Graptolites have been identified by Rickards as N. persculptus fauna, immediately underlying the shelly beds (Wongwanich et al. 1990, p. 6). It indicates that the Hirnantia beds from southern Thailand are correlative with that of the Langkawi Islands. However, the components of the shelly fauna between Langkawi and southern Thailand are a little different; the Hirnantian brachiopods as well as Dalmantina mucronata are absent at Langkawi. On the other hand, the Hirnantia fauna of southern Thailand does not fully agree with those of the Shan State of Burma and Yichang, Yangtze as suggested by Cocks & Fortey (1997). Many distinctive brachiopod taxa of the Hirnantia fauna elsewhere, such as Kinnella, Dalmanella, Cliftonia, Eostropheodonta, Plectothyrella and some others, are not present in these collections. Stratigraphically, we assume that the Hirnantia beds of southern Thailand are probably the equivalent of the upper part of N. persculptus Zone. 3.i. Yukon, Canada

The Ashgill graptolite biostratigraphy of the Peel River, Yukon, was reported by Lenz & McCracken (1982, 1988), and Chen & Lenz (1984). The Dicellograptus ornatus Zone of the Yukon Territory contains Dicellograptus ornatus, D. minor, Amplexograptus latus, Rectograptus abbreviatus, Anticostia cf. fastigata, Appendispinograptus supernus, A. longispinus hvalross, Climacograptus? hastatus, Pararetiograptus sinensis Mu (=Retiograptus pulcherrimus Keble & Harris, Lenz 1977; Orthoretiograptus denticulatus Mu, Chen & Lenz, 1984, Lenz & McCracken, 1988) and Arachniograptus laqueus. It roughly corresponds to the Dicellograptus complexus Zone as suggested by Chen & Lenz (1984) and Lenz & McCracken (1988). The P. pacificus Zone is recognized by the presence of abundant P. pacificus and A. supernus. The other species present include Anticostia cf. fastigata, Amplexograptus latus, Rectograptus abbreviatus and Appendispinograptus leptothecalis Mu & Ge. Diceratograptus mirus Mu (=Diceratograptus cf. mirus Mu, Chen & Lenz, 1984), discovered from the top of the P. pacificus Zone indicates the existence of the D. mirus Subzone in the Yukon. Diceratograptus mirus is associated there with D. ornatus, P. pacificus, A. supernus, A. cf. longispinus, C? sp. ex. gr. C.? hastatus, Amplexograptus latus and Arachniograptus laqueus etc. All of these species except Arachniograptus laqueus are also present in the D. mirus Subzone of the

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Yangtze region. Different species of Arachniograptus occur in these two regions. The P. pacificus and P. acuminatus zones in the Yukon are juxtaposed across a cryptic unconformity at which the N. extraordinarius and N. persculptus zones are missing. This is most likely a consequence of regression and submarine erosion induced by the Hirnantian glaciation (Melchin, 1987; Lenz & McCracken, 1988). Parakidograptus acuminatus occurs together with Akidograptus ascensus, Normalograptus cf. trifilis, Cystograptus vesiculosus and Neodiplograptus modestus diminutus etc. Of the Yangtze platform zones, this fauna is most similar to that of the P. acuminatus Zone. 3.j. Canadian Arctic islands

Melchin (1987) erected the Anticostia fastigata Zone (originally referred to as the ‘Orthograptus fastigatus’ Zone) from the Canadian Arctic islands and suggested that this Zone is correlative with the Dicellograptus complexus Zone. His Paraorthograptus pacificus Zone contains Amplexograptus latus (Elles & Wood) and Appendispinograptus supernus (Elles & Wood) from Huff ridge. In the Arctic islands, the P. pacificus Zone is succeeded immediately by an interval containing ‘Neodiplograptus bohemicus (Marek)’ and Normalograptus shanchongensis (Li) (Melchin, McCracken & Oliff, 1991). Following previous interpretations of the Yangtze region graptolites, Melchin, McCracken & Oliff (1991) referred this fauna to the N. bohemicus Zone. Sˇtorch & Loydell (1996) suggested that these pre-Hirnantia Canadian specimens of N. bohemicus, like the type materials of N. bohemicus (Marek), may be synonymous with Normalograptus persculptus (Elles & Wood). However, Fan’s (Fan Junxuan, unpub. Masters thesis, Nanjing Institute of Geology & Palaeontology, 1998) demonstration that the Chinese ‘N. bohemicus’ from the N. extraordinarius–N. ojsuensis Zone is instead identical with Normalograptus ojsuensis (Koren & Mikhailova) raises the possibility of a similar relation for the Canadian Arctic materials. Based on its location between clearly corresponding P. pacificus and N. persculptus zones, we tentatively correlate the ‘N. bohemicus’ Zone of Melchin, McCracken & Oliff (1991) with the N. extraordinarius–N. ojsuensis Zone in the Yangtze region. Melchin (pers. comm., Dec. 1998) also has recently come to the conclusion that his specimens referred as ‘N. bohemicus (Marek)’ are N. ojsuensis (Koren & Mikhailova). The Normalograptus persculptus Zone from Arctic Canada includes Normalograptus cf. persculptus (Elles & Wood), N. wangjiawanensis (Mu & Lin) and Neodiplograptus shanchongensis (Li). The interval above the N. persculptus Zone in the Canadian Arctic islands lacks A. ascensus and P. acuminatus. Melchin, McCracken & Oliff (1991) placed the base of the postN. persculptus zone (their P. acuminatus Zone) at the

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FADs of N. madernii and N. lubricus. This N. madernii–N. lubricus fauna, which they recognized as a lower subzone of the P. acuminatus Zone, is succeeded by an upper subzone characterized by the appearance of Hirsutograptus sinitzini. Obviously it is impossible to confirm the relations of A. ascensus and P. acuminatus based on these data.

Cuerda, Rickards & Cingolani, seem to be representatives of late Rhuddanian graptolite faunas, although Cuerda, Rickards & Cingolani (1988) suggested they were a P. acuminatus Zone fauna. Among these new forms, Metaclimacograptus robustus has been recognized from the C. vesiculosus Zone at Honghuayuan section in the Yangtze region.

3.k. Eureka, Nevada, USA

3.m. Djado plateau, Niger and Hodh, Mauritania

Continuous graptolite, conodont and chitinozoan sequences through Dicellograptus ornatus, Paraorthograptus pacificus, Normalograptus extraordinarius and N. persculptus zones have been documented recently from Vinini Creek, and the Monitor Range, near Eureka, Nevada (Finney et al. 1999). From the graptolite fauna, the Dicellograptus ornatus Zone may correlate with the Dicellograptus complexus Zone of the Yangtze region. Most of the P. pacificus Zone from Nevada may be correlative to the same zone of the Yangtze region. The Tangyagraptus typicus and Diceratograptus mirus faunules have not yet been found in Nevada. The Normalograptus extraordinarius Zone of Nevada is marked by the FADs of N. extraordinarius (Sobolevskaya) and N. pseudovenustus (Legrand). The FAD of Normalograptus ojsuensis (Koren & Mikhailova) is little earlier than that of N. extraordinarius (Sobolevskaya) from the Vinini Creek section. The vertical distribution of these two species is similar to that of Mirny Creek, southeastern Siberia. Thus, this zone in Nevada is broadly correlative with the N. extraordinarius–N. ojsuensis Zone of the Yangtze region, but is slightly younger at its base. We would instead place the base of the zone at the first appearance of N. ojsuensis. From the Monitor Range, Nevada, Normalograptus persculptus (Elles & Wood) and Glyptograptus laciniosus (Churkin & Carter) occur in the upper N. persculptus Zone. Conodonts of the Amorphognathus ordovicicus Zone as well as chitinozoans occur through P. pacificus and N. extraordinarius zones but exhibit a little change in their taxonomic composition. However, conodonts and chitinozoans considered typical of the Early Silurian appear in the N. persculptus Zone.

Legrand (1993) recognized Normalograptus ojsuensis (=Glyptograptus (Glyptograptus?) ojsuensis Koren & Mikhailova, Legrand, 1993) from the Djado plateau, Niger. Associated forms are Normalograptus sp., inarticulate brachiopods and trinucleid trilobites. Underwood, Deynoux & Ghienne (1998) reported that Normalograptus persculptus (=Persculptograptus persculptus s.l.: Underwood, Deynoux & Ghienne, 1998, fig. 5S) occurs with Normalograptus extraordinarius (=N. cf. extraordinarius typical forms, Underwood, Deynoux & Ghienne, 1998, fig. 5H,G) from their lower persculptus Zone (at Hodh, Mauritania). It is important that N. persculptus (Elles & Wood) occurs together with N. extraordinarius (Sobolevskaya) in the N. persculptus Zone from Fenxiang (Fig. 4) and Huanghuachang, Yichang. The Djado plateau and Hodh are the only two localities of the latest Ordovician graptolites known from within Gondwana land near the Ordovician South Pole.

3.l. Talacasto, Argentina

Cuerda, Rickards & Cingolani (1988) reported graptolites of the Normalograptus persculptus Zone from the Talacasto region, listing among the fauna N. persculptus, N. angustus, N. normalis and N. rectangularis. However, the last-named species are less known from the N. persculptus Zone elsewhere. The graptolite assemblage from the overlying beds, including Talacastograptus leanzai Cuerda, Rickards & Cingolani, Metaclimacograptus robustus (Cuerda, Rickards & Cingolani) and Lagarograptus praeacinaces

3.n. Darraweit Guim, central Victoria, Australia

Thick clastics (Bolinda Shale and Darraweit Guim Mudstone) and turbidites (Deep Creek Siltstone) occupy the Dicellograptus ornatus–Amplexograptus latus Zone to Parakidograptus acuminatus Zone interval from central Victoria (VandenBerg, Rickards & Holloway, 1984). The D. ornatus–A. latus Zone in the Bolinda Shale contains Dicellograptus minor, D. ornatus, Appendispinograptus supernus, Climacograptus? hastatus, Normalograptus angustus, Paraorthograptus pacificus, Rectograptus amplexicaulis, Anticostia fastigata and ‘Orthoretiograptus denticulatus’, etc. Among them ‘O. denticulatus’ may be Pararetiograptus sinensis Mu. The graptolite fauna indicates that this zone may be equivalent to the D. complexus Zone through P. pacificus Zone interval in the Yangtze region. The N. extraordinarius–N. ojsuensis Zone includes one of the zonal fossils. N. extraordinarius as well as N. angustus associates with the trilobite Songxites darraweitensis (Campbell). Normalograptus persculptus occurs at the top of the Deep Creek Siltstone associated with N. normalis and N. angustus. VandenBerg, Rickards & Holloway (1984) described their Parakidograptus acuminatus sp. cf. P. acuminatus acuminatus (Nicholson) based on a single whole rhabdosome (NMV PL698). They reported there is no direct evidence of its stratigraphic relationship with the

Ordovician–Silurian biozonation of Yangtze region, China beds containing N. persculptus. From the text-figure and description of this specimen, we are inclined to the view that P. acuminatus sp. cf. P. acuminatus acuminatus (VandenBerg, Rickards & Holloway, 1984, fig. 12) is P. acuminatus praematurus (Davies). Davies (1929) suggested originally that P. acuminatus praematurus occurs within the N. persculptus Zone. Rickards (1988) in his discussion of the position of the base of the Silurian, lowered the base of the P. acuminatus Zone to include these P. acuminatus praematurus-bearing strata. So far, P. acuminatus praematurus has not yet been found from China. We are not sure whether the occurrence of P. acuminatus praematurus in the Deep Creek siltstone indicates the existence of the Yangtze A. ascensus Zone in central Victoria, although this is likely if Melchin, Koren & Williams (1998) are correct in their interpretation of the ranges of these taxa. We interpret the occurrence of P. acuminatus praematurus, with its more ‘primitive’ features, to indicate the presence in the Victorian succession of a stratigraphically lower level within the Silurian than that marked by the FAD of P. acuminatus. Acknowledgements. The present paper is supported by the Basic Science Research Program of the Chinese Academy of Sciences (KZ952-J1-401 and KZ952-J1-023), National Natural Science Foundation (no. 49872005 and 49672083) as well as the Special Funds for Major State Basic Research Project (G200077700). Partial support was also provided by the Department of Geological Sciences, New York State University. We thank Dr M. Melchin for his helpful comments and discussion, and J. and J. Enterprise for providing good indoor working facilities in the town of Harvard, Massachusetts, USA. The graptolite illustrations were drafted by Miss Wu Minghua and the brachiopods were photographed by Mr Deng Dongxing.

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