Upper Darriwilian graptolite and conodont zonation in the global stratotype section of the Darriwilian stage (Ordovician) at Huangnitang, Changshan, Zhejiang, China

Palaeoworld 15 (2006) 150–170

Research paper

Upper Darriwilian graptolite and conodont zonation in the global stratotype section of the Darriwilian stage (Ordovician) at Huangnitang, Changshan, Zhejiang, China Xu Chen a,∗ , Yuan-Dong Zhang a , S.M. Bergstr¨om b , Hong-Gen Xu c a

State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, CAS, Nanjing 210008, PR China b Department of Geological Sciences, The Ohio State University, Columbus, OH 43210, USA c Zhejiang Regional Geological Survey, Hangzhou 311201, PR China Received 17 May 2005; received in revised form 14 March 2006; accepted 10 July 2006

Abstract Investigation of the previously little known upper Darriwilian graptolite biozone succession in the Darriwilian GSSP at Huangnitang, Changshan, China has led to the recognition of three biozones above the basal Darriwilian Undulograptus austrodentatus Biozone, namely the Acrograptus ellesae Biozone, the Nicholsonograptus fasciculatus Biozone, and the Pterograptus elegans Biozone. The Hustedograptus teretiusculus Biozone has not been identified and the post-Darriwilian Nemagraptus gracilis Biozone is separated from the P. elegans Biozone by a stratigraphic interval without stratigraphically diagnostic graptolites. Several levels in the succession have yielded index conodonts, making it possible to recognize international conodont biozones in part of the sequence. The virtually complete Darriwilian graptolite biozone succession, the good to excellent preservation of many graptolite taxa, the comprehensive construction of facilities to facilitate the study of the section, and the designation of this section as a protected National Geopark make Huangnitang an internationally unique and important geological site. © 2006 Published by Elsevier Ltd on behalf of Nanjing Institute of Geology and Palaeontology, CAS. Keywords: Graptolites; Conodonts; Biostratigraphy; Darriwilian; Middle Ordovician; China

1. Introduction For more than a century (Hall, 1899), the term Darriwilian has been used as a series or stage designation in the standard classification of the Ordovician of Australia. It was originally based on the graptolite succession in Victoria, where in recent stratigraphic schemes (Cas and VandenBerg, 1988; VandenBerg and Cooper, 1992) it includes a stratigraphic interval from the base of the Undulograptus austrodentatus Biozone to the base of the

∗

Corresponding author. E-mail address: [email protected] (X. Chen).

Nemagraptus gracilis Biozone. The Darriwilian graptolite faunas are quite diverse in Victoria as shown by the fact that close to 70 species were recorded from the unit by VandenBerg and Cooper (1992). The Darriwilian has commonly been classified as the topmost stage in the Australian Lower Ordovician. Because units in the Australian graptolite biozone succession can be recognized in many areas also outside Australia and New Zealand, especially in the so-called Pacific Graptolite Realm, these units have been widely used for local and regional correlations. In view of this, and the fact that it can be defined precisely in terms of graptolite zones, the term Darriwilian was selected as a stage designation in the new global stage and series classifi-

1871-174X/$ – see front matter © 2006 Published by Elsevier Ltd on behalf of Nanjing Institute of Geology and Palaeontology, CAS. doi:10.1016/j.palwor.2006.07.001

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Fig. 1. Index map of the geographic location of important Darriwilian sections and localities, as well as major geotectonic units, in China. Ax, Alxa block; Ca, Cathaysian Land; Ch, Chaidam block; Jn, Jiangnan slope belt; Ka, Eastern Kazakhstan Plate; Nc, North China Plate; Sc, South China Plate; SCS, South China Sea; SM, Sibumasu Plate; Ta, Tarim Plate; Tb, Xizang (Tibet) Plate; Ya, Yangtze platform; YK, Yunkai block; Zh, Zhujiang basin; localities: 1, Dawangou; 2, Kalping; 3, Qilianshan; 4, Taoyuan; 5, Qidong; 6, Chongyi; 7, S. Jiangxi; 8, Xingan; 9, Hule; 10, Ningguo; 11, S. Anhui; 12, Taiping; 13, Changhua; 14, Huangnitang; 15, Yushan; 16, W. Zhejiang; 17, N.E. Jiangxi.

cation of the Ordovician System (Webby et al., 2004), in which it represents the upper stage of the Middle Ordovician Series. Because no suitable GSSP section for the lower boundary of the Darriwilian Stage was identified in Australia, its GSSP was selected by the International Subcommission on Ordovician Stratigraphy to be the Huangnitang section near Changshan in the Zhejiang Province in China (Chen and Bergstr¨om, 1995; Mitchell et al., 1997). This decision was ratified by the International Commission on Stratigraphy in the Spring of 1997. Since then, stratigraphical and palaeontological research on the Darriwilian GSSP and its underlying and overlying strata has been carried out by several investigators (Chen et al., 2001, 2003a,b; Zhang and Fortey, 2001; Tang, 2004; Fortey et al., 2005). Chen et al. (2003b) recorded a Didymograptus (Corymbograptus) deflexus Biozone graptolite fauna as well as defined a new lithostratigraphic unit, the Huangnitang Member of the Ningkuo Formation, which includes the limestone interval below the Azygograptus suecicus Biozone in the Jiangshan–Changshan–Yushan (JCY) area (Fig. 1). Chen et al. (2001) published a correlation of the Undulograptus austrodentatus Biozone

and the equivalent conodont biozones of the JCY area with the biozones in Yangtze Platform region. Zhang and Fortey (2001) described some Darriwilian sinograptid graptolites. Tang (2004) studied the chitinozoans from the Darriwilian and its underlying beds in the JCY area. Recently, the State Ministry of Territory and Resources designated the Huangnitang section and nearby geological sites in Changshan County as the Changshan National Geopark, and thus, the Huangnitang GSSP is now very well protected. A new gallery paralleling the Huangnitang section was built for the tourists (Fig. 2), who are only allowed to observe the Huangnitang outcrop from the gallery. On the other hand, scientists from all countries are permitted to study the Huangnitang section and work directly on its rocks outside the gallery (Fig. 2). Obviously, better hotels, restaurants, parking spaces, and other facilities are available in the Changshan County town than at Huangnitang. Recently, Darriwilian graptolite faunas from the previously little studied stratigraphic interval above the U. austrodentatus Biozone at this section have been found by the authors and their co-workers from the Zhejiang Regional Geological Survey. Among these, an early

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Nemagraptus gracilis Biozone fauna collected by Xu Hong-Gen and his co-workers is particularly important. In ascending order, the Darriwilian graptolite biozones now recognized at Huangnitang (Fig. 3) include the Undulograptus austrodentatus Biozone (including the lower Arienigraptus zhejiangensis Biosubzone and the upper Undulograptus sinicus Biosubzone), the Acrograptus ellesae Biozone, the Nicholsonograptus fasciculatus Biozone, and the Pterograptus elegans Biozone. The base of the Nemagraptus gracilis Biozone is indicated by an early N. gracilis fauna, the first appearance of which marks the top boundary of the Darriwilian. Unfortunately, the topmost Darriwilian strata, corresponding to the Hustedograptus teretiusculus Biozone, have been partly destroyed by volcanic dikes and the characteristic fauna of this biozone has not been found. However, most of the Darriwilian graptolite biozone succession is now identified in the Huangnitang Darriwilian Stratotype section (Fig. 3). The fact that almost all of this global stage is exposed in a single section with a virtually continuous graptolite succession makes the Huangnitang outcrop extraordinarily important and it may in some respects be unique in the world. The purpose of the present paper is to describe its previously undocumented upper Darriwilian graptolite succession and to discuss the relations of its graptolite and conodont biozones to successions elsewhere. For the geographic location of places and other features mentioned in the text, see Fig. 1. All the figured graptolite specimens are kept in the type collection of the Nanjing Institute of Geology and Palaeontology (NIGP) and the figured conodont specimens are kept in the type collection of the Orton Geological Museum, The Ohio State University (OSU). 2. Darriwilian graptolite biozonation at Huangnitang In order to clarify the basis for the biozone classification here applied to the Huangnitang succession, the stratigraphic distribution of important graptolite species in other key sections in China and elsewhere is briefly reviewed below. The appearance of principal graptolite species in the stratigraphic interval from the A. ellesae Biozone to the N. gracilis Biozone at Huangnitang is illustrated in Figs. 4–7.

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2.1. The Undulograptus austrodentatus Biozone In the present paper we will not review the U. austrodentatus Biozone faunas, including the faunas from its two biosubzones, because these were described and discussed at length by Chen and Bergstr¨om (1995). The global correlation of this biozone was also dealt with by Chen and Bergstr¨om (1995) and Mitchell et al. (1997) and we refer to those publications for pertinent information about these matters. 2.2. The Acrograptus ellesae Biozone The base of this biozone at Huangnitang is marked by the first appearance datum (FAD) of A. ellesae (Ruedemann, 1904), which is most common in this biozone but might range as high as the H. teretiusculus Biozone elsewhere. In the past, the designation Amplexograptus confertus Biozone has been used for this stratigraphic interval in the Jiangnan slope belt and the Zhujiang Basin as well as in Qilianshan in the Chaidam Block in China. The A. ellesae Biozone has been recognized as a lower biosubzone of the A. confertus Biozone in Ningguo, Taiping, southern Anhui, Changhua, western Zhejiang, and the JCY area (Mu, 1959; Qian et al., 1964; Ge, 1964; Chen and Han, 1964; Chen et al., 1981). It has also been recorded from Taoyuan, central Hunan, Qidong, southern Hunan (Liu, 1973; Liu and Fu, 1985; Bureau of Geology and Mineral Resources of Hunan Province, 1988), Chongyi, southern Jiangxi (Li et al., 2000), and Xing’an, northern Guangxi (Chen et al., 1981). Mu (1974) defined the A. confertus Biozone with the lower A. ellesae Biosubzone as a standard Ordovician graptolite bio-unit in China. Li (1983), and Li et al. (2000) raised the A. ellesae Biosubzone to independent biozone status to replace the A. confertus Biozone in southern Anhui and southern Jiangxi, respectively. Chen et al. (2003a) followed Li’s proposal and employed the A. ellesae Biozone as a standard Ordovician graptolite bio-unit in China. A. confertus should no longer be used as a zonal index fossil since its FAD is not clear, but it is a common species in A. ellesae Biozone. The so-called A. confertus described by Elles and Wood (1907) was restudied by Maletz (1997) and found to be identical with Archiclimacograptus caelatus (Lapworth, 1875),

Fig. 2. Photos showing the Huangnitang section in the Changshan National Geopark. (A) The level of the base of the N. gracilis Biozone in the GSSP section shown by a hammer; (B) the gate of the Changshan National Geopark; (C) the new gallery paralleling the Huangnitang section; (D) the level of the base of the Darriwilian Stage at Huangnitang shown by a hammer; (E) panoramic view of the Huangnitang section with the public gallery. Arrows (a and b) indicate the positions of the base and top of Darriwilian Stage, respectively.

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Fig. 3. (a and b) Chart showing ranges of graptolites and conodonts through the upper Darriwilian succession at Huangnitang.

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Fig. 4. Illustrations of graptolites from the Isograptus caduceus imitatus Biozone (?), and the interval from the Acrograptus ellesae Biozone to the “Hustedograptus teretiusculus Biozone”. (A) Acrograptus cognatus (Harris et Thomas), NIGP137196(AEP249b); (B and C) Pseudamplexograptus confertus (Lapworth), NIGP139779(AEP257), 137197 (III-13); (D) Aulograptus climacograptoides (Bulman), NIGP137198a(AEP249b); (E) Glossograptus acanthus Elles et Wood, NIGP137199 (II-37); (F and M) Didymograptus nanus (Lapworth), NIGP139780(AEP261), 137207 (III-13); (G) Pseudotrigonograptus ensiformis (Hall), NIGP137201a(AEP249b); (H) Allograptus venustus Chu, NIGP137203(AEP249b); (I) Expansograptus abnormis (Hs¨u), NIGP137204(AEP174a); (J) Acrograptus ellesae (Ruedemann), NIGP137202 (II-37); (K) Expansograptus similis (Hall), NIGP137205(AEP174c); (L) Cryptograptus gracilicornis Hs¨u, NIGP137206a (III-13); (N) Expansograptus nitidus (Hall), NIGP137208a(AEP174a); (O) Expansograptus praenuntius (T¨ornquist), NIGP137210(AEP174c); (P) Expansograptus cf. hirundo (Salter), NIGP137211a(AEP174c). Magnifications: (P) ×2.5, the others ×3.1.

which ranges from the A. ellesae Biozone upwards into the Pterograptus elegans Biozone. This species has been recorded from strata referred to the Didymograptus murchisoni Biozone in Wales (Elles and Wood, 1906) and the N. fasciculatus and P. elegans biozones in the Oslo region, Norway (Maletz, 1997). Acrograptus ellesae (Ruedemann, 1904) was originally described from the base of Bed 3 at Deep Kill, New York State, where it is associated with Didymograptus nanus Lapworth, 1875 and D. gracilis T¨ornquist, 1890 [Acrograptus gracilis (T¨ornquist, 1890)]. The A. ellesae Biozone is easy to correlate with its equivalents in both Europe and North America. A. ellesae has also been recorded from western Australia although the graptolite biozonation is not completely

established there (Legg, 1976). The species has not yet been reported from Victoria, but a related species, A. cognatus (Harris and Thomas, 1935), occurs in Da3 in that region (VandenBerg and Cooper, 1992). The latter has also been recorded in the interval from the Expansograptus hirundo Biozone to the “Didymograptus bifidus” Biozone in Kazakhstan (Tzaj, 1974). Sinograptids such as Tylograptus and Allograptus are characteristic forms in the A. ellesae Biozone. Most of the sinograptids are typical Pacific Realm species that are present in the low to middle latitude belt (Chen, 1994). Aulograptus is also common. Maletz (1997) recognized that Aulograptus orientalis (Mu, 1957) is a junior synonym of A. climacograptoides (Bulman, 1931). A few expansograptids, such as Expansograptus

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Fig. 5. Illustrations of graptolites in the Isograptus caduceus imitatus Biozone (?), and the interval from the Acrograptus ellesae Biozone to the “Hustedograptus teretiusculus Biozone” (?). (A) Cryptograptus schaeferi Lapworth, NIGP139746(AEP260), ×3.3; (B–D) Cryptograptus tricornis (Carruthers), NIGP139747(AEP261), NIGP139748(AEP280), NIGP137216 (III-19-H), ×3.3; (E–I) Haddingograptus eurystoma (Jaanusson); (E) NIGP139749(AEP257), ×6.5; (F) NIGP137214 (III-19-H), ×3.3; (G) NIGP139750(AEP257), ×3.3; (H) NIGP139751(AEP258), ×6.5; (I) NIGP139752(AEP257), ×6.5; (J and K). Hustedograptus teretiusculus (Hisinger), NIGP137200a (III-14-H), NIGP139753(AEP280), both ×3.3; (L) Loganograptus leei Hs¨u, NIGP139754(AEP258), ×3.3; (M) Tylograptus spinatus Mu, NIGP139755(AEP258), ×6.5; (N) Sinograptus typicalis Mu, NIGP139756(AEP261), ×6.5; (O) Tylograptus spiniformis Mu, NIGP137213 (III-13), ×3.3; (P) Hustedograptus sp., NIGP137212 (III-19H), ×3.3; (Q and T) Proclimacograptus angustatus (Ekstr¨om), NIGP139757(AEP258), NIGP139758(AEP258), both ×3.3; (R) Haddingograptus oliveri (Bouˇcek), NIGP137217 (III-16-H), ×3.3; (S) Archiclimacograptus caelatus (Lapworth), NIGP139759(AEP257), ×3.3; (U) Nicholsonograptus sp., NIGP137209 (III-14-H), ×3.3; (V) Nicholsonograptus fasciculatus (Nicholson), NIGP139760(AEP257), ×3.3; (W) Pseudophyllograptus angustifolius (Hall), NIGP139761(AEP261), ×3.3; (X) Phyllograptus anna Hall, NIGP139762(AEP260), ×3.3.

praenuntius (T¨ornquist, 1901), range into this biozone from the underlying interval. Bergstroemograptus crawfordi (Harris, 1926) occurs also in the equivalent strata in Qiliangshan (Chen et al., 2001) and in Da2–3 in Australia (VandenBerg and Cooper, 1992). Eoglyptograptus was morphologically defined by Mitchell (1987) based on the type species Fucoides dentatus Brongniart, 1828, which was first described from the Upper L´evis Shale at Point L´evis, Quebec. The illustration given by Mitchell of this species is of E. dentatus (Brongniart) sensu Bulman, 1936 from the D. artus

Biozone in the Holen Limestone of the Baltic Kundan Stage. However, “Eoglyptograptus” dentatus (Brongniart) was then restudied by Maletz and Mitchell (1995) as Undulograptus dentatus (Brongniart) and defined as a senior synonym of Undulograptus intersitus (Harris and Thomas, 1935) from the Darriwilian 2 (Da2) of Australia. Thus, the early occurrence of U. dentatus at the Huangnitang section from the A. ellesae Biozone may roughly correspond to Da2 of the Australia standard. Two glossograptid species, Glossograptus acanthus Elles and Wood, 1908 and Cryptograptus gracilicornis

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Fig. 6. Illustrations of graptolites from the P. elegans Biozone, the “H. teretiusculus Biozone”, and the N. gracilis Biozone. (A and B) Xiphograptus norvegicus (Berry), NIGP137215 (III-19-H), ×3.3, NIGP139763(AEP262), ×6.5; (C) Lasiograptus? sp. NIGP139764(AEP262), ×6.5; (D and F) Dicellograptus sextans (Hall), NIGP135754, 135755, both ×3.3; (E and G) Dicellograptus gurleyi Lapworth, NIGP135760, NIGP139765(AEP262a), both ×3.3; (H) Normalograptus sp., NIGP139766(AEP257), ×3.3; (I) Archiclimacograptus riddellensis (Harris), NIGP137218a (III-19-H), ×3.3; (J) Glossograptus fimbriatus (Hopkinson), NIGP135759, ×3.3.

(Hs¨u, 1934), occur in the present biozone. The former is a cosmopolitan form, which was first described from “lower Llanvirn” strata with “D. bifidus” itself in Great Britain (Elles and Wood, 1908). It occurs from Da1 to Da3 in Australia (VandenBerg and Cooper, 1992), and in “Llanvirn strata” in Kazakhstan (Tzaj, 1974). In China, it is present not only in the Jiangnan slope belt but also in the Zhujiang Basin where it has a longer range (Li et al., 2000). However, it occurs only in the A. confertus Biozone in Qilianshan (Mu et al., 1962). C. gracilicornis is an endemic species which was originally described from the “Climacograptus? gracilicornis” Biozone at Hulo, southern Anhui, the type locality of the Ningkuo Formation (Hs¨u, 1934). It is also recorded from the A. ellesae Biozone at Taiping, southern Anhui (Li in Qian et al., 1964), and from the same biozone at Changhua, western Zhejiang (Ge, 1964). It seems that the “Climacograptus? gracilicornis” Biozone at Hulo is an equivalent to the A. ellesae Biozone in southern Anhui and western Zhejiang, including the JCY area. 2.3. The Nicholsonograptus fasciculatus Biozone The base of this biozone is marked by the FAD of the eponymous species. Unfortunately, only a fragment of Nicholsonograptus sp. has been collected at the base of the present biozone at Huangnitang. However, the occurrence of the genus Nicholsonograptus is restricted to the present biozone in the Jiangnan slope belt, includ-

ing southern Anhui (Hs¨u, 1934; Li in Qian et al., 1964; Li, 1983), western Zhejiang (Ge, 1964), northeastern Jiangxi (Chen and Han, 1964; Xiao and Chen, 1990) and the Zhujiang Basin, including southern Jiangxi (Li et al., 2000). Also in the Oslo region, the genus occurs only within this biozone (Maletz, 1997). In Bohemia, Nicholsonograptus is present in the Expansograptus ferrugineus Horizon in the lower part of the S´arka Formation. Bouˇcek (1973) concluded that all Nicholsonograptus species occur in “lower Llanvirn” strata. Thus, we may employ the designation Nicholsonograptus fasciculatus Biozone based on the occurrence of a specimen of the genus Nicholsonograptus even if the zone index species has not yet been identified at Huangnitang. Apart from 9 species that range from lower strata, there are 13 other species in this biozone (Fig. 3). Three didymograptid species, D. nanus Lapworth, 1875, “D”. spinosus Ruedemann, 1904 and D. sp. occur in the topmost part of the biozone. The former is a characteristic form in “lower Llanvirn” strata in South Wales and the Lake District. On the other hand, “D”. spinosus is a distinctive species in the Paraglossograptus tentaculatus (formerly P. etheridgei) Biozone in eastern New York (Berry, 1962). In both western and eastern North America, the P. tentaculatus Biozone covers a broad stratigraphic interval between the D. bifidus Biozone (Berry, 1962), or the Isograptus Biozone (Finney et al., 1995), and the Hustedograptus teretiusculus Biozone. Cryptograptus tricornis (Carruthers) occurs in the

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Fig. 7. Illustrations of graptolites from near the base of Nemagraptus gracilis Biozone at the Huangnitang and Wuliancun water pool sections, Changshan, China. (A) Nemagraptus exilis (Lapworth), NIGP139767(AEP280), ×3.2; (B, D, G and K) Nemagraptus gracilis (Hall), B.NIGP139768(AEP280); (D) NIGP139769(AEP280); (G) NIGP135752(III-22-H2); (K) NIGP135753 (same slab as 135754, III-22-H2), all four figures ×3.2; (C) “Didymograptus” sp., NIGP135751(III-22-H2), ×3.2; (E) Pseudazygograptus incurvus (Ekstr¨om), NIGP139770(AEP262a), ×3.2; (F and J) Dicellograptus sextans exilis Elles et Wood, NIGP135756(III-22-H2), NIGP139771(AEP280), both ×3.2; (H, I, N) Pseudoclimacograptus scharenbergi (Lapworth), NIGP135762(AEP III-22-H2), ×3.2, NIGP135763(AEP(III-22-H2), ×3.2, NIGP139772(AEP280), ×6.4; (L) Climacograptus eximius Ruedemann, NIGP 135761(III-22-H2), ×3.2; (M, O and U) Normalograptus euglyphus (Lapworth), NIGP135757(III22-H2), ×3.2, NIGP139773(AEP262a), ×6.4, NIGP135758 (III-22-H2), ×3.2; (P) Proclimacograptus? elongatus (Fang), NIGP139774(AEP280), ×3.2; (Q) Normalograptus brevis (Elles et Wood), NIGP139775(AEP280), ×6.4; (R) Climacograptus sp., NIGP139776(AEP280), ×6.4; (S) Archiclimacograptus angulatus (Bulman), NIGP139777(AEP280), ×6.4; (T) Glossograptus ciliatus Emmons, 1855 NIGP139778(AEP280), ×6.4.

N. fasciculatus Biozone in southern Anhui (Li in Qian et al., 1964), Wuning, northwestern Jiangxi (Ni, 1991), and Chongyi, southern Jiangxi (Li et al., 2000). It is also present in the A. confertus Biozone in Qilianshan (Mu et al., 1962), but the specimens are not well preserved. In Alxa and Kalping, Xinjiang, this species is present in the Pterograptus elegans and the H. teretiusculus biozones (Ge et al., 1990; Chen in Zhou et al.,

1990, 1992). This is the interval of peak occurrence of the present species, although its total range may extend into lower Caradoc strata (Elles and Wood, 1908). Cryptograptus uniformis Chen was first described from Darriwilian strata at Guangyuan, Sichuan on the northwestern margin of the Yangtze Platform (Chen in Mu et al., 1979). The occurrence of this species in the N. fasciculatus Biozone provides a more precise correlation of

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the Huangnitang succession. Pseudamplexograptus differtus (Harris and Thomas, 1935) and Haddingograptus eurystoma (Jaanusson, 1960) are herein first recorded from the N. fasciculatus Biozone in the Jiangnan slope belt. The former was originally described from Da3 of Victoria (VandenBerg and Cooper, 1992), and the latter is common in the N. fasciculatus Biozone of the Oslo region (Maletz, 1997). It is also present in the stratigraphically slightly younger Folkeslunda Limestone of ¨ Oland, Sweden from which it was originally described. 2.4. The Pterograptus elegans Biozone At the last level of occurrence of Nicholsonograptus, several new taxa appear (Fig. 3), including Climacograptus cf. uniformis Hs¨u, 1934, Oelandograptus oelandicus (Bulman, 1963), Archiclimacograptus riddellensis (Harris, 1924), Didymograptus miserabilis Bulman, 1931, Proclimacograptus bulmani Maletz, 1997, and Hustedograptus teretiusculus (Hisinger, 1840). Slightly higher stratigraphically are the first occurrences of Sinograptus rastritoides Mu, 1957 and Haddingograptus oliveri (Bouˇcek, 1973). Still higher, in the upper part of the biozone, there are occurrences of Archiclimacograptus cf. marathonensis (Clarkson, 1963) and Xiphograptus robustus (Ekstr¨om, 1937). Among these, P. bulmani, H. oliveri, X. robustus, and O. oelandicus are not recorded from strata younger than the Pterograptus elegans Biozone in northern Europe, and also H. eurystoma and A. riddellensis are best known from the P. elegans Biozone (see, e.g., Maletz, 1997). Because it lacks the species characteristic of the H. teretiusculus Biozone and has a species association very similar to that of the P. elegans Biozone in northern Europe, it is justified to refer this stratigraphic interval to the P. elegans Biozone. The P. elegans Biozone occurs in nearby Yushan County and elsewhere in southern Anhui and western Zhejiang. The apparent absence of the biozone index P. elegans at Huangnitang might be due to local environmental control or collecting failure; it should be noted that this biozone index species is absent in parts of the P. elegans Biozone in several important sections in northern Europe (Maletz, 1997). Fifteen species of the present biozone range upwards from the underlying beds and ten species appear in the present biozone. Didymograptus miserabilis Bulman was originally described from the Cryptograptus schaeferi Band of the upper Llanvirn in Peru (Bulman, 1931). Oelandograptus oelandicus (Bulman) was first found in strata attributed to the E. hirundo Biozone of the early Kundan Stage in southeastern Sweden (Skevington, 1965). Maletz (1997) recorded it only from

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the N. fasciculatus Biozone of the Oslo region. The present specimens from Huangnitang confirm its stratigraphic range as extending into the upper Darriwilian. Archiclimacograptus riddellensis (Harris) was first described from Victoria without information about the precise horizon. Maletz (1997) indicated that it came from Da4a and that the fauna from the type locality described by Harris (1924) is ‘identical’ with that in the P. elegans Zone in Norway that Maletz restudied in considerable detail. He also indicated that the species is restricted to the P. elegans Biozone in Scandinavia and Da4 in Australia. Hughes (1989) recorded A. riddellensis (referred to as Pseudoclimacograptus angulatus sebyensis Jaanusson, 1960) from the D. murchisoni Biozone and the H. teretiusculus Biozone in Wales. Proclimacograptus bulmani Maletz was first described from the P. elegans Biozone in Oslo region (Maletz, 1997) and it is also known from the lower part of the same biozone in western Newfoundland. Haddingograptus oliveri was based on collections from the Didymograptus murchisoni Biozone in the Oslo region (Bouˇcek, 1973), where it ranges from the N. fasciculatus Biozone through the P. elegans Biozone (Maletz, 1997). Archiclimacograptus marathonensis has been recorded from the Holmograptus lentus Biozone of southern Sweden and the N. fasciculatus Biozone in Norway (Maletz, 1997). Xiphograptus norvegicus was first described from the D. murchisoni Biozone of the Upper Didymograptus Shale (now Elnes Formation) in the Oslo-Asker district (Berry, 1964). Maletz (1997) suggested that it is a junior synonym of Xiphograptus robustus (Ekstr¨om) from the Upper Didymograptus Shale (now Almelund Shale) in southern Sweden, where it is common. It is not known from younger strata either in that region or in Norway. Maletz (1997) recorded it only from the P. elegans Biozone. Finally, Haddingograptus eurystoma is a characteristic species in the N. fasciculatus and P. elegans biozones in Scandinavia and is not known from younger strata. As shown in Fig. 8, there is a remarkable similarity between the graptolite faunas of this biozone between Baltoscandia and Huangnitang. There is little doubt that this resemblance reflects a closely similar, if not the same, age. 2.5. The “Hustedograptus teretiusculus Biozone” The FAD of H. teretiusculus at Huangnitang is at the base of the P. elegans Biozone as is the case in Scandinavia (Maletz, 1997). Because this species is known to have a very extended range in northern Europe, from strata as old as the lower P. elegans Biozone to the lower

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part of the Diplograptus foliaceus Biozone (Nilsson, 1977; Maletz, 1997), its presence alone is not a diagnostic index of the H. teretiusculus Biozone (Maletz, 1997). The top part of the Darriwilian in South China has been defined as the Glossograptus hincksii/Gymnograptus linnarssoni Biozone (Hu1) by Mu (1974) but these biozone index species are not known from Huangnitang. In their study of Tarim biostratigraphy, Zhou et al. (1992) suggested that the H. teretiusculus Biozone represents the top part of the Middle Ordovician. Chen et al. (2003a) proposed that this biozone should be considered to represent the uppermost part of the Middle Ordovician in the Chinese standard classification in agreement with the international standard. Diagnostic species of the H. teretiusculus Biozone, such as early dicellograptids, nemagraptids, and dicranograptids, and Gymnograptus linnarssoni (Moberg, 1896) (Fig. 8), have not yet been found in the Huangnitang succession and the apparent absence of species of this biozone calls for an explanation. No graptolites have been collected from most of this part of the Hulo Formation where volcanic dikes have partly destroyed the sediments. The very uppermost part of this interval, immediately below the FAD of N. gracilis, has produced three graptolite species, namely Dicellograptus gurleyi Lapworth (in Gurley, 1896), Normalograptus euglyphus (Lapworth, 1880), and Pseudazygograptus incurvus (Ekstr¨om, 1937). D. gurleyi occurs at the same level, just below the FAD of N. gracilis, in the Dawangou, Xinjiang section and D. gurleyi n. subsp. A (Finney, 1977) occurs at the level of the FAD of N. gracilis in the Calera, Alabama section (Finney, 1984). Hence, its appearance at Huangnitang is in agreement with that in the two Upper Ordovician global auxillary GSSP sections. N. euglyphus ranges from the D. murchisoni Biozone at Dawangou based on a recent restudy of the Dawangou graptolites by the present authors. In Scania, P. incurvus has been recorded from a lower part of the H. teretiusculus Biozone that was formerly referred to as the Climacograptus haddingi (Ekstr¨om’s C. putillus) Biozone (Ekstr¨om, 1937). At Dawangou, the species first

Fig. 8. Comparison of stratigraphic ranges of upper Darriwilian graptolite species present in both the Huangnitang section and in successions in southern Scandinavia (mostly after Maletz, 1995, 1997). Note the close similarity shown by the Pterograptus elegans Biozone faunas between these geographically widely separated areas. Also note that several of the species in the latter biozone are not known to range into the Hustedograptus teretiusculus Biozone. The interval distinguished

as the Pseudoamplexograptus distichus Biozone by Maletz (1995) is here included in the P. elegans Biozone as its fauna is essentially the same as that of the latter biozone apart from the presence of Pseudamplexograptus distichus (Eichwald, 1840), which is known only from Baltoscandia and Wales, and ranges up into the H. teretiusculus Biozone. The right portion of the diagram shows stratigraphic ranges (mainly after Bergstr¨om et al., 2000) of some species characteristic of the H. teretiusculus Biozone in Scandinavia. Except for the longranging Hustedograptus teretiusculus (Hisinger, 1840), none of these species have been found at Huangnitang that indicates that the latter biozone currently cannot be recognized in this section.

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occurs at the top of the H. teretiusculus Biozone and it is common in the N. gracilis Biozone (Chen in Zhou et al., 1990). Based on its known range, the FAD of this species appears to be in the H. teretiusculus Biozone but its presence is not diagnostic of that biozone. Because of the absence of the diagnostic species, it is not appropriate to recognize this zone as a biostratigraphic unit between the top of the N. fasciculatus Biozone and the base of the N. gracilis Biozone at Huangnitang. Temporarily, pending further studies of the interval at the top of the Darriwilian that has produced no graptolites, we employ the term “Hustedograptus teretiusculus Biozone” for this interval. 3. The Nemagraptus gracilis Biozone and the base of the Upper Ordovician In the new global Ordovician classification, the base of the Upper Ordovician is marked by the FAD of Nemagraptus gracilis (Hall, 1847), which coincides with the base of the N. gracilis Biozone (Bergstr¨om et al., 2000). N. gracilis (Hall) occurs at Huangnitang in association with Dicellograptus sextans (Hall, 1843) and D. gurleyi Lapworth. At F˚agels˚ang, the GSSP of the Upper Ordovician, the FAD of D. sextans is a little later than that of N. gracilis itself. However, its FAD coincides with those of N. gracilis and D. gurleyi at Dawangou, Xinjiang, which is the auxiliary stratotype section of the Upper Ordovician (Bergstr¨om et al., 2000). We believe that the base of the N. gracilis Biozone at Huangnitang may be coeval with that of the F˚agels˚ang

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GSSP, or at least very close to it. Hence this level at Huangnitang may be correlated with that of the international standard of the base of the Upper Ordovician (Fig. 9). The vertical ranges of other species, such as D. sextans exilis Elles and Wood, 1904 agree with that of D. sextans (Hall). Pseudoclimacograptus scharenbergi (Lapworth, 1876) and Climacograptus eximius Ruedemann, 1908 are common in the present biozone but may extend into higher strata. However, Normalograptus euglyphus (Lapworth) and Glossograptus fimbriatus (Hopkinson, 1872) occur from the older strata and may extend to the N. gracilis Biozone. Very recently, the present authors made a diverse graptolite collection from gray-yellow mudstones of the Hulo Formation at the Wuliancun water pool, 1.5 km southeast of Huangnitang. This collection, which represents the N. gracilis Biozone, includes Nemagraptus gracilis, Nemagraptus exilis (Lapworth in Gurley, 1896), Dicellograptus gurleyi, D. sextans exilis, Pseudazygograptus incurvus, P. scharenbergi, P. scharenbergi stenostoma (Bulman, 1947), Proclimacograptus? elongatus (Fang et al., 1990), Climacograptus sp., Hustedograptus teretiusculus (Hisinger, 1840), Archiclimacograptus angulatus (Bulman, 1953), Haddingograptus eurystoma (Jaanusson, 1960), Cryptograptus tricornis (Carruthers, 1858), Glossograptus ciliatus Emmons, 1855, and Normalograptus brevis (Elles and Wood, 1906). The presence of N. brevis with a high diversity fauna may indicate that the Wuliancun collection comes from an interval higher up in the N. gra-

Fig. 9. Correlation of the base of Upper Ordovician between the Huangnitang, Dawangou, Calera, and F˚agels˚ang sections (modified from Bergstr¨om et al., 2000, Fig. 8).

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cilis Biozone than the Huangnitang collections from that biozone. 4. Darriwilian conodont biozonation at Huangnitang Considerable efforts have been made to establish a conodont zonation through the Darriwilian succession at Hunagnitang using samples from limestone interbeds in the dominantly shaly sequence. Regrettably, many of these samples were poorly productive, or contained only taxa of minor, if any, biostratigraphic significance. The latter was the case with the samples from the early Darriwilian Undulograptus austrodentatus Biozone in the Ningkuo Formation. These, which did not produce conodonts useful for the recognition of conodont biozones, were detailed by Wang and Bergstr¨om (1995) and Bergstr¨om and Wang (1995) and will not be discussed further herein. 4.1. The Yangtzeplacognathus crassus Biozone Samples from the middle and upper part of the A. ellesae Biozone, and the lower part of the N. fasciculatus Biozone, produced a low-diversity conodont fauna (Fig. 3). Among the species present, Yangtzeplacognathus crassus (Chen and Zhang, 1993) is of special importance because this species, which combines a short stratigraphic range with a very wide geographic distribution (Bergstr¨om and Wang, 1998), has in recent years been used as a conodont biozone index in the standard conodont biozone successions in both China and Baltoscandia (Zhang, 1998; L¨ofgren, 2003; Stouge and Nielsen, 2003; L¨ofgren and Zhang, 2003). Its earliest known occurrence in the Huangnitang succession is at the base of the Hulo Formation (Wang and Bergstr¨om, 1995, Pl. 8, Fig. 12; there referred to as Amorphognathus? variabilis). This level is in the lower half of the Acrograptus ellesae Biozone. As shown in Fig. 3, Y. crassus ranges through a large portion of the A. ellesae Biozone into the basal most part of the Nicholsonograptus fasciculatus Biozone. The stratigraphic range of Y. crassus has been used to define the scope of the Y. crassus Biozone in Baltoscandia as well as in China (L¨ofgren and Zhang, 2003). Using the same biozone concept at Huangnitang, this biozone corresponds to a large portion of the A. ellesae Biozone and ends just above the base of N. fasciculatus Biozone. In the Yangtze Platform region, where the Y. crassus Biozone is 3–5 m thick (Zhang, 1998), it is located in the lower-middle portion of the Kuniutan Formation. This

limestone interval, which lacks graptolites, is generally correlated with the A. ellesae Biozone in the graptolite succession (Chen et al., 1995). In southern Sweden, the Y. crassus Biozone has been recognized in strata correlated with the Holmograptus lentus Biozone (also referred to as the lower part of the Didymograptus artus Biozone) (L¨ofgren, 2003) but this graptolite correlation is indirect. That is, in the key F˚agels˚ang succession, this conodont biozone occurs in the non-graptolitic upper part of the Komstad Limestone below an interval having conodonts of the Microzarkodina hagetiana Biosubzone of the Eoplacognathus pseudoplanus Biozone (Stouge and Nielsen, 2003). The top of the range of Y. crassus is 0.3 m below in the basal portion of the Almelund Shale that has produced graptolites of the H. lentus Biozone, including the zone index (Hede, 1951). This shows that the top of the Y. crassus Biozone is below the top of the H. lentus Biozone. Cooper and Lindholm (1990, Fig. 1), as well as L¨ofgren (2003), correlated this Swedish graptolite interval with the Australian Da2 (“Glyptograptus” intersitus Biozone) and hence, there is good agreement between graptolite/conodont biozone relations in this interval in Sweden and in China. Additional support from the Huangnitang succession for this correlation is provided by Paroistodus horridus (Barnes and Poplawski, 1973). In Argentina, where the evolution of P. horridus from P. originalis has been documented by Albanesi and Barnes (2000), this takes place in the upper part of the Lenodus variabilis Conodont Biozone corresponding to the lower part of the stratigraphically broad Paraglossograptus tentaculatus Graptolite Biozone. Albanesi and Ortega (2003, Fig. 1) correlated this interval with the Holmograptus lentus Biozone. Although this suggests that strata containing the geographically very widespread P. horridus are not older than the H. lentus Biozone as it is recognized in Argentina, further studies are needed to tie its FAD precisely into the graptolite biozone succession, especially in the Atlantic Realm. 4.2. The Histiodella holodentata Biozone The sparse conodonts collected in samples from most of the Nicholsonograptus fasciculatus Biozone do not include biostratigraphically diagnostic species. However, in all probability this interval corresponds to the Histiodella holodentata Biozone as it is recognized elsewhere in the interval between the Yangtzeplacognathus crassus and Histiodella kristinae biozones. We use the provisional designation “Histiodella holodentata Biozone” for this part of the succession pending further studies.

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4.3. The Histiodella kristinae Biozone A stratigraphically important and taxonomically diverse conodont fauna, including more than 600 specimens, was isolated from sample AEP 261,

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which was collected from the basal most part of the Pterograptus elegans Biozone (Fig. 3). Its significant species include Eoplacognathus cf. pseudoplanus (Viira) (Fig. 10Q–V), Histiodella holodentata (Ethington and Clark, 1982) (Fig. 10W), H. kristinae

Fig. 10. Schematic camera lucida drawings of selected conodont taxa from sample AEP 261. All figured specimens are kept in the type collection of the Orton Geological Museum, The Ohio State University. (A) Parapaltodus arcuatus Stouge, 1984, OSU 52316, lateral view, ×50; (B) Panderodus sulcatus F˚ahreus, 1966, OSU 52317, lateral view, ×100; (C–F) Triangulodus maocaopus Zhang, 1998, OSU 52318–52321, four different elements of the apparatus, all lateral view and ×50; (G–H) Protopanderodus varicostatus (Sweet and Bergstr¨om, 1962), OSU 52322–52323, two different elements of the apparatus, lateral views, ×50 and ×25, respectively; (I) Drepanodus arcuatus Pander, 1856, OSU 52324, lateral view, ×50; (J–M) Periodon macrodentata (Graves and Ellison, 1941), OSU 52325–52328, four different elements of the apparatus; (J and L) ×100; (K and M) ×50; (K and L) illustrate the range of variation in the denticulation of the M element; (N and O) Paroistodus horridus (Barnes and Poplawski, 1973), OSU 52329–52330, drepanodiform and oistodiform elements, respectively, lateral views, ×50; (P) Polonodus cf. clivosus (Viira, 1974), OSU 52331, Pa element, upper view, ×50; (Q–V) Eoplacognathus cf. pseudoplanus (Viira, 1974), OSU 52332–52336, five different elements of the apparatus; (Q and R) are upper views, others lateral views (R and S are two views of OSU 52333); (Q) ×50, others ×100; (W) Histiodella holodentata (Ethington and Clark, 1982), OSU 52337, P element, lateral view, ×100; (X and Y) Histiodella kristinae Stouge, 1984, OSU 52338–53339, P elements, lateral views, ×100.

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Stouge (Fig. 10X–Y), Paroistodus horridus (Barnes and Poplawski, 1973) (Fig. 10N and O), Periodon macrodentata (Graves and Ellison, 1941) (Fig. 10J–M), Polonodus cf. clivosus (Viira) (Fig. 10P), and Triangulodus maocaopus Zhang (Fig. 10C–F). The stratigraphical ranges of the key species in the Baltoscandic, Newfoundland and Yangtze Platform successions are summarized in Fig. 10. Brief comments on these taxa are given below. Of particular biostratigraphic importance is the presence of morphologically advanced specimens of H. holodentata associated with specimens of H. kristinae and elements morphologically intermediate between these taxa. As was first demonstrated by Stouge (1984), the former species (referred to by him as H. tableheadensis) evolves into the latter. The AEP 261 sample is interpreted to represent the level of this speciation event that marks the base of the H. kristinae Biozone. Hence, the evidence from Huangnitang suggests that the base of this conodont biozone corresponds closely to the base of the P. elegans Graptolite Biozone. This is a new and important tie-point between the conodont and graptolite biozone successions. Stouge (1984) demonstrated this evolutionary event in collections from the uppermost part of the Lower Member of the Table Head Formation in western Newfoundland. In a monographic study of Scandinavian conodonts, Rasmussen (2001) recorded the FAD of H. kristinae (just above the last occurrence of H. holodentata) in his P. graeai Biozone, which he suggested corresponds to a level just below the base of the Pterograptus elegans Biozone although direct graptolite evidence for this correlation was not available in his sections. As noted above, Stouge and Nielsen (2003) recorded conodonts of the Microzarkodina hagetiana Biosubzone of the Eoplacognathus pseudoplanus Biozone about 0.5 m above the base of the Almelund Shale in the F˚agels˚ang succession. In Hede’s (1951) description of the F˚agels˚ang core, the Holmograptus lentus and Nicholsonograptus fasciculatus Biozones, each about 1 m thick, underlies the base of the Pterograptus elegans Biozone, as defined by the FAD of P. elegans at 2.17 m above the base of the Almelund ¨ Shale. In the limestone succession at Gillberga on Oland, south-eastern Sweden, the thickness of the M. hagetiana Biosubzone is about 2.8 m (L¨ofgren, 2000). K-bentonite bed-based correlations between F˚agels˚ang and Gillberga (Bagnoli and Stouge, 1999) suggest that the thicknesses of early-middle Darriwilian units at Gillberga are quite similar to coeval ones at F˚agels˚ang. No conodont work has yet been carried out on shale samples from the pre-P. elegans Biozone part of the Almelund Shale that has a thickness of about 2 m. However, if the thickness of the F˚agels˚ang M. hagetiana Biosubzone is similar to that at

Gillberga, its top would be just above the base of the P. elegans Biozone, hence at precisely the same stratigraphic level as in the graptolite biozone succession at Huangnitang. Hence, the conodonts, and especially the evolution of the Histiodella lineage, provide not only a reliable basis for correlation of this level between Newfoundland, Scandinavia, and China but also, they give strong support for the equivalence of the base of the P. elegans Biozone as it is recognized in the Huangnitang succession and in Scandinavia. It is also of interest to note that Zhang (1998) documented the replacement of H. holodentata (H. tableheadensis in Zhang) with H. kristinae in the Kunuitan Formation on the Yangtze Platform, where it is located at the base of her M. ozarkodella Biosubzone. In her Fenxiang, Cili, and Maocaopu sections, the FAD of H. kristinae, and the base of the eponymous biozone, is about 0.5, 11, and 7 m, respectively below the top of the formation. Because the Gunuitan Formation has not yielded any biostratigraphically useful graptolites, the conodonts furnish a useful tie-point (essentially the base of the P. elegans Biozone) between the graptolite biozone succession and the Kunuitan limestone sequence. Although providing less precise correlation, the ranges of P. cf. clivosus, P. horridus, P. macrodentata, Eoplacognathusa cf. pseudoplanus, and Triangulodus maocaopus are in agreement with the biostratigraphic evidence from the Histiodella lineage. Polonodus clivosus was first described (as Ambalodus clivosus) from the Aluojan Substage of the Kundan Stage in Estonia (Viira, 1974), where it ranges through strata now referred to the Eoplacognathus pseudoplanus Biozone (Viira and M¨annik, 1997, Table 42). The species is also known from coeval strata in Sweden (L¨ofgren, 1978; Rasmussen, 2001), China (Zhang, 1998), and possibly Poland (Dzik, 1976) and Newfoundland (Stouge, 1984). Although similar to one of Viira’s original illustrations (Viira, 1974, Fig. 38a), the single specimen from Huangnitang is not quite typical of the species and it is referred to as P. cf. clivosus pending recovery of additional specimens. Paroistodus horridus ranges from the Lenodus variabilis Biozone in China (Zhang, 1998) and Argentina (Albanesi and Barnes, 2000) and coeval strata in the Histiodella holodentata Biozone of western Newfoundland (Stouge, 1984). It is also known in many other regions (for references, see L¨ofgren, 1995). The top of its range is not well established but the record by Albanesi and Barnes (2000, Fig. 2) from the Pygodus anitae Biosubzone of the Eoplacognathus suecicus Biozone may be the youngest published so far. The Huangnitang spec-

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imens are quite typical of the taxon called P. horridus horridus by Albanesi and Barnes (2000). Periodon macrodentata was originally described (as Ozarkodina macrodentata) from the middle part of the Fort Pe˜na Formation in West Texas (Graves and Ellison, 1941). Because the same sample yielded the holotype of Histiodella sinuosa, the index of the H. sinuosa Biozone (Sweet, 1984), the holotype of P. macrodentata comes from that conodont biozone. Those beds also represent the Paraglossograptus tentaculatus Graptolite Biozone. Periodon macrodentata has a pandemic distribution in early-middle Darriwilian strata but has frequently been identified as P. aculeatus Hadding, its late Darriwilian descendent. Eoplacognathus pseudoplanus, which was first described (as Ambalodus pseudoplanus) from the Aluojan Substage of the Kundan Stage of Estonia (Viira, 1974), has been used as a biozone index in Baltoscandia (L¨ofgren and Zhang, 2003) and the Yangtze Platform of China (An et al., 1981, 1985). One of our Pb ele-

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ments (Fig. 10Q) is very fragmentary but it is similar to some published illustrations of the species (Viira, 1974, Fig. 45b; Zhang, 1998, Pl. 9, Figs. 4 and 5; L¨ofgren and Zhang, 2003, Figs. 3, 9 and 10). Another, more complete, Pb element (Fig. 10S) is different in several respects from that of typical E. pseudoplanus. Furthermore, the associated M element (Fig. 10U) has a much more strongly developed denticulation along the anterior margin than the faint serration typical of this species (L¨ofgren and Zhang, 2003, p. 735, Fig. 13, p. 18). In view of these differences, these specimens are herein referred to as E. cf. pseudoplanus. Triangulodus maocaopus was first described (Zhang, 1998) from the Yangtze Platform, where it ranges from the Lenodus variabilis Biozone to the top of the M. ozarkodella Biosubzone of the Eoplacognathus pseudoplanus Biozone. In her synonymy list, Zhang (1998) listed several records from northern Europe and North America, where the distribution of the species is still poorly known.

Fig. 11. Stratigraphic ranges in Baltoscandia, western Newfoundland, and Yangtze Platform of some important conodont species present in sample AEP 261 from the base of the Pterograptus elegans Zone at Huangnitang. Based on the overlapping ranges and the presence of Histiodella kristinae Stouge, 1984 this sample is interpreted to represent strata coeval with the Microzarkodina ozarkodella Biosubzone of the Eoplacognathus pseudoplanus Biozone in Baltoscandia and China, and the basal Histiodella kristinae Biozone in western Newfoundland. Range data based on many sources; for Baltoscandia mainly L¨ofgren (1978, 2003), L¨ofgren and Zhang (2003), Rasmussen (2001), and Stouge and Nielsen (2003); for western Newfoundland Stouge (1984); and for the Yangtze Platform mainly Zhang (1998).

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Fig. 12. Correlation between conodont and graptolite biozones in some important successions discussed in the text. Scandinavian units mainly after L¨ofgren (2003), L¨ofgren and Zhang (2003), and Maletz (1995, 1997); Newfoundland conodont biozones after Stouge (1984); and Yangtze Platform conodont biozones after Zhang (1998). Note that at Huangnitang, the base of the Yangtzeplacognathus crassus Conodont Biozone is just below the base of the Acrograptus ellesae Graptolite Biozone and the base of the H. kristinae Conodont Biozone is very close to the base of the Pterograptus elegans Graptolite Biozone.

Based on the published ranges of its species elsewhere (Fig. 11), we conclude that the Huangnitang sample AEP 261 comes from a level corresponding to the basal part of the Microzarkodina ozarkodella Biosubzone of the E. pseudoplanus Biozone in Baltoscandia, the base of the H. kristinae Biozone in western Newfoundland, and the Microzarkodina ozarkodella Biosubzone of the Dzikodus tablepointensis Biozone on the Yangtze Platform. Our interpretation of the relations between the conodont and graptolite biozones recognized at Huangnitang and in the successions in Scandinavia, Newfoundland, and the Yangtze Platform is shown in Fig. 12. 5. Conclusions The succession of Darriwilian graptolite biozones may be virtually complete at Huangnitang, the GSSP of the Darriwilian Stage. In ascending order they are the Undulograptus austrodentatus (including the Arienograptus zhejiangensis and the Undulograptus sinicus biosubzones), the Acrograptus ellesae, the Nicholsonograptus fasciculatus, and the Pterograptus elegans biozones. Decisive evidence of the presence

of the late Darriwilian Hustedograptus teretiusculus Biozone is lacking, the Nemagraptus gracilis Biozone being separated from the P. elegans Biozone by a nongraptolitic interval. These Darriwilian biozones may be correlated with those of neighboring regions in China as well as with those of other continents (Fig. 13). In terms of conodont biostratigraphy, the Yangtzeplacognathus crassus and Histiodella kristinae biozones are positively identified, and the interval between these biozones is tentatively referred to as the “Histiodella holodentata Biozone.” The conodont biostratigraphy is in good agreement with the graptolite zonation, and the Huangnitang succession provides important tie-points between standard graptolite and conodont biozones. Hence, it is shown that the top of the Y. crassus Conodont Biozone (base of the H. holodentata Conodont Biozone) is at essentially the same stratigraphic level as the base of the N. fasciculatus Graptolite Biozone at Huangnitang, and that the base of the H. kristinae Conodont Biozone corresponds to a level just above the base of the P. elegans Graptolite Biozone. The nearly complete graptolite biozone succession at Huangnitang, and the established direct relations of its

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Fig. 13. Correlation diagram showing relations between graptolite biozones in the Huangnitang succession and those in some global key sequences. Note that in a recent paper, Maletz (2005, text, Fig. 8) correlated the base of the British E. hirundo Biozone with the top of the U. austrodentatus Biozone. However, he provided no data in support for his interpretation of this very restricted scope of the E. hirundo Biozone.

graptolite biozones to international conodont biozones, make this remarkable Darriwilian GSSP particularly important as a global reference locality. It is therefore with great satisfaction that we note that the local authorities have made most considerable efforts not only to preserve this locality but also to facilitate its study by scientists and the general public. Acknowledgments The present study was supported by the Chinese Academy of Sciences (KZCX2-SW-129) and the Natural Science Foundation of China (Grant nos. 40372007 and 40532009). A part of the expenses for the conodont study by S.M. Bergstr¨om was defrayed by the Department of the Geological Sciences, The Ohio State University. We thank Yu Guohua (Zhejiang) for participating the field work, and Luo Tiantian (Nanjing) and Ren Yugao (Nanjing) for completing some of the figures. We are also indebted to A. L¨ofgren for useful comments on some conodont identifications. The present manuscript was kindly reviewed by Dr. Roger Cooper and Dr. Guillermo L. Albanesi. References Albanesi, G.L., Barnes, C.R., 2000. Subspeciation within a punctuated equilibrium evolutionary event: phylogenetic history of the LowerMiddle Ordovician Paroistodus originalis–P. horridus complex (Conodonta). J. Paleontol. 74, 492–502.

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