Lower-Middle Triassic conodont biostratigraphy of the Mingtang section, Nanpanjiang Basin, South China

    Lower-Middle Triassic conodont biostratigraphy of the Mingtang section, Nanpanjiang Basin, South China Lei Liang, Jinnan Tong, Haijun Song, Ting Song, Li Tian, Huyue Song, Haiou Qiu PII: DOI: Reference:

S0031-0182(16)30282-6 doi: 10.1016/j.palaeo.2016.07.027 PALAEO 7918

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

1 October 2015 16 July 2016 20 July 2016

Please cite this article as: Liang, Lei, Tong, Jinnan, Song, Haijun, Song, Ting, Tian, Li, Song, Huyue, Qiu, Haiou, Lower-Middle Triassic conodont biostratigraphy of the Mingtang section, Nanpanjiang Basin, South China, Palaeogeography, Palaeoclimatology, Palaeoecology (2016), doi: 10.1016/j.palaeo.2016.07.027

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ACCEPTED MANUSCRIPT Lower-Middle Triassic conodont biostratigraphy of the Mingtang section, Nanpanjiang Basin, South China

State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China

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Lei Lianga, Jinnan Tonga*, Haijun Songa, Ting Songa, Li Tiana, Huyue Songa, Haiou Qiub

University of Geosciences, Wuhan 430074, China

School of Material Science and Chemical Engineering, China University of Geosciences, Wuhan

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430074, China

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* Corresponding author. E-mail addresses: [email protected] (J. Tong)

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ABSTRACT

The well-exposed stratigraphic sequences on the Great Bank of Guizhou (GBG) in South

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China provide important information for the understanding of various environmental settings during the Permian-Triassic mass extinction and the subsequent recovery, including one of the oldest Triassic marginal reef complexes, which is viewed as an important indicator of biotic recovery following the end-Permian mass extinction. Here we report a systematic conodont study on a newly exposed Lower-Middle Triassic section, the Mingtang section, at the margin of the GBG to provide more data for the time scale of the post-extinction recovery. Twenty-four species in 11 conodont genera are identified and assigned to Hindeodus parvus, Neoclarkina discreta, Neospathodus dieneri, Novispathodus waageni, Neospathodus triangularis-Triassospathodus homeri, Chiosella timorensis, Nicoraella germanica, and Nicoraella kockeli zones in ascending order. These zones 1

ACCEPTED MANUSCRIPT correlate well with the conodont successions in South China and over the world. The Induan-Olenekian boundary can be well defined by the first occurrence of Novispathodus waageni

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in the lower-middle part of the Luolou Formation, while the Early-Middle Triassic boundary was

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placed at 0.2 m below the top of the Luolou Formation according to the first occurrence of Chiosella timorensis. These conodont data suggest that the Tubiphytes-reef began to recover in the Bithynian (early Middle Triassic), later than benthonic organisms such as foraminifers and

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calcareous algae.

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Key words: Conodont biostratigraphy, Early Triassic recovery, Nanpanjiang Basin

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1. Introduction

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The Permian-Triassic (P-Tr) mass extinction witnessed the most severe biotic crisis in the

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Phanerozoic while over 90 % of marine species were wiped out and the marine ecosystem structures were altered from the Paleozoic type to Mesozoic-Modern type (Raup, 1979; Sepkoski, 1984; Erwin,

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1994; Benton and Twitchett, 2003; Payne and Clapham, 2012; Song H.J. et al., 2013). Following the mass extinction, the catastrophic climate and environment, including high temperature (Sun et al., 2012; Romano et al., 2013), anoxia (Song et al., 2012; Grasby et al., 2013; Tian et al., 2014), and elevated weathering rates (Algeo et al., 2010, 2011), persisted or repeatedly occurred in the Early Triassic (Song et al., 2014; Wei et al., 2014; Tian et al., 2015a, b), accompanied by the puzzling biotic recovery, particularly the tempo and mechanism. Some clades such as ammonoids, conodonts and foraminifers began to recover in the early Smithian (Brayard et al., 2009; Orchard, 2007; Song et al., 2011; Stanley, 2009), whereas other clades recovered later in the Middle Triassic (Hallam, 1991; Retallack et al., 1996; Flügel, 2002; Payne et al., 2006a; Chen and Benton, 2012). The 2

ACCEPTED MANUSCRIPT influence of stressed environmental conditions on the biotic recovery has been discussed at length (Algeo et al., 2011; Pietsch and Bottjer, 2014; Song et al., 2014). Associated biotic and

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environmental studies should provide direct evidence to solve this dispute.

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The Great Bank of Guizhou (GBG) in South China is an ideal carbonate platform for the study of the biotic and environment changes that occurred in different geological settings. It was located in the northern part of the Nanpanjiang Basin, South China, as an isolated platform from the Late

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Permian to Late Triassic, providing perfect carbonate records with a series of well-exposed sections

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from platform interior to the basin (Lehrmann et al., 1998). Previously, the extinctions and survivals of foraminifers (Song et al., 2009) and ostracods (Forel et al., 2009) as well as the proliferation of

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microbialites (Lehrmann, 1999; Lehrmann et al., 2003; Kershaw et al., 2007; Yang et al., 2011, 2015)

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were reported in the platform interior during the P-Tr transition. A distinct negative δ13C excursion

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was recognized from the shallow water section (Heping) of the GBG (Krull et al., 2004). Besides the negative excursion around the P-Tr boundary, the Early Triassic large perturbations of isotopic

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carbon, reported in a series of sections of the GBG (Payne et al., 2004; Tong et al., 2007; Meyer et al., 2011, 2013; Song H.Y., 2013), draw considerable attention to the recovery through the Early Triassic. Subsequently, the oxygen isotopes of the conodonts were used to reconstruct the temperature changes in the Triassic ocean (Sun et al., 2012). The Ce-anomaly and Th/U data of conodonts along with framboidal pyrites revealed multiple oceanic anoxic events in the Early Triassic (Song et al., 2012; Tian et al., 2014). Intensified weathering in the Early Triassic was indicated by the 87Sr/86Sr ratios recorded in the conodonts at the Guandao section (Song et al., 2015). In addition, research on foraminifers suggested a quick recovery in the early Smithian (Song et al., 2011), while the increase in diversity and abundance of fossils began around the Early-Middle 3

ACCEPTED MANUSCRIPT Triassic boundary (Payne et al., 2006b) and reef ecosystem reoccurred in the Middle Triassic (Payne et al., 2006a).

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Studies of conodont biostratigraphy mainly focused on the P-Tr boundary in the platform interior

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(Krull et al., 2004; Chen et al., 2009; Jiang et al., 2014), while complete Early Triassic conodont successions were reported in the basin-margin sections (i.e. Guandao and Bianyang sections, Wang et al., 2005; Orchard et al., 2007; Yan et al., 2013), which correlated with the shallow sections based

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on the excursion of carbon isotopes (Payne et al., 2004; Meyer et al., 2011). Here we present a

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conodont study at a newly-exposed platform-marginal Lower-Middle Triassic section, aiming to establish a conodont succession in the platform-marginal area on the GBG and provide a time

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constraint for the post-extinction recovery in the Triassic.

2. Geological setting and lithostratigraphy

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Serial Upper Permian-Middle Triassic sections are exposed in the GBG and adjacent areas. The Dawen, Dajiang, Lalaicao, Guandao and Bianyang sections are distributed from south to north,

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presenting the spatial variations of the Early Triassic successions from shallow platform interior to the basin (Lehrmann et al., 1998, 2001; Payne et al., 2004; Meyer et al., 2011). The Mingtang section is located between the Lalaicao section and Guandao section, and it records exceptional lithological facies at the margin of the GBG (Fig. 1). In ascending order, the uppermost Permian to Middle Triassic succession of the Mingtang section consists of the Dalong Formation, Luolou Formation and Poduan Formation. The exposed Dalong Formation at the section is composed of about 10 m of black-grayish thin-bedded siliceous mudstone (Bed 1 in Fig. 3). Macrofossils, such as ammonoids, brachiopods and bivalves, are collected from this formation. The lowermost part of the Luolou Formation is dominated by yellow-grayish thin-bedded mudstone, whereas the upper part is characterized by grayish thick-bedded limestone intercalated with thick-bedded brecciated limestone. The lower part of the mudstone member at the base of the Luolou Formation yields abundant brachiopods, bivalves and ammonoids while its upper part contains a low-diversity Claraia-Lingula fauna. These lithologic 4

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and biotic characteristics indicate a low-energy inter-shelf basin environment. The upper part of the Luolou Formation is composed of thin/medium-bedded limestone intercalated with thick-bedded breccia, suggesting a depositional condition on the slope of the GBG. The Poduan Formation is characterized by thick-bedded Tubiphytes boundstone and grianstone with interbeds of brecciated limestone. Based on the study of the microfacies, the limestone is mainly Tubiphytes-enriched grainstone (Fig. 2), which is a major microfacies type of reef complex at the platform margin of the GBG (Payne et al., 2006a).

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3. Materials and Methods

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Eighty two samples from the Luolou Formation (221.2 m) to the lower part of the Poduan Formation (88.8 m) were collected in order to extract conodonts. All conodont samples, each 2-5 kg,

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were sampled at ~4 m intervals. After coarsely crushing to ~2x2 cm pieces, the samples were dissolved in 10% acetic acid, and then filtered through 20-mesh and 160-mesh sieves before

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separation in 2.78-2.80 g/mL heavy liquid. The conodont fossils were carefully selected from the

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prepared samples under a stereo-microscope, before photographing using a Scanning Electronic

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Microscope (Quanta200, SU8010) at the China University of Geosciences (CUG). A total of 2636 conodont specimens were extracted, including the P1 elements of Hindeodus, Neoclarkina,

Nicoraella.

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Neospathodus, Novispathodus, Triassospathodus, Chiosella, Cornudina, Spathicuspus and

4. Conodont zonation

Eight conodont zones from the Griesbachian (Induan) to the Pelsonian (Anisian) can be well defined in order to refine the stratigraphic sequence. The distribution of the main conodont taxa and the conodont zonation of the section are shown in Fig. 3. The conodont zones are described as follows and the fossils are shown in Figs. 4-6.

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ACCEPTED MANUSCRIPT 4. 1 Hindeodus parvus Zone

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The Hindeodus parvus Zone ranges from the base to the middle part of Bed 8 at the section

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(Fig. 3) and its base is defined by the first occurrence (FO) of Hindeodus parvus while the

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uppermost horizon is defined by the FO of Neoclarkina discreta. Associated taxa include Hindeodus inflatus, H. anterodentatus, and H. praeparvus. Since the strata below Bed 8 at the Mingtang section

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are composed of mudstones and no conodonts have been retrieved from the rocks, the First Appearance Datum (FAD) of Hindeodus parvus, which is the definition of the Permian-Triassic

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boundary (Yin et al., 2001), is uncertain at this section.

Hindeodus parvus widely occurred around the GBG, from the platform interior sections, such as

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Heping section (Lehrmann et al., 2003), Dawen section (Chen et al., 2009) and Dajiang section (Jiang et al., 2014), to the marginal basin sections, such as Guandao section (Wang et al., 2005) and

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Bianyang section (Yan et al., 2013). The FO of H. parvus is reported inside the microbialites at the Heping and Dawen sections (Lehrmann et al., 2003; Chen et al., 2009), whereas Jiang et al. (2014)

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believed that H. parvus firstly occurred below the microbialite at the Dajiang section. At the marginal basin sections such as Guandao section, H. parvus first occurred once the rocks become carbonate from mudstone in the lowermost Luolou Fm. (Wang et al., 2005; Lehrmann et al., 2015), like the Mingtang section. In South China, Hindeodus parvus was first reported at Jiangyou in Sichuan (Wang and Dai, 1981) and Lichuan in Hubei (Wang and Cao, 1981), and later from many other P-Tr boundary sections in Sichuan (Zhang et al., 1984), Zhejiang (Zhang, 1984; Wang et al., 1996; Jiang et al., 2007; Zhang et al., 2009a), Jiangsu (Duan, 1987), Jiangxi (Yang and Sun, 1990), Guizhou (Metcalfe and Nicoll, 2007; Chen et al., 2009; Yan et al., 2013; Jiang et al., 2014; Zhang et al., 2014), Guangxi (Yang et al., 1986; Brosse et al., 2015) and Hubei provinces (Wang and Xia, 6

ACCEPTED MANUSCRIPT 2004; Zhao et al., 2013). The FAD of this species has been accepted as the definition of the P-Tr boundary for its world-wide distribution and proven earliest Triassic age (Yin et al., 1986, 2001),

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though there is an argument recently that this species co-occurs with some typical Permian taxa

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such as fusulinids (Zhang et al., 2014).

4.2 Neoclarkina discreta Zone

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The Neoclarkina discreta Zone ranges from the upper part of Bed 8 to the base of Bed 9. The

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lower limit of the zone is defined by the FO of Neoclarkina discreta and the upper limit by the FO of Neospathodus dieneri. Unfortunately, the conodonts are very rare in this interval at the section.

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The index fossil is only collected from a sample in the upper part of Bed 8, together with Hindeodus

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praeparvus, H. anterodentatus and Clarkina cf. planata. The Neoclarkina discreta Zone was firstly

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used in Spiti, India, as the latest Griesbachian zone (Orchard and Krystyn, 1998). In China, this zone has been reported at the Daxiakou section in Hubei (Zhao et al., 2013) and the Jiarong section

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in Guizhou (Chen et al., 2015; Fig. 7), but not found at the other sections around the GBG. Neoclarkina discreta was an evolutionary link between Clarkina krystyni and Sweetospathodus kummeli (Orchard, 2007). Besides, Clarkina carinata was ever used as a late Griesbachian index fossil, i.e. Clarkina carinata zone, and it typically co-occurred with C. krystyni, C. planata, Neoclarkina discreta (Sweet, 1970a, b; Orchard and Tozer, 1997; Yang et al., 1999; Shen et al., 2010). However, C. carinata is a long-ranging element from the uppermost Permian to Smithian, whereas Neoclarkina discreta had a distinctive age range, thus a successive conodont zones would be useful from the Clarkina krystyni Zone (late Griesbachian), Nc. discreta Zone (latest Griesbachian), to Sweetospathodus kummeli Zone (early Dienerian).

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ACCEPTED MANUSCRIPT 4.3 Neospathodus dieneri Zone

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The Neospathodus dieneri Zone ranges from the lower part of Bed 9 to Bed 10. The FO of the

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index species defines the lower limit of the Ns. dieneri Zone and the FO of Novispathodus waageni

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defines its upper limit. Three subzones of the Ns. dieneri Zone were identified at the Chaohu section (Zhao et al., 2007) and in the Three Gorges area (Zhao et al., 2013) based on the morphotypes of Ns.

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dieneri. These three morphotypes are also recognized at the Mingtang section, but Ns. dieneri M3 co-occurred with Ns. dieneri M2 in Bed 10. The lack of Ns. dieneri M2 in the lower strata is

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probably a result of the lack of conodonts in the brecciated limestone of Bed 9. Meanwhile, the zonal species was also reported from the Guandao section (Wang et al., 2005) and Bianyang section

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(Yan et al., 2013).

Since Ns. dieneri was identified by Sweet (1970a) in Kashmir, this zone has been widely used over

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the world, for instance in Pakistan (Sweet, 1970b), India (Goel, 1977), Malaysia (Metcalfe, 1992), United States (Paull, 1988) and South China (Wang and Cao, 1981; Yang et al., 1986; Wang et al.,

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2005), and it indicates an age of Dienerian.

4. 4 Novispathodus waageni Zone

The base of the Novispathodus waageni Zone is defined by the FO of Nv. waageni, and the top is by the FO of Neospathodus triangularis. It ranges from Bed 11 to Bed 17 at the section. The conodonts found in this zone are not very rich, but Nv. waageni is distinctive and the associated elements include Neospathodus dieneri and Pachycladina obliqua. The Novispathodus waageni Zone is also well defined at the Guandao section (Lehrmann et al., 2015), but not distinguished at the Bianyang section yet (Yan et al., 2013). 8

ACCEPTED MANUSCRIPT The Novispathodus waageni Zone is defined as the first conodont zone of the Olenekian Stage (Tong et al., 2003) and it has been identified widely in South China (Yang et al., 1986; Wang and

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Wang, 1995; Wang et al., 2005; Zhao et al., 2007, 2013; Ji et al., 2011), and over the world, such as

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in Pakistan (Sweet, 1970b), India (Goel, 1977; Orchard and Krystyn, 2007), United States (Solien, 1979), Vietnam (Thang, 1989), Canada (Orchard, 2008) and Australia (Metcalfe et al., 2013).

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4. 5 Neospathodus triangularis- Triassospathodus homeri Zone

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The lower limit of this zone is the FO of Neospathodus triangularis or Triassospathodus homeri. The upper limit is the FO of Chiosella timorensis. This zone ranges from Bed 18 to the

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upper part of Bed 26. The conodonts from this zone are relatively rich and diverse. The associated

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conodonts include Triassospathodus symmetricus, Novispathodus abruptus, Neospathodus clinatus,

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Ns. brevissimus, Chiosella gondolelloides and Cornudina sp. These species were also found at Guandao section (Lehrmann et al., 2015) and Bianyang section (Yan et al., 2013).

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The zonal species are typical Spathian elements and co-existed in the Garencuo Formation in Tibet (Wu et al., 2007), Yongningzhen Formation of Guizhou (Yang and Chu, 1992) and the Luolou Formation of Guangxi and Guizhou (Yang et al., 2001; Wang et al., 2005). Outside of China, these species were recognized in India (Goel, 1977), United States (Solien, 1979), Kashmir (Chhabra and Sahni, 1981), Vietnam (Thang, 1989), Oman (Orchard, 1995), Albania (Meço, 1999) and Romania (Orchard and Dinaru, 2007).

4. 6 Chiosella timorensis Zone

The Chiosella timorensis Zone begins with the FO of Chiosella timorensis and ends with the FO of Nicoraella germanica. This zone ranges from the upmost part of Bed 26 to the lower part of 9

ACCEPTED MANUSCRIPT Bed 27. Co-occurring conodonts are Novispathodus abruptus, Chiosella gondolelloides, Spathicuspus spathi and Cornudina sp. This zone was also reported at Guandao section (Wang et al.,

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2005; Orchard et al., 2007; Lehrmann et al., 2015).

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The zonal species was first identified by Nogami (1968) in Timor and then widely found in Canada (Orchard and Tozer, 1997), Romania (Grãdinaru et al., 2006; Orchard and Dinaru, 2007), Oman (Orchard, 1995), India (Krystyn et al., 2004) and United States (Goudemand et al., 2012).

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Meanwhile, this species was also recognized in South China (Yang et al., 2001; Wang et al., 2005;

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Orchard et al., 2007; Ji et al., 2011) and Tibet (Tian, 1982). The FO of Ch. timorensis at the Desli Caira (Romania) is exactly at the basal Anisian ammonoid zone and was suggested as the index

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4. 7 Nicoraella germanica Zone

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fossil of the Olenekian-Anisian boundary (Grãdinaru et al., 2006).

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The lower limit of this zone is defined by the FO of Nicoraella germanica and the upper limit is defined by the FO of Nic. kockeli, ranging from the middle part of Bed 27 to the lower part of Bed

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29. The fauna was dominated by the zonal species and associated with Novispathodus abruptus, Chiosella timorensis, Ch. gondolelloides, Neogondolella sp., Spathicuspus spathi and Cornudina sp. Nicoraella germanica was first identified by Kozur (1972) and reported in Germany, Poland (Zawidzka, 1975) and United States (Kozur, 1980). In China, the species is also known from the Leikoupo Formation in Sichuan (Wang and Dai, 1981) and Guizhou (Sun et al., 2006), the Guanling Formation in Yunnan (Zhang et al., 2009; Huang et al., 2011) and the Baifeng Formation in Guangxi (Yang et al., 1999).

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ACCEPTED MANUSCRIPT 4. 8 Nicoraella kockeli Zone

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The FO of Nicoraella kockeli defines the base of this zone in the lower part of Bed 29 and its

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upper limit is not defined because no key conodonts are found above Bed 29 at the section.

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Associated elements include Chiosella timorensis, Ch. gondolelloides, Gladigondolella tethydis and Nicoraella germanica. The conodonts became relative sparse in this interval probably because of

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the Tubiphytes–reef facies.

Nic. kockeli was first reported in Germany (Tatge, 1956; Kozur, 1972), and then in United States

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(Carey, 1984), Malaysia (Metcalfe, 1990), Austria (Kozur et al., 1994) and Poland (Narkiewicz and Szulc, 2004). In South China, this zone yields the Panxian Fauna in Guizhou (Sun et al., 2006) and

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the Luoping Biota in Yunnan (Zhang et al., 2009; Huang et al., 2011), indicating an age of Pelsonian (middle Anisian).

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In addition to Nicoraella, Paragondolella is among the representative of younger Anisian fauna (Orchard, 2007). The FO of Paragondolella bulgarica and Nicoraella germanica are

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contemporaneous in Poland (Narkiewicz and Szulc, 2004). Pa. bulgarica co-occurred with Nic. kockeli in the United States (Carey, 1984), Poland (Narkiewicz and Szulc, 2004) and South China (Sun et al., 2006). Thus, the Nic. kockeli Zone could correlate with the upper part of Pa. bulgarica Zone (Fig. 7).

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ACCEPTED MANUSCRIPT 5. Discussion

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5.1. Permian-Triassic boundary

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The FAD of Hindeodus parvus has been accepted as the definition of the Permian-Triassic

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boundary (Yin et al., 2001). The recent report of Zhang et al. (2014) that Hindeodus parvus co-exists with the Permian fusulinids is to be further studied (e.g. possible reworking of underlying

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rocks). A hiatus is proven between the lowermost Triassic microbialites and uppermost Permian

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skeleton limestones in the shallow facies on the GBG, such as the Dajiang, Langbai and Dawen sections (Payne et al., 2007; Collin et al., 2009; Jiang et al., 2014; Yin et al., 2014; Lehrmann et al.,

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2015), and regression (Wignall et al., 2009; Yin et al., 2014) and ocean acidification (Payne et al.,

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2007; Lehrmann et al., 2015) are believed to be the causes. Furthermore, an irregular truncation

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surface is also observed inside the Hindeodus changxingensis Zone at the Bianyang section in the marginal basin facies of the GBG (Yan et al., 2013), indicating that the sea-level falling and/or

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acidification influenced the deep-water area as well, though to a lesser extent. Consequently, it is understandable that the P-Tr boundary strata at the Zhongzhai section, located at the junction between marine and non-marine settings (Zhang et al., 2014), were influenced as well. At the Mingtang section, the FO of H. parvus is in the first limestone bed of the Luolou Formation and the underlying strata are mudstones among which no conodonts are retrieved. Thus, it is uncertain whether the FO of Hindeodus parvus represents the Permian-Triassic boundary at this section. In addition, the carbon isotopes show no significant shift like that at the Meishan section, the Global Stratotype Section and Point of P-Tr boundary (Yin et al., 2001). Consequently, the P-Tr boundary probably lies inside the mudstone member below the limestone of the Luolou Formation.

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ACCEPTED MANUSCRIPT The P-Tr boundary is also characterized by a mixed fauna of the Permian relicts and Triassic newcomers at the Meishan section (Sheng et al., 1984; Yin et al., 2001). Similar mixed fauna also

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exists in the mudstone member below the FO of Hindeodus parvus at the Mingtang section. The

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interval containing the mixed fauna ranges from Bed 3 to Bed 4, composed of yellow-grayish mudstone, at the Mingtang section, and it is characteristic of Permian brachiopods and Triassic bivalves. The lower part (Bed 3) is predominated by abundant Permian-type brachiopods, such as

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Paracrurithyris pigmaea, Paryphella orbicularis, P. triquetra, P. nasuta and P. transversa while

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the upper part (Bed 4) is by bivalve Claraia aurita and brachiopod Lingula sp., which were disaster taxa in the aftermath of the end-Permian mass extinction (Schubert and Bottjer; 1995; Rodland and

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Bottjer, 2001). More Triassic bivalves, such as Claraia griesbachia, C. concentrica, C. wangi, C.

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guizhouensis, Unionites fassaensis, occur in Beds 5 and 6. Consequently, the Permian-Triassic

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boundary is placed between Beds 3 and 4 at the Mingtang section. 5.2 Induan-Olenekian Boundary

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The FAD of Novispathodus waageni was proposed as the definition of the Induan-Olenekian boundary (IOB) at Chaohu by Tong et al. (2003) and it has been widely accepted with its global correlation (Wang et al., 2005; Krystyn et al., 2007; Metcalfe et al., 2013; Chen et al., 2015; Lehrmann et al., 2015). In addition, the peak of the large positive δ13C excursion is coincident with the IOB identified by the FO of Nv. waageni at the Mingtang section (Song H.Y. et al., 2013). Similar positive δ13C shifts were reported from the West Pingdingshan section in Anhui, Guandao and Jiarong sections in Guizhou (Payne et al., 2004; Zuo et al., 2006; Tong et al., 2007; Song HY et al., 2013; Chen et al., 2015), and the Zuodeng and Jinya sections in Guangxi, South China (Galfetti et al., 2007). Besides, the positive δ13C excursion around the IOB was reported at the Mud section 13

ACCEPTED MANUSCRIPT in India, the Bith Formation in the Arabian Platform (Clarkson et al., 2013), the L’Om Picol/Uomo section in Italy (Horacek et al., 2007a) and the Zal section in Iran (Horacek et al., 2007b). Among

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these sections, the FO of Nv. waageni is just slightly prior to the peak of the positive δ13C excursion

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(e.g. Tong et al., 2005, 2007). At the Mingtang section, the FO of Nv. waageni is 2.5 m below the peak of positive δ13C excursions. Consequently, the Induan-Olenekian boundary can be well defined at the Mingtang section according to the FO of Nv. waggeni and the positive excursion of carbon

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isotopes (Fig. 3).

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The FO of Novispathodus pingdingshanensis was used as the index fossil of the Smithian-Spathian boundary (SSB) according to a high-resolution conodont study in Chaohu (Liang

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et al., 2011) and Jiarong (Chen et al., 2013). However, no Nv. pingdingshanensis has been found at

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the Mingtang section below the FO of Triassospathodus homeri, a typical late Spathian conodont

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element, in Bed 18. However, the SSB is accompanied by a sharp positive shift of δ13C, indicating that the δ13C excursion can be a key reference for the SSB (Liang et al., 2011). Therefore, the

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significant positive shift of δ13C at the lower part of Bed 16 marks the SSB at the Mingtang section.

5.3 Early-Middle Triassic Boundary

Agreement on placing the Olenekian-Anisian Boundary (OAB) at the FAD of Chiosella timorensis was reached by the International Commission on Stratigraphy in 2002 (Kozur, 2003), and it has been widely used in Canada (Orchard and Tozer, 1997), South China (Wang et al., 2005; Lehrmann et al., 2006; Ji et al., 2011; Yao et al., 2011; Fig. 7) and Romania (Orchard and Dinaru, 2007). At the Mingtang section, Ch. timorensis first occurred in the top of Bed 26 in the Luolou Formation. Thus, we place the base of the Middle Triassic at this level.

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ACCEPTED MANUSCRIPT Nicoraella germanica is a species which characterizes the Bithynian in Europe and it occurs in the Silberlingites mulleri Zone in Nevada, United States (Kozur, 1980; Orchard and Tozer, 1997). In

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South China, this species was taken as the marker of the substage boundary between Aegean and

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Bithynian in the lower Anisian at the Guandao section (Lehrmann et al., 2006; Fig. 8). The FO of Nic. germanica at the Mingtang section is in the lower part of Bed 27, marking the base of the Bithynian Substage.

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The FAD of Nicoraella kockeli was used to mark the base of the Pelsonian Substage in the German

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Basin (Kozur, 1972). The usage of Nic. kockeli as the indicator of the base of Pelsonian is supported by its wide distribution in Europe (Kozur, 1972, 1980; Meço, 1999; Narkiewicz, 2013) and South

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China (Wang et al., 2005; Sun et al., 2006; Zhang et al., 2009b). Therefore, we use the FO of Nic.

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kockeli in the lower part of Bed 29 to define the Bithynian-Pelsonian boundary at the Mingtang

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section.

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5.4 Timing of the recurrence of Tubiphytes-reefs

The metazoan-reef gap after the Permian–Triassic crisis comprises the complete Early Triassic and the early Anisian, a time interval of at least 5 Myr (Chen and Benton, 2012; Flügel, 2002). The Tubiphytes boundstone is one of the oldest reef complexes in the Triassic (Flügel, 2002; Payne et al., 2006b). This type of bio-reefs was reported in Italy (Gaetani and Gorza, 1989; Harris, 1993), Romania (Popa et al., 2014) and Hungary (Velledits et al., 2011), as well as South China (Enos et al., 1997). The Tubiphytes boundstones and grainstones are recognized at the margin of the GBG in South China (Enos et al., 2006; Payne et al., 2006a). The reef complex that developed on the margin of the GBG was approximate 1 km long and 800 m thick, and the age of the reef was deduced based

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ACCEPTED MANUSCRIPT on the occurrence of Tubiphytes grains at the basin-marginal Guandao section (Payne et al., 2006a). At the Mingtang section, the typical Tubiphytes-enriched grainstone of a reef complex initially

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occurred 5.3 m above the Early-Middle Triassic boundary in the conodont Nicoraella germanica

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Zone, indicating that the first occurrence of the Tubiphytes-reef at the GBG was of a Bithynian age (Anisian).

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6. Conclusions

A systematic study of the conodont succession at the Mingtang section provides more

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information for the time scale of the platform-marginal setting of the GBG. Eight conodont zones

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are established from the Early Triassic to the early Middle Triassic. These zones are correlated

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well with the basin-marginal sections of the GBG, i.e. Guandao and Bianyang sections. The Induan-Olenekian boundary was placed at the lower part of the Luolou Formation, 13.4 m above

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the base of Bed 10, based on the first occurrence of Novispathodus waageni. The FO of Chiosella timorensis, at 0.2 m below the top of the Luolou Formation, defines the Early-Middle Triassic

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boundary at the Mingtang section. The Tubiphytes-reefs, occurring 5.3 m above the OAB at the Mingtang section on the margin of the GBG, belong to the Bithynian (Anisian) in age.

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ACCEPTED MANUSCRIPT Acknowledgements We thank Dongdong Li, Xincheng Qiu and Kui Wu for help in the processing of conodonts,

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Wenting Ji and Hao Yang for photographing, as well as Hui Shi and Hongbing Fu for their field

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assistance. We are also grateful to Laishi Zhao and Haishui Jiang for their help in identification of conodonts, and to Roger Pierson and Guangrong Shi (Deakin University, Australia) for improving the English expression. This research is supported by the National Nature Science Foundation of

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China (Nos. 41530104, 41272372, 41302271 and 41402302), the Fundamental Research Funds for

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the Central University (CUG 160842) and the ―111‖ project (B08030). This is a contribution to the IGCP-630.

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Fig. 1. A. The Early Triassic paleogeographic map of South China (Modified from Lehrmann et al.,

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2006); B. Location map of the Mingtang section; C. Geological map of the studing area. The red star indicates the Mingtang section while the black dots show the sections mentioned in the paper.

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Fig. 2. Outcrop photographs from the Mingtang section. A. The panorama of the Mingtang section, B. Banded chert and claystone in the uppermost Dalong Formation, C. Thin bedded limestone in

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Fig. 3. Lower-Middle Triassic stratigraphical sequence of the Mingtang section. UP. Upper Permian, CX. Changhsingian, D. Dalong Formation, Aeg. Aegean, Bith. Bithynian.

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Fig. 4. SEM photographs of the key conodont elements from the Mingtang section. Scale bar = 200 μm. 1-5, 23 Neospathodus dieneri Sweet: 1,5 Morphotype 3, lateral view, 1 from Bed 10, 5 from Bed 18, 2-4 Morphotype 2, 2a lateral view, 2b upper view, from Bed 16, 3, 4 lateral view, from Bed 10, 23 Morphotype 1, lateral view, from Bed 10; 6-7, 12, 17 Neospathodus clinatus Orchard and Sweet: lateral view, 6, 17, 20 from Bed 26, 12 from Bed 18; 8 Novispathodus waageni waageni (Sweet): lateral view, from bed 10; 9-10 Spathicuspus spathi (Sweet): lateral view, from Bed 26; 11 Spathicuspus sp.: lateral view, from Bed 17; 13 Novispathodus abruptus (Orchard): lateral view, from Bed 25; 14-16 Neospathodus triangularis (Bender): 14, 15a, 16a lateral view, 15b, 16b upper view, from Bed 26; 18 Hindeodus anterodentatus (Dai, Tian and Zhang): lateral 43

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lateral view, from Bed 29; 33 Nicoraella germanica (Kozur): lateral view, from Bed 29; 34

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Fig. 5. SEM photographs of the key conodont elements from the Mingtang section. Scale bar = 200 μm. 1, 3-6, 10 Triassospathodus symmetricus (Orchard): 1a, 3a, 4a, 5a, 6a, 10 lateral view, 1b, 3b,

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(Orchard): 7a, 13a lateral view, 7b, 13b upper view, from Bed 26; 8, 11, 12, 15 Triassospathodus homeri (Bender): 8 lateral view, from Bed 18, 11a, 12a lateral view, 11b, 12b upper view, from Bed

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23, 15a lateral view, 15b upper view, from Bed 26. Fig. 6. SEM photographs of the key conodont elements from the Mingtang section. Scale bar = 200 μm. 1-4, 6-9 Nicoraella germanica (Kozur): lateral view, from Bed 29; 5 Pachycladina obliqua Staesche: lateral view, from Bed 14; 10, 17-19 Chiosella gondolelloides (Bender): lateral view, 10, 18 from Bed 27, 17, 19 from 26; 11 Neospathodus linearis Zhao and Orchard: lateral view, from Bed 18; 12 Nicoraella sp.: lateral view, from Bed 29; 13 Triassospathodus sosioensis (Kozur, Krainer & Mosher): lateral view, from Bed 26; 14 Neogondolella sp.: upper view, from Bed 27; 15 Triassospathodus symmetricus (Orchard): 15a upper view, 15b lateral view, from Bed 29; 16, 20-22 Chiosella timorensis (Nogami): 16, lateral view, from Bed 26, 20 lateral view, from Bed 27, 44

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Fig.7 Correlation of the Early-Middle Triassic conodont zones of Mingtang section with over the

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world. For the substages, A. Aegean, B. Bithynian, P. Pelsonian; for the generic names, H. Hindeodus, Bo. Borinella, Ch. Chiosella, C. Clarkina, Co. Conservatella, Ds. Discretella, Ic. Icriospathodus, I. Isarcicella, Ng. Neogondolella, Nic. Nicoraella, Ns. Neospathodus, Nv. P.

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from Lehrmann et al. (2015) and those of the Bianyang section are from Yan et al. (2013). A. Aegean, B. Bithynian, CX Changhsingian, Dien. Dienerian, UP Upper Permian, Ch. Chiosella, D.

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Discretella, H. Hindeodus, Ic. Icriospathodus, Nc. Neoclarkina, Nic. Nicoraella, Ns. Neospathodus, Pa. Pachycladina, Pc. Parachirognathus, Sw. Sweetospathodus, Tr. Triassospathodus. See Fig. 3 for other lithologic legends.

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Eight conodont zones in Early-Middle Triassic are identified at platform margin

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Tubiphytes-reef re-occurred in Nicoraella germanica zone of Bithynian (Anisian)

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