Changhsingian conodont succession and the end-Permian mass extinction event at the Daijiagou section in Chongqing, Southwest China

Journal of Asian Earth Sciences 105 (2015) 234–251

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Changhsingian conodont succession and the end-Permian mass extinction event at the Daijiagou section in Chongqing, Southwest China Dong-xun Yuan a,b, Jun Chen c, Yi-chun Zhang b, Quan-feng Zheng b, Shu-zhong Shen b,⇑ a

School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China c State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China b

a r t i c l e

i n f o

Article history: Received 4 October 2014 Received in revised form 5 March 2015 Accepted 1 April 2015 Available online 6 April 2015 Keywords: Conodonts Changhsingian End-Permian mass extinction Permian–Triassic boundary South China

a b s t r a c t Previous studies suggested rapid evolution of conodonts across the Permian–Triassic boundary (PTB), and the end-Permian mass extinction pattern varies in different sections in South China. Here we document a high-resolution conodont succession from a carbonate facies of the Changhsingian Stage and across the PTB at the Daijiagou section, about 35 km north to Chongqing City, Southwest China. Two genera and twelve species are identified. Seven conodont zones are recognized from the uppermost part of the Lungtan Formation to the lowest Feixianguan Formation. They are the Clarkina liangshanensis, C. wangi, C. subcarinata, C. changxingensis, C. yini, C. meishanensis, and Hindeodus parvus zones in ascending order. Based on the high-resolution biostratigraphical framework at Daijiagou, the end-Permian mass extinction was rapid and it began in the base of the Clarkina meishanensis Zone. Associated with the extinction, a negative excursion of d13Ccarb started in the middle part of Clarkina yini Zone with a progressive shift of 1.6‰ to the middle part of the Clarkina meishanensis, followed by a sharp shift of 3.51‰ from the Clarkina meishanensis Zone to the Hindeodus parvus Zone. Our study also suggests that the Triassic index species Hindeodus parvus co-occurred with Hindeodus changxingensis and Clarkina zhejiangensis and directly overlies the Clarkina meishanensis Zone at the Daijiagou section. All these data from the Daijiagou section and some previous studies of other sections in Sichuan, Guizhou provinces and Chongqing City suggest that the first occurrences of Hindeodus parvus are slightly earlier than the sharp negative excursion of d13Ccarb and the FAD at the Meishan GSSP section. We consider that the slight difference of the end-Permian mass extinction, chemostratigraphy and conodont biostratigraphy at Daijiagou and its adjacent areas is most likely subject to different lithofacies, fossil preservation, and the constraint on the stratigraphic resolution rather than a different tempo of the end-Permian mass extinction in a global sense. The whole Changhsingian conodont succession at Daijiagou provides a high-resolution correlation with other equivalent sections in the Palaeotethys. The controversial results of biostratigraphy and chemostratigraphy between the sections investigated in this paper and the Meishan GSSP section also provide some important implications that accurate chronocorrelation requires the evaluation of multiple, varied stratigraphcal signals rather than relying solely on the FAD of the Triassic index species Hindeodus parvus for recognizing the Permian–Triassic boundary (PTB). Growth series of abundant specimens for each species are figured. The taxonomy of some Hindeodus species in the PTB interval is updated. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The end-Permian mass extinction has been widely documented as the largest in life’s history (Sepkoski, 1981, 1984; Erwin, 2006; Bambach, 2006). Many fossil groups such as fusulinids, rugose

⇑ Corresponding author. Tel./fax: +86 25 83282131. E-mail address: [email protected] (S.-z. Shen). http://dx.doi.org/10.1016/j.jseaes.2015.04.002 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

corals, and brachiopods suffered great losses or became extinct around the PTB. By contrast, conodonts were relatively less affected in terms of diversity, but responded in high-speed evolution under environmental stress in terms of durations of conodont zones. Therefore, high-resolution conodont zones have been recognized around the PTB in some key sections in South China (Ji et al., 2007; Metcalfe and Nicoll, 2007; Zhang et al., 2007; Jiang et al., 2007, 2011, 2014; Chen et al., 2009a; Shen et al., 2011; Yuan and

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Shen, 2011; Yuan et al., 2014) and they have been widely used for calibration of the end-Permian mass extinction and correlation for the PTB interval (Metcalfe and Nicoll, 2007; Shen et al., 2011; Wang et al., 2014). Although a high-resolution conodont-based biostratigraphical framework and a rapid end-Permian mass extinction have been well documented, the details of the conodont zones, identification of conodont species, and the extinction patterns remain controversial (Wang et al., 2014; Yuan et al., 2014). For instance, the conodont index species, Hindeodus parvus, that was defined as the base of the Triassic (Yin et al., 2001), has been documented to occur below the PTB at the Zhongzhai section in Guizhou Province based on the last occurrence of the fusulinid Palaeofusulina sinensis and numerous Permian brachiopods (Zhang et al., 2014), highprecision radiometric dates, and d13Ccarb excursions (Shen et al., 2011). Various conodont species including the Lower Triassic Clarkina carinata, the early Changhsingian C. subcarinata etc. are identified from the uppermost Changhsingian in different sections or the same sections by different researchers (Ji et al., 2007; Jiang et al., 2011; Yuan et al., 2014). The end-Permian mass extinction has been calibrated within 61 ± 48 kyr as a single catastrophic extinction (Burgess et al., 2014), but others suggested the extinction consists of two phases respectively at Bed 25 and Bed 28 at the Meishan section (Song et al., 2013) and many different horizons in South China (see Wang et al., 2014, Fig. 8). In addition, the magnitude of the d13Ccarb excursion is also variable although the excursion itself has been recognized globally (Cao et al., 2010; Korte and Kozur, 2010; Shen et al., 2013). Such variations and discrepancies are likely derived from different lithofacies in different sedimentary settings or sampling biases in South China. Thus, studying the PTB sections in different lithofacies is very important to unravel the spatial and temporal patterns of the end-Permian mass extinction. South China possesses numerous complete marine sequences from the uppermost Permian to lowest Triassic. They include sections with shallow (e.g., the Dukou section in Sichuan Province, the Huangzishan section in Zhejiang Province), moderately deep (e.g., the Meishan sections in Zhejiang Province), deep water (e.g., the Shangsi section in Sichuan and the Majiashan section in Anhui Province), shallow marine carbonate platform facies (e.g., the Dajiang and Dawen sections in Guizhou Province), and reefal carbonate facies (e.g., the Laolongdong section in Chongqing City) in the late Changhsingian. In addition, a great number of Permian– Triassic transitional sections have been documented from South China. Most of the previous conodont studies in South China focused on the PTB interval (Zhang, 1987; Wang, 1994, 1995; Orchard et al., 1994; Lai et al., 1999, 2001; Nicoll et al., 2002; Xia et al., 2004; Wang and Xia, 2004; Luo et al., 2006, 2008; Zhang et al., 2007; Ji et al., 2007; Jiang et al., 2007, 2011, 2014; Metcalfe and Nicoll, 2007; Metcalfe et al., 2007; Chen et al., 2008, 2009a; Yuan and Shen, 2011) except for the Meishan section (Zhao et al., 1981; Mei et al., 1998; Zhang et al., 2009; Yuan et al., 2014) and the Shangsi section (Li et al., 1989; Shen et al., 2011). Only a few other sections have been investigated in detail for conodonts through the whole Changhsingian (e.g., Tian, 1993a, 1993b; Nafi et al., 2006). In this paper, we report and discuss our detailed study on the conodont biostratigraphy, extinction pattern, and carbon isotope profile on the basis of the Daijiagou section in northern Chongqing City and provide a comparison with other key sections in terms of conodont zonation and the end-Permian mass extinction pattern. This section represents a carbonate facies in the Changhsingian with siliciclastic facies on the topmost 1 m overlain

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by marls containing abundant conodonts, some surviving brachiopods, and Triassic bivalves with a few volcanic ash interbeds around the PTB. 2. Geological setting The Daijiagou section is located at Daijiagou Village, Beibei District, Chongqing City, near a mine mining the coal from the Lungtan Formation. It is approximately 35 km north of Chongqing (Fig. 1). Geologically, it belongs to the southern part of the Huaying Mountains anticlinorium of eastern Sichuan complex fold belt on the northern shelf of the South China Block (Zeng et al., 2008; Mu et al., 2009). The Permian sequence at the section is well exposed and composed of the Guadalupian massive carbonate Maokou Formation, the Wuchiapingian coal-bearing clastic Lungtan Formation, the Changhsingian carbonate Changhsing Formation, and the Induan Feixianguan Formation. Brachiopods are extremely abundant in the Lungtan and Changhsing formations (Shen et al., 1995; Zeng et al., 1995; Chen et al., 2005; Shen and Shi, 2007). In this study, we measured and sampled the sequence for conodont and carbon isotope studies from the Changhsing to Feixianguan formations. The PTB interval has been studied in great detail. Several other PTB sections close to the Daijiagou section have been previously studied by other colleagues (Fig. 1). The Laolongdong section is about 2 km southwest to the Daijiagou section and its PTB interval is marked by a thick microbiolite unit directly overlying the Changhsingian reefal facies (Reinhardt, 1988; Fan et al., 1996; Wignall and Hallam, 1996; Kershaw et al., 1999, 2002; Wu et al., 2006; Liu et al., 2006; Qi and Liao, 2007; Jiang et al., 2010), which is not developed at the Daijiagou section. Some conodonts and a d13Ccarb profile were reported by Mu et al. (2009) and Qi and Liao (2007), but in relatively low resolution. Kershaw et al. (2002) reported Hindeodus parvus from the reefal carbonates at the Baizhuyuan section, which is about 50 km away from the Daijiagou section, but the precise PTB is not clear at that section. A few well exposed PTB sections in the suburb of Chongqing City are similar to the Daijiagou section in terms of the lithofacies around the PTB. Detailed studies of brachiopods from the Changhsing Formation at the Beifengjing and Liangfengya sections (Yang et al., 1987; Shen and He, 1991; Shen et al., 1995; Shen and Shi, 2007; Clapham et al., 2013) and conodonts from the PTB interval at the Liangfengya section have been presented (Yuan and Shen, 2011), and an extinction pattern was briefly discussed by Wignall and Hallam (1996) and Shen and Shi (2002). All those sections together provide important data for studying the changeover of faunas and the extinction patterns in southwestern China. 3. Conodont zonation Fifty-five samples were collected from the Changhsing Formation and the lowest part of the Feixianguan Formation at the Daijiagou section and 41 samples are productive in conodonts (Fig. 2). Samples in the overlying horizons of the Feixianguan Formation were collected, but do not contain conodonts. Based on sample-population approach (see discussions by Mei et al., 2004; Shen and Mei, 2010), two genera (Clarkina and Hindeodus) with twelve species (Clarkina liangshanensis, C. subcarinata, C. changxingensis, C. yini, C. meishanensis, C. zhejiangensis, Hindeodus typicalis, H. eurypyge, H. changxingensis, H. praeparvus, H. parvus and H. sp.) and three transitional forms are recognized. Seven conodont zones are established from the upper part of the Lungtan

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Fig. 1. Maps showing the location and stratigraphy of the Daijiagou section in Chongqing, southwestern China.

Formation to the lowest part of the Feixianguan Formation. They are, in ascending order, the Clarkina liangshanensis Zone in the upper Lungtan Formation, the C. wangi, C. subcarinata, C. changxingensis, C. yini zones in the Changhsing Formation, and the C. meishanensis and Hindeodus parvus zones in the lowest Feixianguan Formation (Fig. 2).

3.1. Clarkina liangshanensis Zone This zone is recognized by the presence of the named species at the upper part of the Lungtan Formation (Plate 1, Figs. 1–5). Because only several carbonate interlayers were collected in this formation and the upper part of the Lungtan Formation is partly covered, the base and top boundary of this zone is unclear at the Daijiagou section. This zone has been widely reported in the middle part of the Lungtan and Wuchiaping formations in South

China (Mei et al., 1994; Mei and Wardlaw, 1996; Shen and Mei, 2010). 3.2. Clarkina wangi Zone This zone is named based on some samples containing specimens with transitional characters from Clarkina wangi to Clarkina subcarinata. Its base is unclear because typical Clarkina wangi is not found from the underlying horizon. The top of the zone is defined by the first occurrence of C. subcarinata from Sample DJG-85 m (Fig. 2). Hindeodus typicalis is also present in this zone. This zone belongs to the upper part of Clarkina wangi Zone of Jin et al. (2006), and the C. wangi Zone has been reported from the base of Talung and Changhsing formations at Shangsi and Meishan sections (Shen et al., 2011). However, the C. wangi Zone is absent in some sections (e.g., Penglaitan, Laibin in Guangxi Province) in some regions of South China (Shen et al., 2007).

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Fig. 2. Stratigraphic column of the Daijiagou section showing samples, conodont species ranges, and zonation.

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Plate 1. (1–5) Clarkina liangshanensis (Wang, 1978). All from Sample DJG018, registration nos. NIGP161925–161929. (6–16) Clarkina wangi transitional to C. subcarinata, all from Sample DJG-88.2 m, registration nos. NIGP161930–161940.

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3.3. Clarkina subcarinata Zone This zone is defined by the first occurrence of Clarkina subcarinata from Sample DJG-85 m at the base and the first occurrence of C. changxingensis from Sample DJG015 (Fig. 2). The precise boundary between the C. subcarinata and C. changxingensis zones is difficult to determine because Clarkina specimens are rare in the uppermost part of the C. subcarinata Zone. Some individuals at the top of the zone have transitional characters from C. subcarinata to C. changxingensis. This zone has been widely reported in South China and Iran (Shen and Mei, 2010). Some previous reports of this species (e.g., Jiang et al., 2007) from the uppermost part of the Changhsingian are questionable based on our taxonomic appraisal.

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et al., 2011; Zhang et al., 2014). This zone is recognized by the first occurrence of H. parvus (Plate 5, Figs. 25–30) from Sample DJG0.58–0.73 m at the base. The upper boundary of the zone is unclear because more than ten samples above the sample DJG1.40–1.50 m do not contain conodonts at the Daijiagou section. Clarkina zhejiangensis, Hindeodus typicalis, H. changxingensis, and H. praeparvus coexist with H. parvus in the lower part of this zone. H. parvus is rare at the basal part of this zone, but H. praeparvus and H. changxingensis are very abundant. Some Hindeodus sp. (Plate 6, Figs. 11–28) specimens are also found from the lower part of this zone.

4. Lithostratigraphy, biostratigraphy, correlation of conodont zones and the Permian–Triassic boundary

3.4. Clarkina changxingensis Zone This zone is defined by the first occurrence of Clarkina changxingensis from Sample DJG015 (Plate 2, Figs. 12–20) at the base and the first occurrence of C. yini from Sample DJG013 at the top (Fig. 2). It ranges about half of the Changhsing Formation in terms of rock thickness because sedimentation rate is relatively high at the Daijiagou section. Some individuals at the upper part of the zone have transitional characters from C. changxingensis to C. yini. Hindeodus typicalis is present in the entire zone. H. eurypyge are rare in the topmost part of this zone. This zone has been widely reported in the Palaeotethys and it is an important mark for the middle Changhsingian (Yuan et al., 2014). 3.5. Clarkina yini Zone This zone is defined by the first occurrence of Clarkina yini from Sample DJG013 at the base and the first occurrence of C. meishanensis from Sample DJG005 at the top. The thickness (9 m) of this zone is larger than that at the Meishan section (Fig. 2). Therefore, the Daijiagou section is slightly more expanded in the upper part of the Changhsingian. A few specimens of C. ?meishanensis or C. changxingensis transitional to C. meishanensis are found in the middle part of this zone. Hindeodus typicalis (Plate 3, Figs. 21–25) and H. eurypyge (Plate 3, Figs. 29–31) are associated with Clarkina yini. This zone belongs to the upper part of Changhsingian. The middle part of this zone is associated with the onset of the gradual decline of the d13Ccarb excursion at Meishan, which probably indicates environmental deterioration before the end-Permian mass extinction. 3.6. Clarkina meishanensis Zone This zone is defined by the first occurrence of Clarkina meishanensis (Plate 4, Figs. 1–10) from Sample DJG005 at the base and the first occurrence of Hindeodus parvus from Sample DJG0.58– 0.73 m at the top (Fig. 2). The top of this zone is the horizon with a sharp changeover from Clarkina-dominated to Hindeodusdominated population of conodonts (Fig. 3). A few specimens of Hindeodus eurypyge also occur in the Clarkina meishanensis Zone. This zone represents the topmost part of the Changhsingian Stage. It has a very short range and can be used for precise calibration of the end-Permian mass extinction interval. 3.7. Hindeodus parvus Zone The FAD of Hindeodus parvus at the Meishan section D has been defined as the PTB (Yin et al., 2001). However, previous studies suggested that this species may be slightly diachronous around PTB (Baud, 1996; Shevyrev, 2006; Hermann et al., 2010; Jiang

The PTB interval is well-exposed at the Daijiagou section (Fig. 3). Almost the whole Changhsing Formation consists of medium to thick-bedded packstone in the lower part and massive grainstone in the middle and upper parts with numerous brachiopods and calcareous sponges, bryozoans and algae of reefal facies which is very close to that at the nearby Laolongdong section (Qiang et al., 1985). The Changhsing Formation is topped with 1 m siliceous mudstone containing highly diverse Permian brachiopods and bryozoans and rare trilobites. The boundary (0 m in the stratigraphic column, see Fig. 2) between the Feixianguan and Changhsing formations is marked by a 5-cm ash layer. The lowest one meter of the Feixianguan Formation consists of wackstone or marls with a few clay interbeds. Conodonts are very abundant around the PTB. Hindeodus parvus begins to occur at Sample DJG0.58–0.73 m, which usually indicates the PTB level at Daijiagou (Fig. 3). However, three other species including Clarkina zhejiangensis, Hindeodus changxingensis, and H. praeparvus also begin to occur in this sample, which is immediately overlying the Clarkina meishanensis Zone. All those three species occur slightly below the Hindeodus parvus Zone at Meishan and many other sections (Jiang et al., 2007; Metcalfe et al., 2007; Yuan et al., 2014). Thus, the first occurrence of H. parvus from the sample DJG0.58–0.73 m is likely an earlier occurrence than the FAD of H. parvus at Meishan (see more discussion below), which is similar to that at the Zhongzhai section in Guizhou Province (Zhang et al., 2014). In fact, a specimen similar to H. parvus was also reported from Bed 27b below the PTB at the Meishan section (Jiang et al., 2007) although the species assignment of the specimen may need to be re-considered based on a population approach. The overlying sample (Sample DJG0.86–1.00 m) contains H. parvus only, though not abundant. Therefore, we consider that this sample is biostratigraphically in the H. parvus Zone. Conodont zonation throughout the Changhsingian and across the PTB has been studied from the Meishan (Zhao et al., 1981; Mei et al., 1998; Zhang et al., 2009; Yuan et al., 2014), Shangsi (Li et al., 1989; Shen et al., 2011; Jiang et al., 2011), Ganxi sections (Nafi et al., 2006), Rencunping and Cili in northwestern Hunan Province (Tian, 1993a, 1993b), and Zhongzhai in Guizhou Province (Metcalfe and Nicoll, 2007; Metcalfe et al., 2007; Zhang et al., 2014). These sections are located in different regions on the Yangtze platform. The conodont zonation previously documented is generally comparable with our studies from the Daijiagou section, but a considerable revision on their taxonomy has been done by Yuan et al. (2014). Fig. 4 shows the correlation between the Daijiagou section and other sections after the Clarkina taxonomy has been updated. Notably, the critical Clarkina zhejiangensis–Hindeodus changxingensis Zone indicating the onset of the end-Permian mass extinction (Metcalfe et al., 2007) is overlapped with the Hindeodus parvus Zone (Plate 5, Figs. 25–30), which begins to occur in the same level with the zonal

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Plate 2. (1–11) Clarkina subcarinata (Sweet in Teichert et al., 1973). All from Sample DJG-82 m, registration nos. NIGP161941–151951. (12–20) Clarkina changxingensis (Wang and Wang in Zhao et al., 1981). All from Sample DJG015, registration nos. NIGP161952–161960.

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Plate 3. (1–9) Clarkina yini Mei in Mei et al., 1998. All from Sample DJG012, registration nos. NIGP161961–161969. (10–15) Clarkina zhejiangensis Mei, 1996. All from Sample DJG0.58–0.73 m, registration nos. NIGP161970–161975. (16–27) Hindeodis typicalis (Sweet, 1970). (16–19) from Sample DJG-60.9 m, registration nos. NIGP161976–161979; 20, from Sample DJG-80 m, registration no. NIGP161980; 21, 24, from Sample DJG-5.0 m, registration nos. NIGP161981, 161982; 22, 25, from Sample DJG-1.8 m, registration nos. NIGP161983, 161984; 23, 27, from Sample DJG-6.0 m, registration nos. NIGP161985, 161986; 26, from Sample DJG-85 m, registration no. NIGP161987. 28. Hindeodus eurypyge Nicoll et al., 2002. from Sample DJG-5.0 m, registration no. NIGP161988. (29–31) Hindeodus eurypyge Nicoll et al., 2002. 29, 31, from Sample DJG-5.0 m, registration nos. NIGP161989, 161990; 30, from Sample DJG-1.8 m, registration no. NIGP161991.

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Plate 4. (1–10) Clarkina meishanensis (Zhang et al., 1995). All from Sample DJG005, registration nos. NIGP161992–162001.

species at Daijiagou. Therefore, the first occurrences of H. parvus and their stratigraphic relationship with H. changxingensis and Clarkina zhejiangensis are temporally variable in view of high

resolution timescale due to lithofacies changes and fossil preservation. This also explains the record that the first occurrence of Hindeodus parvus is apparently above the PTB at the Shangsi

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Fig. 3. Photo showing lithology and conodont biostratigraphy across the PTB interval at the Daijiagou section. A: Clarkina meishanensis, B: C. zhejiangensis, C, F, H: Hindeodus parvus, D: H. praeparvus, E: H. changxingensis, G: Hindeodus sp. As shown in this figure, a distinct changeover from Clarkina-dominated to Hindeodus-dominated population of conodonts occurs in the lower part of the extinction interval and Hindeodus species clearly have a Lilliput Effect through the extinction interval.

section in Sichuan Province (Li et al., 1989; Jiang et al., 2011; Shen et al., 2011). Based on our updated taxonomy from Tian (1993a, 1993b), four conodont zones (Clarkina wangi, C. subcarinata, C. changxingensis, and C. yini zones in ascending order) in Changhsingian can be recognized in the northwestern Hunan Province. Clarkina meishanensis and Clarkina zhejiangensis–Hindeodus changxingensis zones are not recognized in this area due to coarse samples. Similarly, five conodont zones can be recognized from the Ganxi section, among which the C. subcarinata Zone can be subdivided from the C. wangi Zone based on the specimens illustrated by Nafi et al. (2006). Conodont zones near the PTB at the Ganxi section are not available because they did not sample the interval above the C. meishanensis meishanensis Zone (Fig. 4).

5. Carbon isotope It has been widely documented that the PTB interval recorded a major negative excursions of both d13Ccarb and d13Corg (see a review by Korte and Kozur, 2010; Shen et al., 2013). This negative shift began at about 60 kyr below Bed 25 in the Clarkina yini Zone at the Meishan sections with a progressive depletion of 2‰, which is followed by an abrupt 5‰ decline in the Clarkina meishanensis Zone marking the onset of the extinction at the top of Bed 24e (Burgess et al., 2014; Yuan et al., 2014). In order to configure the carbon isotope excursion, 41 whole rock samples were collected from the Daijiagou section and were analyzed in the isotope laboratory of the Nanjing Institute of Geology and Palaeontology with standard methodology (see Supplemental data). The results show that d13Ccarb values in the most part of Changhsingian are stable on the average +3.34‰, which are consistent with the background values in the Changhsingian of many other sections. The d13Ccarb values began to decline progressively from 2.937‰ at about 13 m below the top of the Changhsing Formation in the middle part of the Clarkina yini Zone to 1.316‰ at 0.15 m above the top of the Changhsing Formation, and followed by a sharp negative shift to

the lowest value 2.197‰ at 1.5 m above the base of the Feixianguan Formation. A recovery has been incompletely detected after the most negative value at +1.5 m, but the overlying strata do not contain sufficient carbonate for isotopic analysis (Fig. 5). This d13Ccarb profile is highly consistent with that documented from the Meishan section, where the most negative value is at the top of Bed 24e (Cao et al., 2009; Shen et al., 2013) (Fig. 5A and D). This result shows that the biostratigraphy and chemostratigraphy are not exactly consistent in high-resolution samples between the Daijiagou and Meishan sections because the lowest d13Ccarb value at Dajiagou is about 1.0 m above the first occurrence of the index species Hindeodus parvus. By contrast, the lowest value is at about 0.17 m below the FAD of H. parvus at the Meishan GSSP section (Fig. 5). At Laolongdong nearby the Daijiagou section, the most negative value is in the middle-upper part of the microbialite unit, a facies that results from the extinction (Kershaw et al., 2002; Liu et al., 2006; Qi and Liao, 2007; Mu et al., 2009), which is slightly above the first occurrence of the index conodont H. parvus (Fig. 5B). This most negative value is in the middle part of the microbialite unit, which is above the first occurrence of H. parvus at Dajiang and Dawen in the Guandao area of Guizhou Province (Payne et al., 2004, 2010; Chen et al., 2009a) (Fig. 5C). Thus, the precise relationship between first occurrences of H. parvus and the most negative excursion of d13Ccarb varies among different sections. This may be interpreted as that either the first occurrence of H. parvus is diachronous or d13Ccarb excursion based on whole carbonate rock suffered from subsequent diagenesis, both could cause a slight difference in recognizing the PTB and extinction interval in different sections. This inconsistency between biostratigraphy and chemostratigraphy at Daijiagou together with the Zhongzhai section recently reported by Zhang et al. (2014) raises some common questions for biostratigraphy. At what degree can we use a defined GSSP for correlation? And which standard is better than the other? Specifically, the sharp negative shift of d13Ccarb at Meishan has been widely documented in numerous sections all over the world, so should be a global signal and represents an isochronous marker in the extinction interval slightly below the PTB (see a review

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Plate 5. 1, 3. Hindeodus eurypyge Nicoll et al., 2002. 1, from Sample DJG0.45–0.50 m, registration no. NIGP162002; 3, from Sample DJG0.15–0.20 m, registration no. NIGP162003. 2. Hindeodus eurypyge Nicoll et al., 2002. from Sample DJG0.45–0.50 m, registration no. NIGP162004. (4–15) Hindeodus changxingensis Wang, 1995. All from Sample DJG0.58–0.73 m, registration nos. NIGP162005–162016. (16–24) Hindeodus praepavus (Kozur, 1996). All from Sample DJG0.58–0.73 m, registration nos. NIGP162017– 162025. (25–30) Hindeodus parvus (Kozur and Pjatakova in Kozur, 1975). All from Sample DJG0.58–0.73 m, registration nos. NIGP162026–162030.

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Plate 6. (1–10) Hindeodus parvus (Kozur and Pjatakova in Kozur, 1975). All from Sample DJG1.30–1.34 m, registration nos. NIGP162031–162040. (11–35) Hindeodus sp. 11– 28, from Sample DJG1.05–1.10 m, registration nos. NIGP162041–162058; 29–35, from Sample DJG1.40–1.50 m, registration nos. NIGP162059–162065.

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Fig. 4. Correlation of conodont zones among the Daijiagou, Meishan, Ganxi sections, and the sections in northwestern Hunan Province. Conodont specimens are the most common centromorphotypes from the corresponding horizons at the sections, but variations of different specimens in the same and different horizons in different sections are also present.

by Korte and Kozur, 2010; Shen et al., 2013). If we use this standard, then the first occurrences of the Triassic index species Hindeodus parvus vary in the interval below and above the negative excursions in different sections. On the other hand, if we use the first occurrence of H. parvus as the marker for the base of Triassic, the negative excursions occur in different horizons from the latest Changhsingian Clarkina meishanensis Zone to the upper part of the H. parvus Zone (see blue and red lines for correlation in Fig. 5), which should not reflect changes of global carbon cycle. We consider that the most important point to understand a GSSP is that the boundary at a stratotype is defined at the level of a single stratigraphically signal, that is the FAD of the index taxon. However, accurate chronocorrelation requires the evaluation of multiple, varied stratigraphic signals (e.g., the lowest occurrence of a specific taxon, an isotopic negative excursion, a high-precision date, a palaeomagnetic reversal, distinct sea-level change etc.) rather than relying solely on a single signal, such as that on which the level of the GSSP was placed. The point in time at the GSSP is of little use for accurate, high-resolution correlation (Finney, 2013). Assuming the identification of the conodont index species is correct and the d13Ccarb values based on whole carbonate rock did not suffer subsequent diagenesis, the sharp negative excursions shown in Fig. 5 should be isochronous, but the first occurrences of Hindeodus parvus is diachronous because fossil record is always incomplete everywhere (see MacLeod, 2005, p. 8, fig. 7).

Nevertheless, we consider that the presence of some minor diachroneity of an index species in different sections does not suggest that it is always necessary to change the GSSP stratotype. The PTB GSSP at Meishan is probably so far the most tested standard among all defined GSSPs due to the most extensive studies on the end-Permian mass extinction. There is some minor diachroneity for the first occurrences of Hindeodus parvus which is unlike the Devonian/Carboniferous boundary (Kaiser, 2009) and the Ordovician/Silurian boundary (Rong et al., 2008), but the minor diachroneity does not affect the PTB correlation much and calibration of the end-Permian mass extinction in a global sense if multiple signals are considered.

6. End-Permian mass extinction pattern The Daijiagou section contains abundant benthic and conodont fossils, and therefore is highly suitable for studying the endPermian mass extinction pattern at the carbonate facies. Sixty brachiopod species were described from the Changhsingian at the Daijiagou section (Shen et al., 1995; Zeng et al., 1995; Chen et al., 2005; Shen and Shi, 2007). The disappearance pattern of brachiopods (Fig. 6) clearly displays a rapid extinction pattern at the end of Changhsingian. Among the 40 species, 24 species became extinct on the top of the last 1 m siliciclastic mudstone from the topmost part of the Clarkina yini Zone to the base of the Clarkina

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Fig. 5. d13Ccarb profile from upper Changhsingian to lowest Triassic at the Daijiagou section and its correlation with other sections in South China (see Supplemental data). The gray bar shows the whole negative shift of d13Ccarb in different sections, which is associated with the end-Permian mass extinction. This excursion is characterized by a gradual, progressive negative shift beginning at the basal part of Bed 23, followed by a sharp negative shift at Bed 24e at Meishan. The four sections shown here suggest a general consistent pattern in d13Ccarb profile. However, it is quite clear that the sharp negative excursions of the three sections in Sichuan and Guizhou provinces are all in the middle part of the Hindeodus parvus Zone (A–C), but the same sharp excursion is in the Clarkina meishanensis Zone at the Meishan section (D). Red line shows the correlation based on the first occurrences of Hindeodus parvus; blue line indicates the correlation based on the most negative d13Ccarb excursions. The different heights of the gray bar indicate different sedimentary rates of different sections. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. End-Permian mass extinction pattern at the Daijiagou section in Chongqing City. Some brachiopods that disappeared in the lower part of the Changhsing Formation are not plotted. Ranges of taxa in blue are all Permian brachiopod and conodont taxa except the Lingula species and in orange are all Triassic-type bivalve and conodont taxa. Rang lines with arrows indicate that those taxa can extend up and down stratigraphically.

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meishanensis Zone. Rare trilobites have been found in the siliciclastic mudstone too. Only three brachiopod species (Paracrurithyis pigmaea, Acosarina minuta, Spinomarginifera chenyaoyanensis) survived the main extinction interval and extended into the basal Triassic. This major extinction horizon is slightly earlier based on the position of the most negative excursion of d13Ccarb than that at the Meishan section, which is at the top of Bed 24e (Clarkina meishanensis Zone). This is usual because the extinction pattern is easily biased by the Signor–Lipps effect due to lithofacies change, fossil preservation, and collecting bias (Wang et al., 2014). The extinction pattern is generally consistent with that at the Beifengjing section in Chongqing City (Wignall and Hallam, 1996; Shen and Shi, 2002). Previous studies also suggested that conodonts suffered distinct population and size changes across the extinction interval although their diversity did not change much (Luo et al., 2006, 2008; Jiang et al., 2011). Our study indicates that the population change from Clarkina-dominated to Hindeodus-dominated populations occurs between samples DJG0.45–0.50 m and DJG0.58– 0.73 m at the Daijiagou section, which is in the topmost part of the Clarkina meishanensis Zone (Figs. 3 and 6) and slightly higher than the major extinction level of benthic brachiopods, but before the most negative value of d13Ccarb excursion. This critical changeover is also in the topmost part of Clarkina meishanensis Zone in the middle part of Bed 30 at the Liangfengya section (Yuan and Shen, 2011), between Unit D and Unit F at the Chaotian section (Ji et al., 2007), and in the middle part of C. meishanensis Zone at the Huangzhishan section (Chen et al., 2008, 2009b), in the lower part of the microbialite unite at the Laolongdong section (Qi and Liao, 2007), which are generally correlatable. The level with the changeover from Clarkina-dominated to Hindeodus-dominated populations generally indicates the onset of the end-Permian mass extinction (Figs. 3 and 6). Clarkina has a distinct ‘‘miniaturization’’ at Bed 24e at the Meishan section, which is associated with the burst of Hindeodus individuals (Luo et al., 2006, 2008). Our conodont populations recovered from the Daijiagou and Liangfengya sections indicate that Clarkina does not have an obvious Lilliput Effect across the PTB in terms of individual size. However, Hindeodus individuals gradually become smaller from Sample DJG0.58–0.73 m to DJG1.30–1.34 m (Fig. 3), which clearly indicates the effect of extinction at Daijiagou. The Daijiagou section also shows some substantial differences from the nearby Laolongdong section, where a thick microbialiate unit is well developed immediately above the Changhsingian reefal facies (Qiang et al., 1985; Reinhardt, 1988; Fan et al., 1996; Kershaw et al., 1999, 2002; Wu et al., 2006; Mu et al., 2009; Jiang et al., 2010), but not present at the Daijiagou section. However, a comparable environmental deterioration is displayed across the PTB at Daijiagou as indicated by the appearances of a few disaster taxa such as the bivalve Eumorphotis multiformis, the brachiopod Lingula and Paracrurithysi pigmaea (Fig. 6), and occurrences of abundant rusty pyrite crystals in the rocks. 7. Conclusions Two genera (Clarkina and Hindeodus) with twelve species have been identified and seven conodont zones are recognized from the uppermost part of the Lungtan Formation to the lowest Feixianguan Formation at the Daijiagou section near Chongqing City. They are the Clarkina liangshanensis, C. wangi, C. subcarinata, C. changxingensis, C. yini, C. meishanensis, and Hindeodus parvus zones in ascending order. This conodont succession provides a high-resolution correlation with other equivalent sections in the Palaeotethys.

Detailed biostratigraphy and chemostratigraphy at Daijiagou and some other sections (Laolongdong in Chongqing, Dajiang and Dawen in Guizhou Province, etc.) in South China indicate that the first occurrences of Hindeodus parvus are before the sharp negative d13Ccarb excursions. This is in contrast to the FAD of Hindeodus parvus at the Meishan GSSP section which is 0.17 m above the sharp negative d13Ccarb excursion at Bed 24e. The Daijiagou section and some other sections in South China suggests that a minor diachroneity of the first occurrences of Hindeodus parvus in different sections and a slightly different temporal pattern of the end-Permian mass extinction. However, we consider that these differences are more likely due to the common phenomena of incomplete fossil records and the Signor–Lipps effect. Thus, more investigations based on different sections in different regions are very useful to narrow down the confidence interval of the endPermian mass extinction.

8. Systematic paleontology Synonyms of Clarkina and Hindeodus species have been listed and discussed in many previous papers, and hence are not repeated herein. Class CONODONTA Eichenberg, 1930 Order OZARKODINIDA Dzki, 1976 Family GONDOLELLIDAE Lindström, 1970 Genus Clarkina Kozur, 1989 Clarkina wangi (Zhang, 1987) Plate 1, Figs. 6–16 Remarks: This species has been described in detail by Mei et al. (2004) and Yuan et al. (2014). The specimens in the sample are slightly different from typical Clarkina wangi by their gradually decreasing carina, which we regarded as a transitional character between C. wangi and C. subcarinata. The denticles on carina of typical C. wangi are equal in height. The specimens are different from C. subcarinata by their relatively broad platform and a high carina. Clarkina subcarinata (Sweet in Teichert et al., 1973) Plate 2, Figs. 1–11 Remarks: The cusp and denticles of the present species are nearly the same in height in its primary stage. However, specimens of advance stage display somewhat transitional to Clarkina changxingensis. In those specimens, a gentle concave between the cusp and the posterior denticles is usually present. C. subcarinata is different from C. changxingensis by its more fused denticles and absence of a prominent gap between the cusp and the posteriormost denticle. Specimens of this species from the Daijiagou section are slightly different from the holotype that has nearly parallel lateral margins. Clarkina meishanensis (Zhang et al., 1995) Plate 4, Figs. 1–10 Remarks: Clarkina meishanensis is different from C. changxingensis by its robust cusp and deep furrows. The specimens of this species from the Daijiagou section are slightly different from those of the Meishan section by their more fused carina and more parallel lateral margins of the platform, but are identical with those from the Liangfengya section (Yuan and Shen, 2011). We consider those minor differences as geographical variations in different regions or different ontogenetic stages of the species. Clarkina zhejiangensis Mei, 1996 Plate 3, Figs. 10–15

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Description: A species of Clarkina characterized by a P1 element with a symmetrical platform. Lateral margins of adults are subparalell. Cusp is erect, usually as large as or slightly higher and larger than posterior denticles. Denticles are usually low and small, and fused in many specimens. Remarks: This species differs from Clarkina meishanensis by its small cusp, moderately deep furrows, and relatively flat platform. It can be readily distinguished from Clarkina yini by its subparalell lateral margins, usually rounded posterior margin, maximum width at middle portion, low carina, and more discrete denticles in adults in top view. Family: ANCHIGANATHODONTIDAE Clark, 1972 Genus: Hindeodus Rexroad and Furnish, 1964 Hindeodus eurypyge Nicoll, Metcalfe and Wang, 2002 Plate 3, Figs. 28–31 and Plate 5, Figs. 1–3. Description: A species of Hindeodus characterized by a carminiscaphate P1 element. Cusp is erect and higher than the nearest denticle. Usually, there are more than eight denticles behind cusp. Denticles gradually decrease posteriorly. The terminal denticle is small and low. Posterior end of P1 element is high and distinctly rounded. The leading edge of cusp is nearly vertical. There is no denticle before cusp in most specimens. Basal cavity is moderately expanded. Cup surface is smooth. Remarks: This species is not clearly defined based on its ‘‘pyge’’ posterior margin only, because some specimens of a few other species also possess this character. A specimen (Nicoll et al., 2002, Fig. 8.1) figured as H. eurypyge is very much like H. parvus in view of its high and strong cusp. The abundant occurrence of this species can be commonly used as the secondary index marker close to the PTB, but rare characteristic specimens are also found from the much lower horizons at Daijiagou and Liangfengya based on the characters of its holotype (Yuan and Shen, 2011). This species needs more specimens to study its population characters. Here we tentatively identified those specimens from the late Changhsingian with the typical ‘‘pyge’’ character as H. eurypyge.

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Remarks: Kozur (1975) established this species, but did not designate its holotype. He subsequently designated two different holotypes respectively in 1976 and 1977 (Kozur and Pjatakova, 1976; Kozur, 1977). The holotype designated by Kozur (1995) is similar to transitional forms between H. praeparvus and H. parvus. The holotype designated by Kozur (1977) is more characteristic and widely applied by subsequent studies. In order to discriminate them, Kozur (1996) named two subspecies, H. parvus parvus and H. parvus erectus, respectively. However, the latter subspecies is rarely used. Nicoll et al. (2002) considered the holotype in 1976 is valid based on priority, but the index species marking the PTB GSSP at the Meishan D section is identical with the holotype designated by Kozur (1977). Moreover, they probably have different ranges. Therefore, it needs further study to determine how to define this species. Hindeodus sp. Plate 6, Figs. 11–35 Description: A species of Hindeodus characterized by a carminiscaphate P1 element. Cusp is erect, unexpanded and only slightly larger than the nearest denticle. Usually, there are four denticles behind cusp. Denticles are discrete, strong, and nearly equal in height except for the posteriormost one. There is no denticle before cusp. Basal cavity is from not expanded in juvenile to strongly expanded in adult. Cup surface is smooth in most individuals, but has light breach at the edge in some specimens. Cup is thickened in a few adult specimens. Remarks: This species is different from Hindeodus parvus by its relatively small cusp and from H. praeparvus by its fewer but more robust denticles. This species may be related to H. postparvus in terms of its stratigraphic position, but no population of H. postparvus has been illustrated so far. Hindeodus sp. is similar to one population identified as H. parvus by Yuan and Shen, (2011, pl. 4, Figs. 9–24). Hindeodus sp. might be a transitional form between Hindeodus to Isarcicella or represents a new species of Hindeodus. Acknowledgements

Hindeodus changxingensis Wang, 1995 Plate 5, Figs. 4–15 Remarks: This species is considered as a descendant of Hindeodus julfensis, which was established based on the specimens from northwestern Iran (Sweet in Teichert et al., 1973; Wang, 1994, 1995). However, the population of H. julfensis has not been illustrated so far. Therefore, the relationship between H. changxingensis and H. julfensis is not clear yet. A few specimens with characteristics of H. julfensis in the H. changxingensis populations are present in South China (e.g., Wang, 1995, Plate 3, Fig. 1; Jiang et al., 2007, Plate 4, Figs. 29 and 30). Perri and Farabegoli (2003) assigned H. changxingensis to Isarcicella, but this viewpoint has not been accepted by other colleagues. Hindeodus praeparvus Kozur, 1996 Plate 5, Figs. 16–24 Remarks: This species is different from Hindeodus parvus by having more denticles and a relatively low cusp. However, abundant specimens with transitional characters between H. praeparvus and H. parvus are present around the PTB, which are difficult to be differentiated. Hindeodus parvus (Kozur and Pjatakova in Kozur, 1975) Plate 5, Figs. 25–30; Plate 6, Figs. 1–10

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