Palaeoecology of the Conodonts Hindeodus and Clarkina During the Permian±Triassic Transitional Period

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Palaeogeography, Palaeoclimatology, Palaeoecology 171 )2001) 63±72

Palaeoecology of the conodonts Hindeodus and Clarkina during the Permian±Triassic transitional period

Xulong Laia,b,*, Paul Wignallc, Kexin Zhanga

aFaculty of Earth Sciences, China University of Geosciences, Wuhan, Hubei 430074, People's Republic of China bDepartment of Geology, University of Leicester LE1 7RH, UK cDepartment of Earth Sciences, University of Leeds, LS2 9JT UK Received 9 March 2000; accepted for publication 26 March 2001

Abstract Detailed investigation of the distributions of the Hindeodus and Clarkina )Neogondolella) conodont faunas has been undertaken on the P/T boundary strata of the Meishan section, Zhejiang Province, South China, in association with detailed facies analysis. This reveals that Hindeodus increased and Clarkina sharply declined in abundance during a phase of relative sea-level rise which began at the end-Permian )Bed 25) and extended into Early Triassic. This was associated with the development of anoxic conditions. This distribution does not accord with previous suggestions that Hindeodus was a nearshore, shallow water taxon. The decline of the supposed deeper water genus Clarkina is also somewhat surprising but it may relate to the inhibition of a nektobenthic genus by dysoxic±anoxic bottom waters. The widespread facies and geographic distribution of Hindeodus suggests it was a pelagic type unaffected by anoxic bottom waters. Hence, the Hindeodus lineage provides a reliable criterion for identi®cation of the Permian±Triassic boundary. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Permian±Triassic boundary; Conodonts; Palaeoenivironment; Palaeoeoclogy

1. Introduction versial. Orchard )1996) considered Clarkina to be a deep-water genus and Hindeodus a shallow-water Yin et al. )1988) proposed the ®rst appearance of genus. Therefore, Baud )1996) proposed the use of the conodont Hindeodus parvus as the marker for the the deeper water Clarkina instead of Hindeodus to basal Triassic; an idea that has been widely accepted identify the basal Triassic. During the past two )e.g. Paull and Paull, 1994). As the main fossils used decades, numerous studies of the biofacies and for correlation of the Permian±Triassic )P/T) bound- palaeoecology of conodonts during the P/T transi- ary, conodonts play an important role in the study of tional period have been published. To date, most this interval, with the genera Clarkina )Neogondo- conodont specialists consider Clarkina )or Neogondo- lella) and Hindeodus being particularly important. lella) to be an offshore, outer shelf or basinal, deep- However, the palaeoecology of these taxa is contro- water taxon )Clark, 1974, 1981; Carr et al., 1984; Clark and Hatleberg, 1983; Clark and Carr, 1984; Hatleberg and Clark, 1984; Hirsch, 1994; Wardlaw * Corresponding author. Address: Faculty of Earth Sciences, and Collinson, 1984; Tian, 1993a,b; Wang, 1996; China University of Geosciences, Wuhan, Hubei 430074, People's Republic of China. Wang and Wang, 1997; Orchard, 1996; Orchard and E-mail address: [email protected] )X. Lai). Krystyn, 1998; Krystyn and Orchard, 1996; Baud,

64 X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72 1996). However, the biofacies of Hindeodus is much 2.1. Palaeoenvironmental analyses more controversial. Some consider it limited to nearshore, shallow marine facies )Wardlaw and Collinson, Three systems tracts have been recognized in the 1984; Tian, 1993a; Hirsch, 1994; Wang, 1996; Orch- uppermost Permian and lowermost Triassic at ard, 1996; Krystyn and Orchard, 1996; Baud, 1996), Meishan )Fig. 1). while others regard it as a widespread genus both in Bed 24d consists of three parasequences; each term of biofacies and/or geographic occurrence parasequence begins with a 1 cm thick dark-grey )Clark, 1981; Clark and Hatleberg, 1983; Hatleberg siliceous-mud bearing limestone. The middle part of and Clark, 1984; Rexroad, 1993; Wang and Zhong, each parasequence is comprised of a 4 cm thick, 1995; Kozur et al., 1996; Kozur, 1996; Lai, 1997; Lai bioclastic packstone, and the upper part is a 5 cm et al., 1998). New investigations of conodont biofa- thick packstone. From bottom to top within each para- cies have been undertaken on the P/T boundary strata sequence, the colour becomes lighter, bioclastic of the Meishan Section, Zhejiang Province, South grains become bigger and the bioclasts change China )Yin et al., 1996) Ð the global stratotype systematically from deeper water types )including section and point )GSSP) of the Permian±Triassic slender non-fusunilind foraminiferas, spicules and Boundary )PTB) and are presented here. A new ecolo- echinoderm) to shallower water types )including gical model for Hindeodus and Clarkina is proposed, thicker-shelled brachiopods, calcareous algae and and the data from several P/T boundary sections else- bigger non-fusulinid foraminiferas). There is a wavy where in the worldwide are re-evaluated. boundary at the top of Bed 24d where a thin bed of Note that the genus name for the gondolellid cono- iron oxide-rich calcareous mud occurs. The truncation donts near the P/T boundary is controversial. Kozur fossil at a wavy surface also occurs at the top of Bed )1989) proposed the genus Clarkina, and several 24d. Microfacies change sharply across the wavy authors accept this genus name instead of Neogon- boundary, which Zhang et al. )1996) considered to dolella. However, Orchard and Rieber )1999) be a type II sequence boundary. reconstructed the multielement taxonomy of Neogon- Bed 24e consists of single upward shallowing dolella, and placed Clarkina in synonymy with parasequence. The lower part is a 1 cm thick, dark- Neogondolella. Most work on the Meishan section grey, siliceous, muddy limestone with horizontal uses Clarkina instead of Neogondollea )e.g. The bedding. The middle part is a 4 cm thick, wavy- Permian±Triassic Boundary Working Group, 1999; bedded bioclastic wackestone and the upper part is a Yin and Tong, 1998; Mei et al., 1998, Wardlaw and 5 cm thick bioclastic wackestone showing reverse Mei, 1998), and we have maintained this `convention' grading. The bioclastics mainly consists of fusulinids, here, although we acknowledge that the validity of non-fusulinid foraminifers and brachiopods. this genus is doubtful. Beds 25 and 26 are the P/T boundary claystone. Generally, Bed 25 has been called the `white clay bed' and Bed 26 the `black clay bed' )Yin et al., 2. Palaeoenvironmental changes and conodont 1996). According to previous studies, both beds are evolution in the Meishan section composed of montmorillonite-illite claystones with subordinate kaolinite, and are of volcanic origin As the one of four candidates for the GSSP, the )Yin et al., 1992; Yin and Zhang, 1996). There are Meishan section has received detailed multidisciplinary pyrite laminae at the bottom and top surfaces of Bed analysis during the past two decades. It is one of the 25. Bed 26 is organic-rich, dark in colour, with ®ne best section for the study of conodont palaeoecology lamination, although Planolites burrows also occur. across the Permian±Triassic transitional period. The taxa are dominated by pelagic types mainly Based on systematic analyses of lithofacies, microfa- consisting of ophiceratid ammonites. A few dysaero- cies, biofacies and sea-level changes, the palaeoenvir- bic benthic taxa are presented including non-fusulinid onmental changes can be related to the distribution of foraminifera and tiny, thin-shelled brachiopods. the conodonts Hindeodus and Clarkina shown in Bed 27 is a 16 cm thick grey, argillaceous micrite Fig. 1. with scattered pyrite grains and a pervasively 中国科技论文在线 http://www.paper.edu.cn

X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72 65

Fig. 1. Palaeoenvironmental changes and conodont evolution across the Permian±Triassic boundary in the Meishan section, Zhejiang, China. 1. claystone; 2. marl; 3. mud-bearing micrite; 4. siliceous micrite; 5. bioclastic micrite; 6. anoxic environment; 7. horizontal bedding; 8. normal graded bedding; 9. high temperature quartz; 10. zircon; 11. microspherule; 12. fusulinid; 13. non-fusulinid foraminifer; 14. conodont;15. calcareous alga; 16. brachiopod; 17. bivalve; 18. ammonite.

burrowed fabric. Based on analysis of the microfacies, The minor omission surface between beds 24d and fauna, and in comparison with other sections )e.g. 24e may mark a preceeding phase of relative sea-level Shangsi section in Sichuan, Lai et al., 1996), Bed 27 lowstand, as interpreted by Zhang et al. )1996), or at Meishan records a highly condensed record. In alternatively a phase of non-deposition caused by a conodont sampling both Zhang )Zhang et al., 1995) sudden pulse or deepening. Regionally, the ±Tr strata and Wang )1994) subdivided this 16 cm bed into four mark a major phase of onlap in China )Jin et al., 1994) equally thick partitions. and elsewhere )Wignall et al., 1996; Hallam and Bed 28 is a 4 cm thick illite-montmorillonite mixed Wignall, 1999). This transgression was associated claystone, with abundant pyrite crystals, together with the development of oxygen-poor deposition at hexagonal bipyramid quartz, zircon and micro- Meishan, as indicated by the presence of abundant spherules of volcanic origin. pyrite, the dysaerobic paper-pecten genus Claraia Bed 29 is grey, medium-bedded, dolomitic calcimi- )Wignall and Hallam, 1993), prasinophytes )cf. Yin crite with minor mud and silt. It yields abundant et al., 1992), and a reduced or absent ichnofauna. specimens of Ophiceras, Claraia and thin-shelled Anoxia initially developed at the base of Bed 25, brachiopods. Many non-fusulinid foraminifera of the but oxygenation appears to have improved somewhat genus Earlandia can be seen in thin-section )Wignall in Bed 27 which is pervasively bioturbated, before and Hallam, 1993). declining once more into the non-burrowed beds of bed 28 and higher. 2.2. Facies interpretation 2.3. Conodont evolution across the P/T transitional The rapid loss of bioclasts and development of ®ne- period grained carbonate strata suggests a deepening event is recorded in the P±Tr boundary interval in Meishan. Conodont abundances change markedly across the 中国科技论文在线 http://www.paper.edu.cn

66 X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72 occurrence that does not support the contention that Table 1 Numerical distribution of Hindeodus Pa elements and Clarkina Pa it was a nearshore shallow water taxon. It is surprising across the Permian±Triassic boundary at Meishan section )sample that the supposed deeper water genus Clarkina size: 2 kg each sample) declined abruptly during the transgression rather than increased its abundance. Beds Hindeodus Pa Clarkina Pa

28 20 0 27d 5 0 3. Palaeoecology of hindeodid and gondolellid 27c 6 0 conodonts near the P/T boundary 27b 6 2 27a 6 0 26 13 66 3.1. Palaeoecology of Clarkina 25 3 52 24e 2 155 Clark )1974, 1981) considered that Neogondolella ¯ourished in quiet, nutrition-de®cent, deeper water P±T boundary at the Meishan, with a modest increase environments ).50 m). Wardlaw and Collinson in Hindeodus and a sharp decline of Clarkina in the )1984) similarly interpreted Neogondolella and early Triassic )Table 1). The relationship between the Xaniognathus as distal offshore genera, as did Carr absolute and relative frequencies of Hindeodus, et al. )1984) as did most subsequent workers )Wang Clarkina and palaeoenvironmental changes are 1996; Wang and Wang, 1997). In a similar vein, Tian shown in Fig. 1. )1993a,b) proposed that P/T gondolellids )Neogondo- Clarkina-dominated conodont biofacies are lella or Clarkina) were deep-water, planktonic, common in the Late Permian conodont record, and free-swimming taxa. Swift )1995) similarly this is also the case in Bed 24a. However, from Bed concluded that Late Permian gondolellid and xanio- 25, the number of Clarkina decline sharply, and it is ganthid taxa were obligate deep-water taxa. Orchard no longer dominant from Bed 27a to the basal Triassic )1996) also concluded that, throughout the Permian, )Bed27c±28). Only a very few gondolellid conodonts Neogondolella is most common in offshore, deep- were recorded from Triassic strata. Generally, the water, and/or cooler water marine environments. replacement of Clarkina by Hindeodus is a common There is thus a consensus in the offshore preference feature of many P/T sections elsewhere in South of Clarkina. Thus the decline of this genus during the China and the rest of the world )e.g. Iran, Salt P/T transgression is somewhat unusual and indicates Range, Europe, West United States). However, basal factors other than water depth were restricting its Triassic conodont assemblages dominated by distribution. This decline coincides with an anoxic Clarkina are known from south Tibet )Orchard et event suggesting that Clarkina was in¯uenced by al., 1994; Jin et al., 1996), Spiti, India )Krystyn and changes at the sea bottom. This implies that Clarkina Orchard, 1996; Orchard and Krystyn, 1998); and the is more likely to have been benthic rather than pelagic Canadian Arctic )Orchard, 1996; Orchard and Tozer, )Fig. 2A). As a benthic free-swimming genus, 1997). These latter data demonstrate that Clarkina Clarkina inhabited the offshore, deeper water, was not consistently replaced by Hindeodus during oxygen-rich environments of the Late Permian, but the end-Permian and earliest Triassic )Lai and Mei, it declined as anoxic conditions developed in the 2000). At Meishan, the replacement coincided with an deeper bottom waters of the P/T transitional period. anoxic event during the end-Permian and earliest We suggest that it only dominates in deep-water P/T Triassic )Wignall and Hallam, 1993), and it has sections where oxygen-rich environments persisted been suggested that the decline of the gondolellid )Fig. 2B). conodonts was directly caused by the anoxic environ- The geographical distribution of Clarkina also ment that started at Bed 25 in Meishan )Lai et al., supports a benthic ecology )Table 2). Although the 1998; Lai and Zhang, 1999). genus Clarkina is widespread, few species are cosmo- At Meishan, Hindeodus increases in abundance politan. At the species level, for example, the very during a phase transgression and anoxia, an common Late Permian gondolellid conodonts of 中国科技论文在线 http://www.paper.edu.cn

X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72 67 water as well. In contrast, Behnken )1975) suggested that Hindeodus, from the Late Permian in Wyoming, to be uppermost photic-zone dwellers, perhaps living in abnormal salinities. Clark )1981) concluded that Hindeodus and Ellisonia were surface dwellers and occurred in both shallow water and deeper water deposits, although Clark and Hatleberg )1983) and Hatleberg and Clark )1984) only found the Griesbachian hindeodid conodonts Isarcicella and H. typicalis in basinal to outer shelf biofacies in the western United States. A different interpretation again was given by Wardlaw and Collinson )1984) based on their work on the Permian Phosphoria Formation; they considered that Hindeodus dominated in lagoonal facies. Tian )1993b), working in western Hunan, China, considered that hindeodids )Hindeodus and Isarcicella) were shallow-water, nektobenthos. Similarly, Hirsch )1994) considered Hindeodus to be an inner shelf Fig. 2. Conodont palaeoecological model across the Permian±Trias- sic boundary. )A) Late Permian oxygen-rich stage; )B) End- dweller, best adapted to eustatic lowstands. Orchard Permian-earliest Triassic anoxic stage 1. Hindeodids )Hindeodus )1996) also concluded that, throughout the Permian and Isarcicella); 2. Clarkina; 3. Oxic; 4. Anoxic. and beyond, Hindeodus and its antecedents ¯ourished in the nearshore, shallower, and/or warmer regions. South China, Clarkina changxingensis, C. de¯ecta Baud )1996) also believed that Hindeodus parvus and C. subcarinata are rarely found in Europe and was a shallow-water species. From their work in America. Kozur )1998) considered C. subcarinata to Spiti, India, Krystyn and Orchard )1996) also be a warm-water species, restricted to the Tethyan proposed that Hindeodus occurred in shallow water realm, and C. de¯ecta and C. changxingensis to be biofacies. tolerant of both warm and cool bottom waters in east- However, many geological records do not support ern Tethys. In fact the temperature-tolerance of this the contention that hindeodids were restricted to shal- genus appears of secondary importance to its low waters. Thus, Zhang )1990) subdivided the Lower preference for deep-water sites. It implies that Triassic of western Guangxi province, China, into two Clarkina is a facies-controlled genus, and supports a depositional types: the deeper water Zuodeng Type benthic ecology for it. and shallow water Taiping type based on lithological characteristics. Griesbachian hindeodid conodonts 3.2. Palaeoecology of Hindeodus )Hindeodus, Isarcicella) occurred in both depositional types. From systematic study of the Lower Triassic The palaeoecology of Hindeodus is much more conodonts from eastern Yunnan, western Guizhou and controversial. There are two main views: one that northern Guangxi, Southwest China, Wang and Hindeodus was a nearshore, shallow water species; Zhong )1995) divided the conodont biofacies into the other that Hindeodus had a wide facies range basinal and platform facies. In platform facies areas, and occurred both in shallow and deeper water H. minutus and H. parvus zone were found, whilst environments. these two species and Isarcicella isarcica were all Based on his work on Permian conodont palaeo- recognized in basinal facies areas. Finally, Kozur ecology in Nevada, Clark )1974) proposed that )1996, 1998) and Kozur et al. )1996) pointed out neogondolellids, anchignathoids )hindeodids) and that the ®rst appearance of H. parvus is not facies Sweetognathus are most abundant in deep-water, related and can be identi®ed both in ammonoid-free nutrient-poor sites of normal salinity with the proviso shallow water deposits and in ammonoid-bearing that anchignathoidids probably ranged into shallow pelagic deposits. 中国科技论文在线 http://www.paper.edu.cn

68 X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72 ; Clarkina Greenland Kozur, 1993; R. Twitchett, Pers. com. C ; Hindeodus Canadian Arctic Beyers and Orchard, 1991; Hendeson, 1993; Orchard and Tozer, 1997 H ; Australia Nicoll and Foster, 1998 Isarcicella West USA Paull and Paull, 1994 I Sicily Gullo and Kozur, 1993; Kouzr, 1996 - nlaub, È kofel Holser et al., 1991; Scho 1991 Gartner Tesero; Bulla Perri and Andraghett, 1987; Perri, 1991 Dorasham Kotlyar et al., 1993 Kuh-e-Ali Bashi Golshani et al., 1986 Hambast )Abadeh) Iran±Jap. Res.Gr., 1981 Narmal Nala Pak. Jap. Res. Gr, 1985 Spiti Krystyn and Orchard, 1996 Guryul Ravine Matsuda, 1981; Kapoor, 1996 Selong China Orchard et al., 1994; Jin et al., 1996 Shangsi China Li et al., 1989; Lai et al., 1996 Meishan China Zhang et al, 1995; Lai et al., 1995 *f.*0 * 0 0 *000 0 * * * ****** * * * ****** * * * * * ***** 0*** * * ***aff.0*00* 0 **** * * ***** * * 0 00000* * * 0 0 *00** * 0 0 *000 0 0 000? * `*' indicates presence, `0' indicates absence; `aff' means af®nity species; `?' means doubtful) Table 2 Geographical distribution of several conodont zonal species near the Permain±Triassic Boundary at Meishan Section )Abbreviation: I. isarcica H. parvus H. typicalis C. changxingensis C. de¯ecta C. subcarinata 中国科技论文在线 http://www.paper.edu.cn

X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72 69 In the Meishan section, Hindeodus increased in other areas, the conodont fauna is dominated by abundance during a transgression, which does not hindeodids )Hindeodus, Isarcicella), whilst gondolel- support the hypothesis of a shallow water ecology. lids are rare during this interval. On the contrary, it suggests that Hindeodus had an In contrast, Clarkina )Neogondolella) still domi- apparently wide facies-tolerance typical of pelagic nated in Spiti, India, the Selong section, Tibet, and taxon. Geographically, the existence of several the Canadian Arctic area during the same period. cosmopolitan species of hindeodids, H. parvus, H. Because there are no detailed palaeoenvironmental typicalis and I. isarcica, during the P/T transitional data available for Spiti and the Canadian Arctic period also supports a pelagic palaeoecology )Table areas, it is hard for us to evaluate if the Clarkina 2). Hindeodids occur both in the warm-water Tethyan )Neogondolella)Ðdominated assemblages in these realm and cold-water Boreal realm. areas are related to oxygen-enrichment environments during the end Permian and Early Triassic. However, both detailed palaeoenvironmental analysis and a 4. Conodont palaeoecological model near the P/T conodont faunal study have been undertaken at the boundary Selong section, Tibet )Orchard et al., 1994; Jin et al., 1996). According to the work of Jin and his collea- If Hindeodus was a pelagic taxon, and Clarkina gues at the Selong section )Jin et al., 1996), the upper- )Neogondolella) a deeper water, nektobenthic taxon most Permian packstone )Waagenites bed) contains then their distribution during the P/T transitional 10±35% bioclastics of brachiopods, ostracods, gastro- period can be seen as a re¯ection of changing benthic pods, calcareous sponges and foraminifera. They oxygenation levels )Fig. 2). The bottom waters during consider this bed to represent carbonate deposits the Late Permian were oxygen-rich environment with formed in shallow, medium-energy shoals and in conodont distribution mainly controlled by facies. In inter-shoal depressions. The Early Triassic Otoceras deeper water sections, both gondolellids and hindeo- Bed in Selong consists of stylo-compacted corroded dids occur, with the number of benthic gondolellids packstone with crinodal and molluscan fragments, greatly exceeded by that of hindeodids. This phenom- that lacks evidence for any oxygen-restriction during enon is apparent in the data from many sections in the deposition. Thus, Clarkina dominates the oxygen-rich world including Meishan )Lai and Zhang, 1999), P±T environments at Selong but disappears from Shangsi, Guryal Ravine )Matsuda, 1981), and the sections elsewhere due to the presence of benthic Salt Range )Sweet, 1970). On the other hand, Hindeo- anoxia. dus dominated in very shallow water environments due to its pelagic ecology, as seen in the Tesero and Bulla sections in the Dolomite area, Italy )Perri, 1991; 5. Conclusion Farabegoli and Perri, 1998) where shallow-water facies, characterized by oolites, were developed Based on the above analysis, a palaeoecological during the Late Permian. model for conodonts near the P/T boundary has During the end Permian and earliest Triassic, the been established. Conodont occurrence and palaeo- benthic genus Clarkina declined in abundance as environmental analysis convince us that Hindeodus anoxic conditions developed in deeper bottom waters. was a pelagic taxon and Clarkina was a deeper As a pelagic taxon, however, Hindeodus could survive water, nektobenthic taxon. Thus, Hindeodus can be despite the anoxic bottom water. Thus, Hindeodus found in facies recording a range of palaeobathymetry dominates in sections where anoxia is recorded during while Clarkina is restricted to the low energy, deeper the end Permian-earliest Triassic. End Permian to water oxygenated settings. This new model can be Early Triassic anoxia is widespread and has been used to interpret the differences in conodont faunas recorded from South China )Hallam, 1994; Wignall recorded in other sections and areas. and Hallam, 1993; Wignall et al., 1995; Wignall and As a pelagic conodont, Hindeodus could migrate Hallam, 1996), the western United States and Italy widely during the Permian±Triassic transitional )Wignall and Hallam, 1992). In these sections and period, and many Hindeodus species were cosmopolitan 中国科技论文在线 http://www.paper.edu.cn

70 X. Lai et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 171 52001) 63±72

species in this interval. Furthermore, the pelagic lithostratigraphy, facies and conodont biostratigraphy. Southern Hindeodus could survive and evolve in warm-water Alps Field Trip Guidebook, Perri, M.C., Spalletta, C. )Eds.), or cold-water, as well as shallow-water or deep-water Geologia Ser 3a vol. 60, 292±307. Golshani, F., Partoazar, H., Seyed-Emami, K., 1986. Permian-Trias- environments even although anoxia developed in sic boundary in Iran. Memorie della Societa Geologica Italiana the bottom water during the P/T transitional 36, 257±262. period. These characteristics of Hindeodus con®rm Gullo, M., Kozur, H., 1993. First evidence of the Scythian cono- that it is an ideal taxon for use in the de®nition of donts in Sicily. N. Jb. Geol. Paleont. Mh. 8, 477±488. the P/T boundary. Hallam, A., 1994. The earliest Triassic as an anoxic event and its relationship to the end-Palaeozoic mass extinction. Pangea: global environment and resources. Canadian Society of Petro- Acknowledgements leum Geologists, Memoir 17, 797±804. Hallam, A., Wignall, P.B., 1999. Mass extinction and sea-level changes. Earth-Science Reviews 48, 217±250. This work was supported by the Natural Science Hatleberg, E.W., Clark, D.L., 1984. Lower Triassic conodonts and Foundation of China )grant no: 49632070), LXL biofacies interpretations: Nepal and Svalbard. Geologica et acknowledge the Chinese Scholarship Council and Palaeontoloica 18, 101±125. the Royal Society for funding this international Hendeson, C.M., 1993. Are Permian±Triassic boundary events collaboration. Thanks are also due to Richard diachronous? Evidence from the Canadian Arctic. Program and Abstracts, Annual Convention of Canadian Society of Aldridge for his valuable suggestions to the draft Petroleum Geology, 137. manuscript. We are also grateful to Mike Orchard Hirsch, F., 1994. Triassic conodonts as ecological eustatic sensors. and R.L. Ethington. 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