Journal of Asian Earth Sciences 80 (2014) 75–83

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Journal of Asian Earth Sciences

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Restudy of conodont biostratigraphy of the Permian–Triassic boundarysection in Zhongzhai, southwestern Guizhou Province, South China

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⇑ Corresponding author.E-mail address: grshi@deakin.edu.au (G.R. Shi).

Yang Zhang a,d, Ke-Xin Zhang b, G.R. Shi a,⇑, Wei-Hong He b, Dong-Xun Yuan c, Ming-Liang Yue d,Ting-Lu Yang d

a School of Life and Environmental Sciences, Deakin University, Melbourne Campus, 221 Burwood Highway, Burwood, VIC 3125, Australiab State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Road, Hongshan, Wuhan 430074, PR Chinac State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, PR Chinad Faculty of Earth Sciences, China University of Geosciences, 388 Lumo Road, Hongshan, Wuhan 430074, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 April 2013Received in revised form 28 October 2013Accepted 29 October 2013Available online 7 November 2013

Keywords:ConodontHindeodus parvusPermian–Triassic boundary (PTB)Zhongzhai sectionSouth China

New conodont samples have been systematically collected at high stratigraphic resolution from theupper part of the Longtan Formation through to the lower part of the Yelang Formation at the Zhongzhaisection, southwestern Guizhou Province, South China, in an effort to verify the first local occurrence ofHindeodus parvus in relation to the Permian–Triassic boundary at this section. The resampled conodontfauna from the Permian–Triassic boundary interval comprises five identified species and two undeter-mined species in Hindeodus and Clarkina. Most importantly, the first local occurrence of Hindeodus parvusis found for the first time from the bottom of Bed 28a, 18 cm lower than the previously reported first localoccurrence of this species at this section. Considering the previously accepted PTB at the Zhongzhai sec-tion, well calibrated by conodont biostratigraphy, geochronology and carbon isotope chemostratigraphy,this lower (earlier) occurrence of H. parvus suggests that this critical species could occur below the Perm-ian–Triassic boundary. As such, this paper provides evidence that (1) the first local occurrences of H. par-vus are diachronous in different sections with respect to the PTB defined by the First Appearance Datum(FAD) of this species at its GSSP section in Meishan, China and that (2) the lower stratigraphic range of H.parvus should now be extended to latest Permian.

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1. Introduction

The end-Permian mass extinction, the most severe of its kind inEarth history (Sepkoski, 1981; Erwin, 1994), has been studied forthe last several decades. However, causes of this mass extinctionremain unclear. In part, the challenge relates to the uncertaintysurrounding the precise determination of the Permian�Triassicboundary at a specific location or section.

Globally, the Permian–Triassic boundary (PTB) has been deter-mined by the First Appearance Datum (FAD) of the conodont spe-cies Hindeodus parvus at the Global Stratotype Section and Point(GSSP) of the Meishan section in Zhejiang Province of South China(Yin et al., 2001), where recent geochronology studies of interdis-persed ash beds have also provided several firm radiometric ageanchor points (Mundil et al., 2004; Shen et al., 2011). Actually, inSouth China, there have been numerous excellent marine Perm-ian–Triassic boundary sections reported in the past three decades(Zhang, 1987; Dai and Zhang, 1989; Tian, 1993; Zhu et al., 1994;Shen et al., 1995; Zhang et al., 1995, 2009; Yang et al., 1999; Wang

and Xia, 2004; Ji et al., 2007; Jiang et al., 2007, 2011; Metcalfe andNicoll, 2007; Chen et al., 2008, 2009), but most of these previousstudies have been focused on shallow-water carbonate platformsections, with few concerning clastic-rock facies from continentalshelf settings. This is despite the significance of clastic-shelf PTBsections in serving as an important bridge in correlating marineand non-marine PTB sequences (Peng et al., 2005). In this regard,the Zhongzhai section is perhaps unique as it is the only knownPTB section in South China to date representing the clastic-shelfrock facies. As such, this section has attracted much attention ina number of recent publications (e.g., Metcalfe and Nicoll, 2007;Shen et al., 2011; Lei et al., 2012).

After a brief note published as an abstract by Nicoll and Met-calfe (2005), Metcalfe and Nicoll (2007) formally reported, for thefirst time, Hindeodus parvus from Bed 30 of the Zhongzhai section,which enabled them to place the PTB at the base of this bed. How-ever, our recent systematic bed-by-bed resampling of the entiresection has discovered that Hindeodus parvus in fact first appearsat Bed 28a, 18 cm lower than the first local occurrence (FLO) of thisspecies previously reported by Metcalfe and Nicoll. This discoveryis important as it would suggest, at first glance, that either the PTBneeds to be lowered to the base of Bed 28a for the Zhongzhai sec-

Fig. 1. (A) Changhsingian lithofacies paleogeography map of South China (after Feng et al., 1997; Xie et al., 2010; Lei et al., 2012). Sections: a, Zhongzhai; b, Heping; c,Huangzhishan; d, Meishan; e, Shangsi. (B) The S–S’ cross-section represents the lithostratigraphy of the Upper Permian to Lower Triassic rock successions in western Guizhouand eastern Yunnan provinces (including non-marine section: Chahe and Zhejue, marine section: Zhongzhai) (after Wang and Yin, 2001).

Fig. 2. Location of the Zhongzhai section in southwestern Guizhou Province, South China.

76 Y. Zhang et al. / Journal of Asian Earth Sciences 80 (2014) 75–83

tion, or the first local occurrence of Hindeodus parvus is actuallylower than the PTB at this section if the previously accepted PTB

is kept unchanged for the Zhongzhai section. As discussed furtherbelow, the second scenario is strongly favored here as the PTB of

Fig. 3. Stratigraphy and lithology of the Zhongzhai section, also showing the detailed collection horizons of conodont fossils near the PTB. (A) Hindeodus parvus Kozur andPjatakova, 1976, Bed 28a. (B) Neochonetes (Zhongyingia) zhongyingensis Liao, 1980, Bed 12. (C) Claraia griesbachi (Bittner, 1899), Bed 32. (D) Rotodiscoceras sp., Bed 27c. Thecarbon isotope data are after Shen et al. (2011).

Y. Zhang et al. / Journal of Asian Earth Sciences 80 (2014) 75–83 77

the Zhongzhai section has been well constrained and calibrated byadditional evidences from both geochronology and d13C isotopechemostratigraphy. Consequently, a significant implication wouldbecome apparent from the present study: if a broad global syn-chronicity can be assumed for the PTB, the first local occurrencesof Hindeodus parvus would have to be diachronous with respectto the PTB at its GSSP section in Meishan and to the first localoccurrence of this species at this section. Further, it also impliesthat the lower stratigraphic range of H. parvus should now be ex-tended to latest Permian.

2. Location and geological setting

Geographically, the Zhongzhai PTB section (26.1529�N and105.2865�E) is situated about 1 km northeast of Zhongzhai (a localtownship), Liuzhi County, southwestern Guizhou Province (Figs. 1and 2). Paleogeographically, the Zhongzhai section was located inthe western part of the Yangtze Basin in South China during thePermian. From Late Permian to Early Triassic, the basin contained

a variety of depositional settings, consisting of shallow-water clas-tic-shelf, shallow-water carbonate platforms, offshore ramps andslope to bathyal deep-water environments (Fig. 1A). Importantly,along the western fringe of the Yangtze Basin and east of the Sich-uan–Yunnan Old Land existed a large epiric sea extending from(present-day) western Guizhou to eastern Yunnan provinces dur-ing the Changhsingian (Late Permian) to Early Triassic (Fig. 1), thusallowing the development of a continuum of both marine and non-marine PTB sequences in this area. In non-marine strata, Wang andYin (2001) initially named the Chahe and Zhejue terrestrial Perm-ian–Triassic boundary (TPTB) sections (Fig. 1), which contain abun-dant plant and sporopollen fossils. Previously, Yao et al. (1980)were first to document in detail a large number of well-preservedand locally fossiliferous PTB sections ranging in depositional faciesfrom coastal or marginal marine through full shallow-marine todeep-water environments, including the Zhongzhai section locatedat the junction between marine and non-marine settings (Fig. 1)(see below for interpretation of Zhongzhai section’s depositionalenvironment).

Fig. 4. 1, Clarkina meishanensis Zhang et al., 1995, upper view, LZ-C-280101. 2, Hindeodus sp.1, lateral view, LZ-C-280201. 3. Hindeodus sp.2, lateral view, LZ-C-280202. 4–20,25–28, Hindeodus praeparvus Kozur, 1996, 4–11, 13–16, 20, 25, 27, 28, lateral view, LZ-C-280203–280218; 12, 17–19, 26, lateral view, LZ-C-280102–280106. 21, Hindeodus cf.H. peculiaris (Perri and Farabegoli, 2003), lateral view, LZ-C-280219. 22–24, Hindeodus bicuspidatus Kozur, 2004, lateral view, LZ-C-280220–280222.

78 Y. Zhang et al. / Journal of Asian Earth Sciences 80 (2014) 75–83

The Permian–Triassic interval strata at the Zhongzhai sectionare comprised of the upper part of the Longtan Formation (Beds1–28) and the lower part of the Yelang Formation (Beds 29–35)(Fig. 3). The compositions of the upper part of the Longtan Forma-tion are calcareous mudstone, argillaceous siltstone, a few sand-stone layers and intercalated limestone, whilst the lower part ofthe Yelang Formation includes purple mudstone, siltstone, sand-stone and a few intercalated limestone layers.

Both the Longtan and the Yelang formations are locally very fos-siliferous, and the fossils are generally well preserved owing to thefine-grained nature of the sediment (Fig. 3). Since Yao’s et al.(1980) initial report of abundant marine fossils from this section,

several other papers have also provided some detailed systematicstudies on some of the faunas: brachiopods (Liao, 1980; Zhanget al., 2013), bivalves (Yao et al., 1980; Gao et al., 2009), conodonts(Nicoll and Metcalfe, 2005; Metcalfe and Nicoll, 2007), gastropods(He et al., 2008), phytoplankton (Lei et al., 2012), as well as a gen-eral account of the stratigraphy of this section (Peng et al., 2005;Glen et al., 2009; Shen et al., 2011).

To date, no specific and detailed sedimentological study hasbeen undertaken to determine the depositional environment ofthe Zhongzhai section. However, in a general account based on aregional combined lithofacies-palaeogeographical mapping exer-cise commissioned by the petroleum industry for the Permian of

Fig. 5. 1, 2, 7, Hindeodus parvus (Kozur and Pjatakova, 1976), 1, 2, lateral view andupper view, LZ-C-280223; 7, lateral view, NIGP159237; 3–6, Hindeodus ?parvus(Kozur and Pjatakova, 1976) 3, 4, lateral view and upper view, LZ-C-280224; 5, 6,lateral view and upper view, LZ-C-280107.

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southwestern China, Feng et al. (1994) showed that the Permian–Triassic boundary strata of the Zhongzhai area was deposited in ashallow-water clastic-shelf environment. This interpretation isconsistent with the presence of economically viable coal measuresin the lower part of the Longtan Formation at the Zhongzhai sec-tion, which are the most significant coal-bearing rocks in this re-gion. The PTB interval strata above the coal measures at theZhongzhai section, from both the upper part of the Longtan andlower part of the Yelang formations, are dominated by mudstoneintercalated with horizontally bedded argillaceous siltstone, whichhas been interpreted to represent an offshore back-barrier, rela-tively low-energy shallow-marine environment (Shen et al.,2011) (Fig. 1). It is therefore not surprising to see that most ofthe brachiopods found within the mudstone beds in the upper partof the Longtan Formation are well preserved (e.g. see the chonetidbrachiopod shown in Fig. 3, still with hinge spines intact).

3. Material and methods

Eighty-two samples (3–5 kg per sample) were systematicallycollected from the claystone, calcareous mudstone, siltstone andlimestone beds of the upper part of the Longtan Formation andthe part of the lower Yelang Formation in the Zhongzhai section.

Table 1Relative abundance (measured by percentage of numbers of specimens) of conodont specieMetcalfe and Nicoll (2007), all others are from this study.

Bed 28

Numbers of specimens Percen

C. meishanensis 1 + 35� 15.9C. tulongensisH. bicuspidatus 3 1.3H. changxingensis 27� 11.9H. eurypyge 41� 18.1H. latidentatusH. parvus, H. ?parvus 4 1.8H. cf. peculiaris 1 0.4H. praeparvus 21 + 72� 41.0H. sp.1 1 0.4H. sp.2 1 0.4H. n. sp. 7� 3.1H. typicalisM. ultima 13� 5.7

Total 227 100

All samples were corroded by 10% solution of acetic acid and thenexamined under a binocular microscope. Importantly, the samplesof Beds 26, 28a, 28b, 29 and 30, which are the key layers of thePermian–Triassic boundary interval, were repeatedly processedand examined (at least three times for each sample) in an effortto search for conodont fossils and for locating the PTB. Finally,some selected specimens were photographed by Scanning ElectronMicroscopy (SEM) at China University of Geosciences (Wuhan) andDeakin University in Melbourne.

All but one specimens figured in this paper are preserved in theMicropaleontology Laboratory, Faculty of Earth Sciences, ChinaUniversity of Geosciences (Wuhan), China, with prefixes LZ-C forspecimens from the Zhongzhai section in Liuzhi County (C forconodont fossils, 2801 for Bed 28a, 2802 for Bed 28b). The excep-tional specimen is provided by one of the authors (YDX) and is reg-istered with NIGP159237 (from Bed 28b) and housed in theNanjing Institute of Geology and Palaeontology, Nanjing, China.

4. The conodont fauna

This Zhongzhai conodont fauna was first reported by Nicoll andMetcalfe (2005) in a very short note, in which they briefly de-scribed the lithology of Permian–Triassic interval strata from Beds26–31, and placed the PTB at the bottom of Bed 30 based on theoccurrence of Hindeodus parvus. Subsequently, Metcalfe and Nicoll(2007) formally reported and figured this fauna from the Zhongz-hai section, which included Hindeodus parvus, H. eurypyge, H.changxingensis, Clarkina tulongensis, C. meishanensis and Merrillinaultima. In the same paper, Metcalfe and Nicoll (2007) also consid-ered the base of Bed 30 to mark the PTB at the Zhongzhai section(Fig. 3).

In the present study, five samples have yielded conodonts. Sam-ple LZ-0101 (Bed 1a), LZ-0102 (Bed 1b) and LZ-26 (Bed 26) containsome conodont fragments which are too poorly preserved for spe-cies identification. However, samples LZ-2801 (Bed 28a) and LZ-2802 (Bed 28b) have yielded well-preserved conodont material.Together, these five samples contain a modest conodont fauna,consisting of: Clarkina meishanensis Zhang et al., 1995, Hindeodusbicuspidatus Kozur, 2004, H. parvus Kozur and Pjatakova, 1976, H.?parvus Kozur and Pjatakova, 1976, Hindeodus cf. H. peculiaris Perriand Farabegoli, 2003, H. praeparvus Kozur, 1996, H. sp.1 and H. sp.2(Figs. 4 and 5, Table 1).

Among these conodont species, the identification of H. parvus ismost critical for this paper but has proved a considerable challengebecause of the fine gradational changes between H. praeparvus and

s in Beds 28 and 30 at the Zhongzhai section. Numbers marked by an asterick are from

Bed 30

tage (%) Numbers of specimens Percentage (%)

1� 1.7

7� 12.140� 69.0

10� 17.2

58 100

Fig. 6. Morphological comparison among Hindeodus parvus Kozur and Pjatakova, 1976, Hindeodus praeparvus Kozur, 1996 and four specimens in this study. 1–9, H. parvus; 1,one of the two original type specimens figured by Kozur and Pjatakova (1976, Fig 1: a), 2, holotype, as originally designated and illustrated by Kozur and Pjatakova (1976, Fig1: b) (this specimen is regarded as the valid holotype for H. parvus, see Nicoll et al., 2002), 3, H. parvus from Meishan PTB GSSP section (Yin et al., 2001, pl. 1, Fig. 4), 4, H. parvusfrom Meishan PTB GSSP section (Metcalfe et al., 2001, Fig. 2), 5, H. parvus from Bed 30 of Zhongzhai section (Metcalfe and Nicoll, 2007, pl. 1, Fig. 1), 6, 7, H. parvus from thisstudy (also figured in Fig. 5.1, 5.2, 5.7); 8, 9, H. ?parvus from this study (also figured in Fig. 5.3–5.6); 10, 11, H. praeparvus Kozur, 1996, 10, holotype, from Kozur, 1996 (pl. 2,Fig. 2), 11, from Meishan D section (Nicoll et al., 2002, Fig. 17: 1d).

80 Y. Zhang et al. / Journal of Asian Earth Sciences 80 (2014) 75–83

H. parvus and the different views currently held by different work-ers as to how these two species should be distinguished one fromanother in an apparent continuum of morphological variations(e.g., see discussion in Kozur, 1996, pp 93, 94; Orchard and Krystyn,1998, p. 352). In our material, two specimens, LZ-C-280223(Figs. 5.1, 5.2 and 6.6) and NIGP159237 (Figs. 5.7 and 6.7), bothfrom Bed 28b, appear to be assignable to H. parvus with good con-fidence. Both specimens are characterized by possessing a highcusp whose height is more than twice that of the average heightof the denticles, broad and largely uniform denticles, and a nearlyvertical posterior margin. These characteristics are highly compa-rable with similar features in the type specimens of H. parvus asoriginally figured by Kozur and Pjatakova (1976) and Kozur(1977) (both reproduced here in Fig. 6.1 and 6.2), as well as withspecimens of H. parvus from the Meishan PTB GSSP section as fig-ured by Yin et al. (2001, here reproduced in Fig. 6.3) and by Met-calfe et al. (2001, here reproduced in Fig. 6.4). Further, we couldnot identify any discernible significant differences between thesetwo specimens and a specimen identified and figured by Metcalfe

and Nicoll (2007) as H. parvus from Bed 30 of the Zhongzhai section(reproduced here in Fig. 6.5).

However, our assignment of two other specimens, LZ-C-280224and LZ-C-280107 (Figs. 5.3–5.6 and 6.8, 6.9), to H. parvus is some-what equivocal as, arguably, they could also be classified as H.praeparvus. In several previous studies where conodont popula-tions have been examined (e.g., Mei et al., 2004; Shen and Mei,2010), it is evident that considerable intra- and inter-specific vari-ations could exist among some conodont species, suggesting aninherent challenge in conodont taxonomy at the species level.Although Hindeodus parvus is generally understood to have a highand slender cusp, an upright posterior margin and even denticles(e.g., Kozur, 1996; Orchard and Krystyn, 1998), but specimens withthese typical features tend to represent the end members of a con-tinuum of strong and continuous intra-specific variations withinthis species, and specimens with these features are usually rarein stratigraphic record (e.g., only a few reliable H. parvus have sofar been reported from the Meishan PTB GSSP section). The twospecimens under question (LZ-C-280224 and LZ-C-280107,

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Fig. 6.8 and 6.9) have nearly uniform denticles, similar to those ofH. parvus shown in Fig. 6.1 and 6.7, but their cusps are relativelylow and somewhat approach those of typical H. praeparvus(Fig. 6.10 and 6.11). These comparisons thus suggest that thesetwo specimens may well represent a transitional form betweenH. praeparvus and H. parvus, although their overall characters aremore characteristic of the latter (see Fig. 6 for detailed morpholog-ical comparisons).

Significantly, although H. parvus occurs in Bed 28, they are veryrare compared to the occurrences of H. praeparvus in the same bed(93 specimens of H. praeparvus were found, co-existing with onlytwo specimens each of H. parvus and H. ?parvus) (Table 1). Thisindicates that the conodont fauna of Bed 28 is undoubtedly domi-nated by H. praeparvus, co-existing with only very rare early H. par-vus elements. This is in sharp contrast to Bed 30 where H. parvushad become dominant, accounting for 69% of all conodonts foundin this bed, with no finding of H. praeparvus (Table 1, conodontsdata from Bed 30 are from Metcalfe and Nicoll, 2007). These simplestatistical differences in species composition and relative abun-dance between the two very different conodont populations ofBed 28 and Bed 30 suggest that significant evolution and faunalreplacement must have occurred at the population level fromBed 28 to Bed 30 (Bed 29 is an ash bed and no conodont have beenfound in this bed). Further, our study indicates that rare elementsof H. parvus could occur in a H. praeparvus-dominated populationalthough their identification may be difficult because of low abun-dance and the continuum of morphological variations between thetwo species. In this case, it is possible that Pa elements of H. parvusoccurring in the H. praeparvus-dominated populations can be mis-taken for H. praeparvus, or vice versa.

5. Discussion

At first glance, the newly discovered first local occurrence (FLO)of H. parvus may suggest that the PTB at the Zhongzhai sectioncould be placed at the base of Bed 28a, instead of at the base ofBed 30 as was previously suggested by Metcalfe and Nicoll(2007). However, this suggestion is not tenable if multiple otherlines of chronostratigraphic and biostratigraphic evidence are ta-ken into account. First, when Nicoll and Metcalfe (2005; see alsoMetcalfe and Nicoll, 2007) reported H. parvus from Bed 30 of theZhongzhai section, they also reported Merrillina ultima from Bed28 of this section. Until now, M. ultima has been considered to bea species characteristic of the Changhsingian (e.g. Kozur, 2005).Furthermore, Professor Shu-zhong Shen (personal communication,July 2013) has advised us that he had previously found Palaeofusu-lina sinenesis from Bed 28, co-existing with H. parvus. Palaeofusulinasinenesis is a critical fusuline species that defines the latestChanghsingian Palaeofusulina sinenesis Zone in South China (Shengand Jin, 1994). Thus, the occurrence of this species concurs withthat of the conodont M. ultima in suggesting that Bed 28 shouldbe assigned to latest Changhsingian, rather than earliest Triassic.This considered, it would be inappropriate to place the PTB of theZhongzhai section at the base of Bed 28a.

Second, maintaining the PTB of the Zhongzhai section at thebase of Bed 30 is also strongly corroborated by both the d13C sig-nals and the U–Pb age dating. According to Shen et al. (2011), thereis no significant change in d13C values from Bed 27 to Bed 28; how-ever, a significant and seemingly sustained negative shift occurredfrom Bed 29 through to the base of Bed 31 when a local minimumof d13C values was reached (Fig. 4 in Shen et al. (2011); reproducedhere in Fig. 3). Shen et al. (2011) correlated this significant negativeshift across Bed 30 of the Zhongzhai section with that of Bed 27 ofthe Meishan section where the GSSP of the PTB is defined. Bed 30of the Zhongzhai section is underlain by a claystone bed (Bed 29)with volcanic ash. This bed has been dated at 252.24 ± 0.13 Ma,

which is statistically indistinguishable from the calibrated PTBage of 252.17 ± 0.06 Ma at the Meishan section (Shen et al., 2011).

Additionally, in the course of this study we have also found anincomplete ammonoid specimen, Rotodiscoceras sp., from Bed 27c,as well as a bivalve species Claraia griesbachi (Bittner, 1899) fromBed 32, both shown in Fig. 3. Biostratigraphically, Rotodiscocerassp. (from Bed 27c) is the zonal taxon of the Pseudotirolites–Rotodis-coceras Zone characteristic of the late Changhsingian in South Chi-na (Zhao et al., 1978; Yang et al., 1987). On the other hand, Claraiagriesbachi from Bed 32, is a typical element of the Claraia wangi–C.griesbachi Assemblage of earliest Triassic in South China (Gao et al.,2009). Therefore, the biostratigraphic evidence from the ammo-noid and bivalve fossils also supports the placement of the Perm-ian–Triassic boundary at the base of Bed 30.

To sum up, considering the multiple lines of evidence availablefrom the Zhongzhai section as summarized above, we suggest thatthe Permian–Triassic boundary of this section be maintained at thebase of Bed 30 as originally proposed by Metcalfe and Nicoll(2007). In this case, the discovery of H. parvus in Bed 28a wouldhave to be treated as an example of this critical species havingits first local occurrence at a local section below the PTB and henceolder than the First Appearance Datum (FAD) of the same speciesat the global PTB GSSP section in Meishan.

Globally, the realization that the first local occurrences (FLO) ofH. parvus may be diachronous is not new, although no previousstudy has reported the occurrence of H. parvus below the Perm-ian–Triassic boundary. Both Baud (1996) and Shevyrev (2006)had both suggested that the occurrences of H. parvus could be dia-chronous in different sections. Hermann et al. (2010), based on theworldwide correlation of PTB carbon isotope records, also ques-tioned the worldwide synchronicity of H. parvus as an absolutetime marker. Similarly, Jiang et al. (2011) have also pointed outthe potential problem that may be caused by using the diachro-nous occurrences of H. parvus as a time marker for the base ofthe Triassic, and they accordingly proposed that the PTB at theShangsi section be placed at the middle of Bed 28a, which is about2 m below the FLO of H. parvus (Bed 29c) at this section (see alsoMundil et al., 2004, p. 1760). In a recent high-resolution cyclo-stratigraphic study of PTB sequences based on astronomical tuningof cyclic sedimentary records from several marine PTB sections inSouth China (Figs. 1 and 6), Huang et al. (2011) compared the agesof the FLOs of H. parvus, and noted the possibility that the FAD ofthis species could be distinctly diachronous across different marineenvironments (notwithstanding that the PTB astronomical timescale (ATS) derived by Huang et al. will now need to be carefullyconsidered in the light of a more recent study by Wu et al.(2013); although this does not affect Huang’s et al. observation thatthe first occurrences of H. parvus in South China are diachronous).

Theoretically, there could be several reasons why the FADs of akey index species such as H. parvus may appear diachronous instratigraphic record (see a recent discussion on this aspect by Land-ing et al. (2013) in relation to Cambrian GSSPs). First, it could be dueto strong facies control in that the timing of appearance of H. parvusin local section (or environment) may have been strongly depen-dent on the attainment of a suitable habitat for this species. Poten-tially, this may explain why the earlier occurrence of H. parvus at theZhongzhai section in a clastic-shelf environment compared to theFAD of H. parvus at the Meishan GSSP section located in an offshorecarbonate ramp setting. Second, the diachronous occurrence of H.parvus in different sections may simply be a reflection of differentialweathering and preservation as a result of erosional truncation, orthe Signor–Lipps effect (Signor and Lipps, 1982), but this effect ap-pears to be geologically negligible as far as the Zhongzhai section isconcerned because there is no physical evidence to suggest any sig-nificant physical break across the PTB beds at the section (see alsoShen et al., 2011). Third, the diachroneity for the FLOs of H. parvus

82 Y. Zhang et al. / Journal of Asian Earth Sciences 80 (2014) 75–83

between different sections (regions) could potentially be explainedby the difference in the timing of their appearance in different areasas a result of differential biogeographic dispersal rates although thisdifference may not have been very significant for H. parvus consid-ering the pelagic lifestyle for this species (Lai and Zhang, 1999).

6. Conclusions

The discovery of H. parvus from Bed 28a of the Zhongzhai sec-tion has yielded three significant stratigraphic implications. First,the first local occurrence (FLO) of H. parvus at this section is18 cm lower than the PTB currently accepted. Second, this studyhas highlighted the inadequateness in using the first occurrenceof H. parvus as an exclusive chronostratigraphic marker for defin-ing the base of the Triassic in marine settings. Instead, other auxil-iary biostratigraphic indicators as well as non-biostratigraphictools, such as d13C chemostratigraphic profile and radiometric ages,where available, must also be considered along with the firstoccurrence of H. parvus – this is a very significant point that re-cently has also been stressed by Finney (2013) as it is applicableto the application of all GSSPs. Third, although some previous stud-ies have reported the first occurrences of H. parvus above the Perm-ian–Triassic boundary in some sections, this is the first study, tothe best of our knowledge, reporting the FLO of H. parvus belowor earlier than the Permian–Triassic boundary in a regional section.

Acknowledgments

We are very grateful to Prof. Shu-zhong Shen, Dr. Hai-shui Jiangand Dr. Robert S. Nicoll for discussion on conodont taxonomy. Wethank Chengchen Du, Qiang Zhang and Wei Zhang for their help infield sampling, Fei Teng for photography. This study was supportedby Natural Science Foundation of China (Grant Nos. 41372030,40872008, 41002012/D0202), the Ministry of Education of China(B08030 of 111 Project, NCET-10-0712), the Foundation of Geolog-ical Survey of China (12120113012500) and in part a researchgrant from the Australian Research Council (to G.R. Shi). DeakinUniversity is also acknowledged for supporting this research (toY. Zhang and G.R. Shi).

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