Quantitative stratigraphic correlation of Tethyan conodonts across the Smithian-Spathian (Early Triassic) extinction event

Earth-Science Reviews 195 (2019) 37–51

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Invited review

Quantitative stratigraphic correlation of Tethyan conodonts across the Smithian-Spathian (Early Triassic) extinction event

T

Yanlong Chena,b, Sylvain Richozb,c, Leopold Krystynd, Zhifei Zhanga,

⁎

a

Shaanxi Key Laboratory of Early Life and Environments, State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi'an 710069, China b Institute of Earth Sciences, University of Graz, Heinrichstrasse 26, 8010 Graz, Austria c Department of Geology, Sölvegatan 12, 22362 Lund, Sweden d Department of Palaeontology, University of Vienna, Althanstraße 14, 1090 Vienna, Austria

ARTICLE INFO

ABSTRACT

Keywords: Tethys Biostratigraphy Oman South China Slovenia Unitary association

Three small-scale extinctions occurred in the Early Triassic with one of them recognized close to the SmithianSpathian boundary. In the last two decades, the end-Permian mass extinction as well as the subsequent recovery have been intensively studied throughout the Tethys region, but correlations within the Lower Triassic are difficult due to conodonts endemism. Here we use paleontological and geochemical methods to document a high-resolution biostratigraphy of the Smithian-Spathian boundary interval from two sections of Oman. In combination with previously published data from both South-Central Europe and South China, a quantitative stratigraphic correlation has been achieved with 7 conodont UA Zones recognized using the unitary association method. Based on conodonts and carbonate carbon isotope data, the Smithian-Spathian boundary is identified in the interval from UAZ4 to UAZ5 close to the last occurrence of Nv. pingdingshanensis in Oman and South China, and within the range of P. inclinata, Ns. planus, Pl. regularis, and Pl. corniger in South-Central Europe. UAZ7 fauna displays a clear diachronism as it starts from South China, arrives a bit later in Oman and even later in western Tethys. Foliella gardenae and Icriospathodus zaksi are reported from Oman for the first time and thus expand the geographical distribution of these rarely reported species.

1. Introduction It has been proposed that the extinction events in the geological record can provide information for conservation biology, and can guide humans to manage climate warming and human-induced losses of species (Ceballos et al., 2015; Payne et al., 2016). The end-Permian and Early Triassic strata provide a unique chance to study mass extinctions and the following delayed recovery. The end-Permian mass extinction is the largest mass extinction in the Phanerozoic (Erwin, 1996; Payne and Clapham, 2012), leading to the extinction of 79% marine invertebrate genera and similarly affecting life on land (Payne and Clapham, 2012). The environmental conditions were poor for life for approximately 5 million years following the end-Permian extinction (Payne et al., 2004; Sun et al., 2012; Clarkson et al., 2016), called as the Early Triassic “cesspool” (Algeo, 2011). The Early Triassic prolonged recovery (Erwin et al., 2002; Chen and Benton, 2012) is characterized by a global coal gap, chert gap, coral gap, elevated weathering rates and the peculiar

occurrence of microbialites (Baud et al., 2007; Algeo and Twitchett, 2010; Brayard et al., 2011; Chen and Benton, 2012; Algeo et al., 2013; Metcalfe et al., 2013; Vennin et al., 2015). Three small-scale extinctions (Stanley, 2009; Brayard et al., 2009), which may be responsible for the prolonged delay until the full recovery from the end-Permian mass extinction, occurred in the Early Triassic, and one of them was firstly recognized as close to the Smithian-Spathian boundary by Hallam and Wignall (1997). The extinction was subsequently documented as associated with extreme climate warming (Joachimski et al., 2012; Sun et al., 2012; Romano et al., 2013; Zhang et al., 2015), anoxia (Sun et al., 2015; Clarkson et al., 2016), positive shift of δ13Ccarb values (Payne et al., 2004; Richoz, 2006; Sun et al., 2015), and conodont size reduction (Chen et al., 2013; Maekawa and Komatsu, 2014). These previous studies highlight the Smithian-Spathian as a key interval for studying the delayed recovery from the end-Permian mass extinction. The compilation of palaeontological data within a precise time scale

⁎ Corresponding author at: Shaanxi Key Laboratory of Early Life and Environments, State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi'an 710069, China. E-mail address: [email protected] (Z. Zhang).

https://doi.org/10.1016/j.earscirev.2019.03.004 Received 26 December 2017; Received in revised form 1 March 2019; Accepted 4 March 2019 Available online 19 March 2019 0012-8252/ © 2019 Elsevier B.V. All rights reserved.

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from a wide geographical area is crucial for a better understanding of the Smithian-Spathian extinction event. However, the construction of a precise time scale using biostratigraphic data can be hampered by ecological control and/or biogeographical exclusions of taxa (Guex et al., 2015). This problem is especially true for the Smithian-Spathian conodonts, which show notable endemism. (cf. Staesche, 1964; Chen et al., 2013, 2015, 2016; Maekawa and Komatsu, 2014; Krystyn et al., 2007; Zhao et al., 2007).

et al., 2013, 2015, 2016; Brosse et al., 2018). From the Permian to Cretaceous, the Arabian shelf formed an epeiric platform at the northern Gondwana margin and a wide belt of shelf carbonates has been deposited on most of the south-eastern Arabian Plate (Fig. 1A; AlJallal, 1995; Insalaco et al., 2006). The platform has a ramp-geometry, characterized by subtidal to supratidal limestones, dolostones and evaporites with prominent carbonate shoal complexes (e.g., Al-Jallal, 1995; Insalaco et al., 2006). During the Late Cretaceous, the overthrusting of the Semail ophiolite carried large pieces of the Neo-Tethyan margin onto the autochthonous shelf succession (Fig. 1B). A variety of depositional settings, including continental slope deposits, proximal and distal basinal deposits, and offshore highs (Bernoulli and Weissert, 1987; Béchennec et al., 1988; Bernoulli et al., 1990; Watts, 1990; Pillevuit et al., 1997; Woods and Baud, 2008; Richoz et al., 2005; Richoz et al., 2010a, 2014; Baud et al., 2012) are now preserved in the Oman Mountains. Both sections studied here were deposited in the proximal Hawasina basin. Thin tectonic slices of the Hawasina nappes are squeezed between the Sumeini nappes (slope deposit) and the Semail ophiolite, cropping out ~700 km along the Oman Mountains (Bernoulli and Weissert, 1987; Béchennec et al., 1990; Pillevuit et al., 1997; Wohlwend et al., 2017). The Radio Tower section (23°03′11.3″ N; 58°17′16.1″ E) is located on the western flank of a small hill supporting a radio tower and overhanging the road linking Wadi Ta'yin to Wadi Rahbah (Richoz et al., 2014; Clarkson et al., 2016). This section includes the mid-Smithian to Anisian part of the Al Jil Formation (Béchennec et al., 1988; Béchennec et al., 1990) and consists of about 40 m of distal turbidite deposits of grey platy limestones and thin shale or marlstone interbeds, representing a distal slope setting. The Wadi Bani Khalid section (22°36′29.45″N; 59° 5′17.13″E), around 100 km ESE of Radio Tower section, is outcropping along the road to the Wadi Bani Khalid pools. It consists of about 46 m thick basinal deposits of the Al Jil Formation. The lower part of this section mainly exposes shales, and the upper part consists of shales, with interbedded calcarenitic to oolitic limestones, and breccias (Fig. 2).

1.1. Conodonts as a critical tool for biostratigraphic correlation Conodonts have been the prime focus of studies on biostratigraphy, paleobiogeography, paleoecology, paleoclimatology, and evolutionary dynamics for more than half a century, because of their wide geographic distribution, fast evolution, and high quantity and quality in the geological record. Conodont species have been proposed as proxies for sub-/stage boundaries for the Early Triassic (e.g., Krystyn et al., 2007; Zhao et al., 2007; Orchard, 2010; Goudemand, 2014). The earliest documentation of Smithian-Spathian conodonts dates back to 1956 in western USA (Müller, 1956), where Smithian species, such as Scythogondolella milleri (Müller); Discretella discreta (Müller), were recovered. Starting with the pioneering work of Staesche (1964), the conodont biostratigraphic framework of the Tethys has become well-established. However, major differences occur between different Tethyan regions. For example, Platyvillosus regularis (Budurov & Pantić) and Foliella gardenae (Staesche) have been frequently reported from Central-South Europe but rarely recovered from other regions of the Tethys. On the other hand, Novispathodus waageni (Sweet), Nv. pingdingshanensis and Icriospathodus collinsoni (Solien) are common in South China (Zhao et al., 2007; Chen et al., 2015), however, these species have never been reported in Central-South Europe. Absence of Novispathodus waageni and Nv. pingdingshanensis has thus prevented a clearcut localisation of the base of the Olenekian stage and that of the Spathian substage there. Moreover, biostratigraphic data are sporadic due to the rarity of fossils recorded in the sediments and often poorly exposed outcrops. For example, due to extensive vegetation and faulting, the Slovenian Smithian-Spathian strata are only in part exposed and the obtained conodont record is discontinous (Chen et al., 2016), making it difficult to construct biochronologic time-scales and worldwide correlations. The first systematic documentation of Early Triassic conodonts from Oman was published by Orchard (1995), who recognized seven new species. However, the biostratigraphy of the Smithian-Spathian boundary interval has not been reported in detail there. Here we provide a high-resolution conodont biostratigraphy across the SmithianSpathian boundary in Oman. We applied unitary association method for the first time to correlate sections from western, southern, and eastern margins of the Tethys based on conodont biostratigraphy.

2.2. Sections of Central-South Europe Sections of Žiri area (Žiri-sortirnica 28, Žiri-road cut 29, Žiri-Vrsnik 61, and Golob 44; Fig. 2) and Mokrice are located in the central and the easternmost part of Slovenia, respectively (Fig. 1; cf. Chen et al., 2016), whereas the Krivi Potok section is located in Serbia (Sudar et al., 2014). Žiri area is part of the geotectonic unit of the External Dinarides, while the Krivi Potok section is located in the Internal Dinarides. The lower Triassic sediments of Mokrice belong to the transitional region between the External and Internal Dinarides. In the Early Triassic period, the Dinarides were part of an extensive carbonate platform (Hips and Pelikan, 2002) or epeiric ramp (Aljinović et al., 2018) on the western margin of Tethys.

2. Geological settings In this study, 9 sections are chosen for the unitary association analysis representing typical sedimentary environments from eastern, western and southern margin of the Tethys. Two sections are in Oman (Radio Tower, and Wadi Bani Khalid; Fig. 1), five sections are in Slovenia (Žiri-sortirnica 28, Žiri-road cut 29, Žiri-Vrsnik 61, Golob 44, and Mokrice; Fig. 1C), one section in Serbia (Krivi Potok) and one in South China (Jiarong; Fig. 1D). Mokrice and Krivi Potok section overlap with the sections in Žiri area, and are not shown in Fig. 2.

2.3. Sections of Nanpanjiang basin During the Early Triassic, the vast shallow-marine carbonate Yangtze Platform of the South China tectonic block was located on the eastern margin of the Tethys. The Nanpanjiang basin was a deepmarine embayment situated on the southern margin of the Yangtze Platform. Plenty of sections are exposed in the Nanpanjiang basin ranging from platform interior to basin. Jiarong section (Fig. 2) was located in the northern Nanpanjiang basin, approximately midway between the Yangtze Platform and the isolated carbonate platform Great Bank of Guizhou (Fig. 1D). The Jiarong section is one of the best documented sections for Smithian and Spathian conodonts within the Nanpanjiang basin (Chen et al., 2013, 2015).

2.1. Sections of Oman In the last two decades, the Arabian peninsula has been an area of intensive study of the end-Permian mass extinction and recovery (e.g, Krystyn et al., 2003; Twitchett et al., 2004; Richoz et al., 2005; Richoz, 2006; Richoz et al., 2010a, 2010b; Brühwiler et al., 2012; Clarkson 38

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Fig. 1. Location of studied sections. (A) Palaeogeography of the Early Triassic world showing the location of Slovenia, Oman and South China (after Romano et al., 2013); (B) simplified geological map of Oman with studied Radio Tower (RT) and Wadi Bani Khalid (WBK) sections (after Richoz et al., 2014); (C) Geotectonic units in Slovenia and its adjacent area with indication of the Žiri area and the Mokrice section (After Chen et al., 2016); (D) Early Triassic paleogeographical reconstruction of Nanpanjiang basin with indication of Jiarong section (after Lehrmann et al., 2006; Chen et al., 2015).

3. Material and methods

insoluble residues were sieved using meshes of 1 mm and 64 μm. Conodonts in the residues were enriched by heavy liquid separation using sodium polytungstate (ca. 2.81 g/cm3) and finally picked from the heavy fraction using a wet brush pen under a binocular microscope. For taxonomic identification, the conodont elements were further studied using a secondary electron microscope (SEM; Zeiss DSM 982 Gemini at Graz University; see Figs. 3–7).

3.1. Previously published data Conodont biostratigraphy of the Smithian and Spathian of southern Europe has been greatly clarified in the past few years. Several short but fossiliferous sections show well documented conodonts in Žiri-sortirnica 28, Žiri-road cut 29, Žiri-Vrsnik 61, Golob 44, Mokrice, and Krivi Potok (Kolar-Jurkovšek et al., 2014; Sudar et al., 2014; Chen et al., 2016). The Smithian and Spathian conodonts from both Chaohu (Zhao et al., 2007) and Jiarong (Chen et al., 2013, 2015) areas represent the best documented conodont biostratigraphy in South China. Conodonts of these two areas showed a high level of similarity (cf. Zhao et al., 2007; Chen et al., 2013, 2015), thus only one of these sections (Jiarong) was chosen as an example for the unitary association analysis. The data from these publications were compiled in a format readable for the free software PAST (Hammer et al., 2001).

3.3. Unitary association analysis The fossil record is often incomplete and the first or last local occurrence of a species can be diachronous, hampering biostratigraphic correlations. The Unitary Association (UA) method is a unique deterministic mathematical model designed to construct a discrete sequence of coexistence intervals of species based on maximal association zones allowing stratigraphers to identify contradictions in the biostratigraphic data, and to recognize UAs unambiguously in stratigraphic sections (Guex, 1991; Galster et al., 2010). The method has been developed and applied in a number of biostratigraphic studies for the Permian and Triassic (e.g. Xiao et al., 2018; Brosse et al., 2016; Chen et al., 2016; Chen and Lukeneder, 2017; Monnet et al., 2015; Angiolini and Bucher, 1999) using the palaeontological analysis free software PAST (Hammer et al., 2001). In this study, the Unitary Association method was applied to

3.2. Conodont sampling and extraction In total, 45 rock samples were collected for conodonts extraction, 24 samples from the Radio Tower (RT) section and 21 from the Wadi Bani Khalid (WBK) section. Rock samples were crashed into small pieces, put into plastic buckets and dissolved in diluted formic acid (ca. 10%). The 39

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Fig. 2. Stratigraphic columns, conodont ranges, biostratigraphical correlations and Carbonate carbon isotope (δ13Ccarb) of sections from Oman (partly after Clarkson et al., 2016), Slovenia (after Chen et al., 2016) and South China (after Chen et al., 2015). Abbreviations: Eu. = Eurygnathodus; Sc. = Scythogondolella; Nv. = Novispathodus; Ng. = Neogondolella; Ns. = Neospathodus; Tr. = Triassospathodus; Ne. = Neostrachanognathus; Ic. = Icriospathodus.

Dienerian Golob 44

Ns. sp. indeterminate Pa. inclinata Ns. planus Ns. robustus

Eu. costatus Eu. hamadai

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Sc. lachrymiformis Foliella gardenae

Tr. symmetricus Sc. milleri Tr. triangularis Nv. pingdingshanensis Ng. ex. gr. jakutensis Ns. curtatus

Nv. waagani Discretella discreta Sc. mosheri Sc. milleri Nv. pingdingshanensis Ng. ex. gr. jakutensis Borinella aff. buurensis Ic. zaksi Foliella gardenae Ng.? n. sp. A Ic. crassatus

Unitary Associations Unitary Association Zones

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Ns. dieneri Eu. costatus

Ns.brevissimus Nv pingdingshanensis Ng sp. Ng. sp. Ic. collinsoni Spathicuspus spathi Tr.symmetricus Ic.crassatus

Discretella discreta Parachirognathus peculiaris Pachycladina inclinata Parachirognathus ethingtoni

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Fig. 3. SEM photos of conodonts from Radio Tower section and Wadi Bani Khalid sections, Oman. 1, 4 & 12, Scythogondolella lachrymiformisOrchard, 2008, 1 from rock sample RT15/14, 4 and 12 from RT20; 2, Scythogondolella mosheri (Kozur & Mostler, 1976), from rock sample RT15/14; 3, Scythogondolella milleri (Müller, 1956), from rock sample RT20; 5–9, Novispathodus pingdingshanensis (Zhao & Orchard, 2007), 5–6 from RT20; 7–9 from rock sample RT25b; 10–11, early ontogenetic stage of Scythogondolella milleri (Müller, 1956), from rock sample RT20; 13, Icriospathodus crassatus (Orchard, 1995), from rock sample RT25c; 14, Triassospathodus brochus (Orchard, 1995), from rock sample RT25c; 15,Triassospathodus symmetricus (Orchard, 1995), from rock sample RT25c; 16–17, Foliella gardenae (Staesche, 1964), 16 from rock sample WBK05/15, 17 from rock sample RT15/3; 18–19, Novispathodus waageni (Sweet 1970), 18 from rock sample RT15/3, 19 from rock sample RT18.

recognize conodont zones and correlate sections of Oman (this study), South-Central Europe (Sudar et al., 2014; Chen et al., 2016) and South China (Chen et al., 2015), which represent fauna from southern, western and eastern Tethys, respectively. Because the Unitary Association

analysis is a method for biostratigraphic correlation, species which were only found in one of these Tethyan sections (e.g., Icriospathodus zaksi), were removed during the construction of the zonation. In total, 27 species were identified as useful for correlations in these sections 41

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Fig. 4. SEM photos of conodonts from rock sample RT25c, Radio Tower section, Oman. 1–6, Neogondolella ex. gr. jakutensis Dagis, 1984, all middle ontogenetic stage; 7, Novispathodus abruptus (Orchard, 1995); 8, 11–13, Triassospathodus symmetricus (Orchard, 1995); 9–10, 14, Neogondolella ex. gr. jakutensis Dagis, 1984, 9 and 14 are early ontogenetic stage; 15, early ontogenetic stage of Triassospathodus symmetricus? (Orchard, 1995).

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Fig. 5. SEM photos of conodonts from Radio Tower and Wadi Bani Khalid sections, Oman. 1–2, Neogondolella ex. gr. jakutensis Dagis, 1984, from RT28, 1 is late ontogenetic stage; 2 is middle ontogenetic stage; 3, Neogondolella n. sp. B, from RT28; 4 and 8, Triassospathodus symmetricus (Orchard, 1995), from RT28; 5, Icriospathodus crassatus (Orchard, 1995), from RT28; 6, Novispathodus pingdingshanensis (Zhao et al., 2007), from WBK10; 7 and 9, Aduncodina unicosta Ding, 1983, from WBK11.

(Fig. 8A and B; see Supplementary material).

4.2. Unitary association analysis 4.2.1. The first analysis The analysis provides 19 residual horizons, 14 maximal cliques, 13 Unitary Associations (Fig. 9A), 4 contradictions, and 0 cliques in cycles (Fig. 8A and B). The fact that there is no clique in cycles indicates that the conodont biostratigraphic data are of good quality (Guex, 1991). However, contradictions reveal conflicts of data between sections. One S3 forbidden sub-graphs were detected during the first analysis (Fig. 8C) involving 3 species: F. gardenae, Tr. symmetricus, Ns. planus (Fig. 8C). Three S4’ forbidden sub-graphs were revealed by the biostratigraphic graph G* (Fig. 8E), involving 6 species: F. gardenae, Sc. milleri, Ng. ex. gr. jakutensis, Tr. symmetricus, N. tahoensis, and Tr. hungaricus. In total, 8 arcs are found between these species. At least some of these arcs must

4. Results 4.1. Conodonts In total, ca. 4640 well-preserved P1 elements were obtained from RT and WBK sections, Oman, including the worldwide rarely reported species Foliella gardenae (Staesche, 1964) and Icriospathodus zaksi (Buryi, 1979). Within these two Omani sections, the fauna is differentiated by the absence of Ic. collinsoni and Ad. unicosta at RT section, and their abundance in the WBK section (Fig. 2). In both sections, late Smithian conodonts are rare, but on the contrary, an abundant and relatively diverse conodont fauna is observed in the Spathian. 43

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Fig. 6. SEM photos of conodonts from Radio Tower and Wadi Bani Khalid sections, Oman. 1, Icriospathodus crassatus (Orchard, 1995), from RT28; 2, Neospathodus ex. gr. planus Chen and Kolar-Jurkovšek, 2016, from RT28; 3, Icriospathodus crassatus (Orchard, 1995), from RT28; 4, Icriospathodus zaksi (Buryi, 1997), from WBK08/8; 5, Neostrachanognathus tahoensis, Koike, 1998, from RT12/2; 6, Aduncodina unicosta Ding, 1983, from WBK11; 7, Icriospathodus crassatus (Orchard, 1995), from WBK05/15; 8–9, Neogondolella? n. sp. A, from WBK05/15; 10–12, Icriospathodus collinsoni Solien, 1979, from WBK05/15; 13–14, Neospathodus curtatus Orchard, 1995, from WBK05/15; 15, Neospathodus brevissimus Orchard, 1995, from WBK11.

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Fig. 7. SEM photos of conodonts from Radio Tower and Wadi Bani Khalid sections, Oman. 1, Triassospathodus homeri (Bender, 1970), from RT40; 2–3, Triassospathodus triangularis (Bender, 1968), 2 from RT38, 3 from BK14; 4–5, Neostrachanognathus tahoensis, Koike, 1998, from Bk14; 6–8, Spathicuspus spathi (Sweet, 1970), all from BK14; 9–10, Icriospathodus zaksi (Buryi, 1997), from WBK08/8.

result from insufficient sampling and/or the ecological control over the distribution of the faunas or biogeographical exclusions of taxa. Here, we use an empirical solution to solve the forbidden sub-graphs by replacing one of these two arcs with a virtual edge in both S3 and S4’ forbidden sub-graphs (Fig. 8D and F). The S3 forbidden sub-graph (Fig. 8C) involves F. gardenae which was believed as an end-Smithian species (Orchard, 2007) in Europe based on published data (Sudar et al., 2014; Chen et al., 2016) and new findings from Plavno section, Croatia (Aljinović et al., 2018). However, our new findings from Oman indicate that this species co-occurs with early Spathian Tr. symmetricus, which occurs higher than Ns. planus in Slovenia (Figs. 2 and 8C). Thus, here we break the arc between F. gardenae and Ns. planus by extending the stratigraphic range of F. gardenae to the interval of Ns. planus (Fig. 8D). All four S4’ forbidden sub-graphs involve Tr. hungaricus, thus we extend the last occurrence of Tr. hungaricus to sample 28c/3 in Slovenia so that it co-occurs with Tr. symmetricus (see supplementary material). Indeed in this sample, which is the FO of Tr. symmetricus, we found Tr. ex. gr. hungaricus. The revision has successfully resolved all the contradictions within these three S4’ forbidden sub-graphs (Fig. 8F).

Normally, some of these Unitary Associations (UAs), which have a very low geographical reproducibility, should be merged with one of their adjacent UAs to increase the ability to correlate between sections. The software PAST gives a reproducibility matrix indicating that UA9 and UA11 were only recognized from Radio Tower and UA10 was only recognized from Wadi Bani Khalid, and thus should be merged with adjacent UAs (Fig. 10A). Based on the reproducibility, the dissimilarity and real arcs between UAs (Fig. 10B; see supplementary material), we decided to merge UA4 with UA3 (D = 1, 1 arc), UA6 with UA5 (D = 0.4762, 3 arcs), UA8 with UA7 (D = 0.2857, 1 arc), UA9 with UA8 (D = 0.2857, 1 arc), and UA11 with UA10 (D = 0.2857, 1 arc). 4.2.3. Biozones The empirical revision of the contradictory biostratigraphic data has resulted in a high resolution of biozone, which can be used as a biochronologic scale. However, the UA9, UA10 and UA11 were only recognized from single sections (Fig. 2; see Supplementary material), thus these unitary associations provide little information for the correlation of these sections. The number of characteristic species of each UA varies from 2 to 9. The merging of UAs with low lateral reproducibility and low dissimilarity has resulted in seven unitary association zones (UAZs; Figs. 2, 9C and 10C), which are recognized as biozones herein. All these biozones are discrete, and the upper and lower boundary of each zone may extend upward or downward respectively in case of further sampling.

4.2.2. The second analysis Using the revised data (see Supplementary material), the second analysis gives 21 residual horizons, 15 maximal cliques, 12 Unitary Associations (Fig. 9B), 0 contradictions, and 0 cliques in cycles. The empirical revision of the original data has successfully solved the contradictions. Comparing to the first analysis, the resolution of the conodont biochronological scheme decreased by one Unitary Association (Fig. 9).

UAZ1. Content: E.hamadai, E.costatus, Ns. dieneri, and Nv. waageni. Characteristic species: E. hamadai, E. costatus and Ns. dieneri. Stratigraphic and geographical distribution: known from sample 45

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Fig. 8. The result of the first unitary association analysis of the original biostratigraphical data and solutions to contradictions. (A) the coexistence graph of the 27 species; (B) the superposition graph of the 27 species, with solid lines from the above-occurring taxon and dashes from the below-occurring taxon; (C and D), S3 forbidden sub-graph, contradictive stratigraphical distributions and the solution to contradiction; (E and F) S4’ forbidden sub-graphs, contradictive stratigraphical distributions and a solution to these contradictions. The dotted lines in (D and F) indicate empirical revision of stratigraphical distributions. Abbreviations: JR = Jiarong; RT = Radio Tower; SL = Slovenia; WBK = Wadi Bani Khalid.

JR/03 to sample JR/04 at Jiarong section, South China; from 44 g/1 to 44 g/13 at Golob 44 section, Slovenia (see Supplementary material).

UAZ4. Content: Sc. milleri, Sc. lachrymiformis, Bo. aff. buurensis, Ns. planus, Ns. robustus, Ns.brevissimus, Nv. pingdingshanensis, Nv. abruptus, Ng. ex. gr. jakutensis, Ng.? n. sp. A (Fig. 6.8–9), Ic. crassatus, Ic. zaksi and F. gardenae. Characteristic species: Bo. aff. buurensis, Nv. pingdingshanensis. Stratigraphic and geographical distribution: recognized from rock sample JRC/47 to JC/5 at Jiarong section, South China; from WBK4/04 to WBK08/11 at Wadi Bani Khalid section, Oman; from RT20 to RT25b at Radio Tower section, Omanand Krivi Potok section, Serbia. Ng.? n. sp. A (Fig. 6.8–9) of Oman is very similar to the Spathian Ng.? n. sp. A reported from Jiarong section (Chen et al., 2013, 2015) and other regions (e.g., Orchard, 2007).

UAZ2. Content: Nv. waageni and Dis. discreta. Characteristic species: Dis. discrete. Stratigraphic and geographical distribution: recognized from rock sample JRC/21 to JRC/39 at the Jiarong section, South China; and from WBK/1 at the Wadi Bani Khalid section, Oman. UAZ3. Content: Nv. waageni, Sc. mosheri, Sc. milleri. Sc. lachrymiformis, F. gardenae, Parachirognathus peculiaris, Pachycladina inclinata, P. obliqua. Characteristic species: Sc. mosheri, Pachycladina inclinata, P. obliqua, Parachirognathus peculiaris. Stratigraphic and geographical distribution: recognized from rock sample WBK08/3 at Wadi Bani Khalid section, Oman; from rock sample RT15/3 to RT18 at Radio Tower section, Oman; from JRC/40 to JB/09 at Jiarong section, South China; Žiri-road cut 29 section, Slovenia; and Krivi Potok section, Serbia (see Supplementary material).

UAZ5. Content: Bo. aff. buurensis, F. gardenae, Ic. crassatus, Ng. ex. gr. jakutensis, Nv. abruptus, Pl. regularis, Pl. corniger, Tr. symmetricus, N. tahoensis, Sc. milleri. Characteristic species: Pl. regularis, Pl. corniger. Stratigraphic and geographical distribution: recognized from Radio Tower, Oman, Mokrice and Žiri-sortirnica 28, Slovenia. 46

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Fig. 9. Results of Unitary Association anlysis and compacted Unitary Association Zones. (A) the Unitary Associations recognized by the first analysis; (B) the Unitary Associations revealed by the second analysis; (C) Unitary Association Zones (UAZ) compacted with the Unitary Associations of the second analysis. Blank squares indicate discontinuities; taxa of C correspond to B.

UAZ6. Content: F. gardenae, Ic. crassatus, Ic. collinsoni, Ng.? n. sp. A, Tr. symmetricus, Ns. curtatus, Ns.brevissimus, N. tahoensis. Characteristic species: Ns. curtatus. Stratigraphic and geographical distribution: recognized from Radio Tower and Wadi Bani Khalid, Oman.

Tr. homeri. Characteristic species: Tr. hungaricus, Tr. homeri, Tr. triangularis, Sp. spathi. Stratigraphic and geographical distribution: recognized from sample JRC/51 to JR/13, Jiarong, South China; Mokrice and Žiri-sortirnica 28, Slovenia; from WBK12 to WBK14, Wadi Bani Khalid, and from RT38 to RT12/2, Radio Tower, Oman.

UAZ7. Content: Ic. crassatus, Ic. collinsoni, N. tahoensis, Ng.? n. sp. A, Tr. triangularis, Nv. abruptus, Sp. spathi, Tr. symmetricus, Tr. hungaricus, and 47

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Fig. 10. Strategies for merging Unitary Associations (UAs) into Unitary Associations Zones (UAZs). (A) reproducibility matrix of the UAs in sections with ranges of uncertainties shown in white squares; (B) dissimilarity and real arcs between UAs; (C) reproducibility matrix of the UAZs in sections.

5. Discussion

only the earliest stage of the excursion due to an observation gap between Žiri-road cut 29 and Žiri-Vrsnik 61 (Fig. 2). UAZ5 and UAZ 6 were not recognized from Jiarong, South China, needing too further study similarly to Slovenia. UAZ4 in both Oman and South China are characterized by Nv. pingdingshanensis, and correlate thus partly with the Nv. pingdingshanensis range Zone recognized in several sections in South China, northern Vietnam and North America (Zhao et al., 2007, 2013; Orchard and Zonneveld, 2009; Chen et al., 2013, 2015). The UAZ5 recognized from the Slovenian area correlates with the negative shift of the δ13C values within the UAZ5 in the Radio Tower section, and correlate with the negative shift of the δ13C values within the uppermost interval between UAZ4 and UAZ7 in Jiarong section, representing the very beginning of the negative shift of the δ13C values. If the carbon isotope curve is taken as reference, UAZ7 in South China seems to be earlier than in Oman and Slovenia as the base of UAZ7 in Jiarong is characterized by a rapid negative δ13C excursion, whereas UAZ7 in both Slovenia and Oman are characterized by relatively steady value of δ13C. This suggests that during this time Slovenia is occupied by endemic fauna, which will be replaced by the more cosmopolitan species of UAZ7 with a delay. In Oman, due to few data in the interval between the top of UAZ6 and the base of UAZ7, the latter could be extended downward until the top of UAZ6, and could then start as well during the δ13C values decrease. This seems to imply that the UAZ7 fauna starts from South China, arrives a bit later in Oman and even later in western Tethys where it replaces the mainly endemic fauna of UAZ5 in Slovenia. This phenomenon could be the results of ecological control and/or biogeographical barriers (Guex et al., 2015).

5.1. Conodont biostratigraphy and correlations Sections from Radio Tower, Wadi Bani Khalid, Slovenia, Serbia and South China are correlated using the Unitary Association method, which is especially suitable for analysing comprehensive and biostratigraphically complicated datasets. As example, Nv. waageni and Sc. mosheri are characteristic species, which are only found together in UAZ3, and thus facilitate the correlation between Radio Tower and Wadi Bani Khalid sections. Nv. waageni was also reported from Western Australian (Metcalfe et al., 2013) and Spiti, India (Krystyn et al., 2004). However, Nv. waageni has a very long stratigraphic range, from basal Smithian to upper Smithian or even to basal Spathian (e.g., Zhao et al., 2007), thus this species alone can provide only a coarse correlation between sections. With the aid of unitary association analysis, three Unitary Association Zones, UAZ1, UAZ2 and UAZ3, are recognized within the range of Nv. waageni and increasing distinctly the biostratigraphic resolution of the previously reported Nv. waageni range zone (e.g., Zhao et al., 2007). The worldwide recognized positive carbonate carbon isotope (δ13C) excursion at the end-Smithian (Payne, 2004; Richoz, 2006; Brühwiler et al., 2009; Sun et al., 2015) is well documented in the four localities presented in Fig. 2. The increase starts during UAZ3 (Slovenia and South China) or in the interval between UAZ3 and 4 (Oman). The maximum of the positive excursion occurs in UAZ3 in Slovenia, in UAZ5 (Radio Tower) and UAZ4 (Wadi Bani Kahlid) and between UAZ4 and UAZ7 in South China. A large discrepancy concerning the peak value of δ13C in the UAZs of the two Omani sections appears, despite their relative proximity. This discrepancy can be explained by the absence of the rare Ng.? n. sp. A and a later inset of Ic. crassatus in Radio Tower section compared to Wadi Bani Khalid. An ecological control on Icriospathodus species seems to appear even on such relative short distance, supported by the large amount of Ic. collinsoni recovered from Wadi Bani Khalid, comparing to their total absence in Radio Tower. If we consider rather the LO of Nv. pingdingshanensis as a more reliable possible upper limit for UAZ4 in Oman, the discrepancy is then no more so important and can also be explained by small gaps or insufficient δ13C resolution. The maximum of the positive excursion seems to occur in UAZ3 in Slovenia, however, this positive δ13C excursion probably represents

5.2. The Smithian/Spathian boundary (SSB) The Smithian/Spathian boundary has still no generally accepted definition (see the review in this volume by Zhang et al., 2019). Different workers have employed differing working definitions, some based on ammonoids (e.g., Komatsu et al., 2016), and others on conodonts, (e.g., Zhao et al., 2007). The worldwide documented positive carbon isotope excursion (e.g., Payne et al., 2004; Richoz, 2006; Brühwiler et al., 2009; Sun et al., 2015; Chen et al., 2016; Clarkson et al., 2016) was previously suggested to be synchronous with the SSB, based on ammonoid biostratigraphy (Brühwiler et al., 2009). Since conodonts have a wider geographical distribution than ammonoids and can be abundant in strata with no ammonoids, palaeontologists were 48

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also trying to find a conodont marker for the SSB. The small sized species Nv. pingdingshanensis has been worldwide recognized around the SSB (Zhao et al., 2007; Orchard and Zonneveld, 2009; Chen et al., 2013), leading to the suggestion that the first occurrence (FO) of Nv. pingdingshanensis is at the base of Spathian (Liang et al., 2011). However, studies based on ammonoids, conodonts and carbon isotopes in northeastern Vietnam indicate that the ammonoid-based SmithianSpathian boundary sensu the FO of the genus Tirolites is higher than the FO of Nv. pingdingshanensis, and occurs immediately after the peak value of the positive carbon isotope excursion (Komatsu et al., 2016; Leu et al., 2019; Goudemand et al., 2019; Zhang et al., 2019), supporting that the FO of Nv. pingdingshanensis could be in the late Smithian (Orchard and Zonneveld, 2009). In both Omani sections and the Jiarong section, South China, the FOs of Nv. pingdingshanensis are around or just below the onset of the positive carbon isotope excursion, indicating that these FOs are late Smithian in age. We consider the positive carbon isotope maximum as being late Smithian in age based on ammonoid biostratigraphy (e.g. Leu et al., 2019; Komatsu et al., 2016; Brühwiler et al., 2011) and that UAZ3 has a typical Smithian conondont fauna and UAZ7 a typical Spathian one. We suggest thus that the SSB should lies within the interval from the base of UA6 in UAZ4 to the base UAZ5 (in Oman and Slovenia), or around the top of the undefined interval between UAZ4 and UAZ7 in Jiarong, South China, immediately above the positive maximum of the carbon isotope excursion. This means that the SSB lies higher than the last occurrence of Nv. pingdingshanensis in Oman and Jiarong, South China.

2015), but Ng. n. sp. B has been known only in Jiarong.

5.3. Comparison of conodont faunas

Acknowledgements

The biochronological scales generated by the unitary association analysis has enabled the comparison of contemporaneous faunas within the Tethys region. In general, the Omani Smithian-Spathian conodont association show similarities not only with the South China sections from the eastern Tethys margin, but also with South-Central European sections from the western Tethys margin, Arctic and western North American sections from the Panthalassa. Both Omani and European sections share Foliella gardenae, which was previously only reported from Central-South Europe (e.g, Kolar-Jurkovšek and Jurkovšek, 1996; Chen et al., 2016). With the new data from Oman, the geographic distribution of Foliella gardenae can be extended to the southern margin of the Tethys. UAZ1, characterized by Eu.costatus and Eu.hamadai, lacks Neospathodus dieneri, and Neospathodus cristagalli in Slovenia (Fig. 2; Fig. 9C). Scythogondolella is occurring in few sections from South China and north Vietnam, (e.g., Zhao et al., 2007, 2013; Yan et al., 2013; Maekawa and Komatsu, 2014; Chen et al., 2015), whereas species of this genus are abundant in Omani sections in UAZ3, UAZ4 and UAZ5, as well as sections of North America (Orchard and Zonneveld, 2009; Clark and Hatleberg, 1983; Clark and Carr, 1984) and in the Arctic (Dagis, 1984; Nakrem et al., 2008; Orchard, 2008). Reasons of the ecological preclusion of Scythogondolella in many sections of South China, and the absence of most Neospathodus species in the Smithian of Slovenia, are still not clear, but may actually be climatically induced (Sun et al., 2012; Romano et al., 2013). Some species as e.g., Discretella discreta, Nv. pingdingshanensis, Nv. abruptus, Nv. waageni and Tr. triangularis occur in both Oman and South China (e.g., Zhao et al., 2007; Chen et al., 2015). For Icriospathodus zaksi (Buryi, 1997), earlier was only reported from easternmost part of Russia (Buryi, 1997), but our data suggest that this species is not endemic to eastern Russia, and has a wider geographic distribution, such as in Oman, Canada, California, and South China (M. J. Orchard, personal communication). Scythogondolella lachrymiformis was rarely reported from South China, but has been reported from Northern Middle Siberia and North America (Dagis, 1984; Orchard, 2008). Ng. n. sp. A has been found in both Oman (this study) and South China (Chen et al.,

The present research was financially supported by the National Natural Science Foundation of China (NSFC 41702010, 41720104002, 41890844, 41425008, 41772002 and 41621003), partly by the Program of Introducing Talents of Discipline to Universities (the 111 Project no. D17013), by China Postdoctoral Science Foundation (2017T100766), and by the Austrian National Committee (Austrian Academy of Sciences) for IGCP, project IGCP 572, 630. S.R. conducted fieldwork in Oman under authorization of the Public Authority for Mining, Sultanate of Oman. We thank Jean Guex (University of Lausanne, Switzerland) for his comments and suggestions on the unitary association analysis. Nicolas Goudemand (University of Lyon) and an anonymous reviewer are acknowledged for their reviews and constructive comments.

6. Conclusions Detailed Smithian-Spathian conodont biostratigraphy has been established for the first time in two Omani sections. To correlate Smithian and Spathian strata across the whole Tethys, biostratigraphic data of Oman are analysed together with previously published data from both South-Central Europe and South China using the quantitative Unitary Association (UA) method. 7 conodont UA zones are recognized as a standard biochronological time scale enabling a wide geographical correlation of sections within the Tethyan region. UAZ7 fauna start from South China, arrive a bit later in Oman and even later in western Tethys. Based on conodont biostratigraphy and chemostratigraphy, the Smithian-Spathian boundary can be constrained within the interval from UAZ4 to UAZ5, just above the maximum peak in δ13C and above the last occurrence of Nv. pingdingshanensis in both Omani sections and in Jiarong, South China. The Omani conodont assemblage is distinguished from all other known Olenekian conodont faunas in that it combines elements of higher (Borinella, Scythogondolella) with those of lower (Eurygnathodus, Foliella, Pachycldina) palaeolatitude; in the Spathian, it shows similarity with sections from both Slovenia and South China. Several important but rarely found species are reported for the first time from the Omani sections, e.g., Foliella gardenae and Icriospathodus zaksi, and thus expand the geographical distribution of these species.

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