The Ladinian–Carnian conodont fauna at Yize, Yunnan, southwestern China, with implications for conodont palaeoecology and palaeogeography

Abstract

Subdivisions of Ladinian–Carnian boundary beds and the lower Carnian strata in South China are challenging owing to a paucity of west Tethyan ammonoids. We investigated a conodont fauna in a continuous section at Yize in eastern Yunnan Province to provide a biostratigraphic solution. Five genera and 24 conodont species are recognized, and five conodont zones are established. The zones are, in ascending order, the Paragondolella inclinata Zone, the Quadralella polygnathiformis Zone, the Quadralella praelindae Zone, the Quadralella auriformis Zone and the Quadralella robusta Zone. The Ladinian–Carnian boundary is provisionally defined by the first occurrences of Quadralella polygnathiformis and Quadralella intermedia in the cherty limestone member of the Zhuganpo Formation. Regional correlations via conodont biostratigraphy indicate that the Zhuganpo Formation is probably diachronous, with a maximal range spanning the upper Ladinian to the lower Carnian. Amongst all common late Ladinian – early Carnian conodont genera, Paragondolella, Quadralella and Mazzaella are probably cosmopolitan. Budurovignathus was restricted to a few basins and probably preferred offshore or deep-water environments.

1. Introduction

The Middle and Late Triassic were critical times in evolution. The global ecosystems recovered fully from the end-Permian mass extinction and started a full radiation both in the ocean and on land (Chen & Benton, 2012; Sun et al. 2012; Benton et al. 2013, 2014; Marshall, 2019). The marine realm saw the onset of the Mesozoic marine revolution, accompanied by an increase in predation pressure in an increasingly complex marine food web (Vermeij, 1977; Harper et al. 1998; Benton et al. 2014; Kelley & Pyenson, 2015; Tackett, 2016). The environmental, biotic and climatic changes have been intensively studied in this interval (e.g. Mutti & Weissert, 1995; Wang et al. 2008; Stefani et al. 2010; Benton et al. 2013; Trotter et al. 2015; Tanner, 2018; Sun et al. 2020). However, lining up key evolutionary events with palaeoenvironmental changes at a global scale requires a robust biostratigraphic framework and high precision supra-regional correlation, which remain challenging in many cases (e.g. Kozur, 2003; Mundil et al. 2010; Chen et al. 2015; Shen & Zhang, 2017; Rigo et al. 2018; Tong et al. 2019; Gradstein et al. 2020).

The Global Stratotype Section and Point for the base of the Carnian Stage is defined by the first appearance datum (FAD) of the ammonoid Daxatina canadensis at Prati di Stuores in NE Italy (Mietto et al. 2012). Correlations between the western Tethys and North America for the Ladinian–Carnian boundary (L-CB) beds are well resolved (Mietto & Manfrin, 1995; Orchard & Tozer, 1997; Gallet et al. 1998; Balini et al. 2000, 2010; Mietto et al. 2007, 2012; Orchard, 2007). However, correlating time-equivalent beds in the eastern and western Tethys has proven challenging. This is largely because the biostratigraphic definition of the L-CB in South China remains ambiguous owing to paucities of the ammonoids Daxatina canadensis and associated Trachyceras species (Tong et al. 2019). Several options were proposed, and some led to discrepant solutions (e.g. Xu et al. 2003; Sun et al. 2005; Zou et al. 2015).

A common practice is to define the L-CB using the FAD of ‘Paragondolella’ (P.) polygnathiformis, which was ratified as the secondary auxiliary marker for the basal Carnian at Prati di Stuores (Yang et al. 1995; Sun et al. 2005; Mietto et al. 2012; Muttoni et al. 2014; Lehrmann et al. 2015). However, the FAD of P. polygnathiformis is difficult to define in practice, and large ontogenetic variation within the species hinders the resolution of this issue further (e.g. Koike, 1982; Koike et al. 1991; Orchard & Balini, 2007; Orchard, 2010; Chen & Lukeneder, 2017; Jiang et al. 2018). Therefore, additional markers for the L-CB are in urgent need.

Subdividing Carnian strata is a difficult task. Conodont zonation, especially in the lower Carnian (Julian), varies significantly in different regions (Koike, 1979; Kozur, 1980 a, 2003; Budurov et al. 1985; Orchard & Tozer, 1997; Chen et al. 2015; Rigo et al. 2018; Yamashita et al. 2018). This is partly due to few studies being carried out and partly due to an increase in endemism in conodonts since late Middle Triassic time (e.g. Budurov et al. 1985; Chen et al. 2015). In North America, two conodont zones, namely the Neospathodus newpassensis Zone and the P. polygnathiformis Zone, were initially established for the lower Carnian in Nevada and British Columbia (Sweet et al. 1971). The Metapolygnathus (Me.) polygnathiformis Zone and the P. inclinata–Mosherella assemblage zone (AZ) were summarized for the lower Carnian in the Western Canada Sedimentary Basin (Orchard & Tozer, 1997). Later, the P. inclinata Zone (which consisted of the P. sulcata Subzone and the Me. acuminata Subzone), the Me. intermedia Zone and the Me. tadpole Zone were recognized in British Columbia (Orchard, 2007). In the western Tethys, three conodont zones, namely, the Carinella diebeli AZ, the Gladigondolella (G.) tethydis AZ and the Gondolella polygnathiformis AZ were established for the upper Ladinian to the lower Carnian in the Salzkammergut region of the North Calcareous Alps (Krystyn, 1980). The P. foliata range zone (RZ) and the P. polygnathiformis interval zone (IZ) were established based on collections from the Alps, Dinarides, Balkans and Himalayas (Budurov & Sudar, 1990). Later, five conodont zones, namely, the Budurovignathus (B.) mostleri IZ, the Me. tadpole IZ, the Me. auriformis IZ, the Me. carnica RZ and the G. tethydis IZ were recognized in the North Calcareous Alps and southwestern Turkey in the lower Carnian (Gallet et al. 1994). Rigo et al. (2018) proposed a new lower Carnian conodont scheme for the western Tethys, consisting of the P. polygnathiformis IZ, the Mazzaella (Ma.) carnica IZ and the P. praelindae IZ. In the Panthalassa Ocean, two zones, namely, the Neogondolella (N.) foliata Zone and N. polygnathiformis Zone, were summarized for the L-CB interval in Japan (Koike, 1979). In central Japan, only the P. tadpole IZ was initially established for the lower Carnian (Yamashita et al. 2018). This was later revised to the Ma. carnica Zone and the P. praelindae Zone in the same section (Tomimatsu et al. 2021). In addition, the Quadralella (Q.) tadpole – G. malayensis AZ was established in Kamura in southern Japan for the lower Carnian (Zhang et al. 2019).

Much progress has been made in the Carnian conodont biostratigraphy in South China (e.g. Wang et al. 1998, 2008; Yang et al. 2002; Sun, Z. Y. et al. 2005, 2016; Jin et al. 2018). Early studies only recognized two conodont zones, the B. diebeli Zone and the P. polygnathiformis Zone, for the entire Carnian (Yao, 1987; Lai & Mei, 2000). The N. polygnathiformis – N. maantangensis AZ, N. polygnathiformis – N. tadpole AZ and N. polygnathiformis Zone were established by Yang et al. (1995) in Guizhou. Yang et al. (1999) noticed that conodont assemblages vary with depositional settings and established the B. diebeli – P. sp. A AZ, the P. polygnathiformis Zone and the ‘Epigondolella’ nodosa Zone for basinal environments and the P. polygnathiformis – P. maantangensis AZ, the P. polygnathiformis – P. tadpole AZ and the P. polygnathiformis Zone for platform environments (Yang et al. 1999; Wang & Wang, 2016). Y. D. Sun et al. (2016) identified the P. foliata – Q. polygnathiformis AZ, the Q. polygnathiformis noah AZ and the Q. ex gr. carpathica AZ in a carbonate ramp section in Guizhou. Zhang et al. (2018 a) summarized eight conodont zones in southwestern China: the P. foliata Zone, the Q. polygnathiformis Zone, the Q. tadpole Zone, the Q. praelindae Zone, the Q. aff. auriformis Zone, the Q. robusta Zone, the Q. noah Zone and the Q. ex gr. carpathica Zone. Jiang et al. (2019) identified the Ma. carnica RZ in northern Sichuan.

The subdivision of the Middle and Upper Triassic strata into substages or finer levels is a longstanding conundrum in South China. To address this issue, we carried out a study in a newly excavated section in eastern Yunnan Province. Here, we define the L-CB in the Zhuganpo Formation (Fm) by using the first occurrence (FO) of Q. polygnathiformis and the FO of Q. intermedia, providing a practical solution for the L-CB definition in the region. A new conodont biostratigraphic framework is established, and the zones can be well correlated to other regions. Our study also reveals that the conodont fauna from southwestern China shows mixed morphological features compared to their North American and western Tethyan counterparts.

2. Geological setting

The South China Block has been a classic area for Triassic research. It was located in the northern low latitudes in the eastern Palaeo-Tethys during Late Triassic time and consisted of the Yangtze Platform, the Cathaysian Platform and the Nanpanjiang Basin (Fig. 1; Yin & Peng, 2000). Marine carbonates with diverse facies were extensively developed in the region during Early Triassic time. The carbonate platform continued to shrink toward Late Triassic time, partly due to the Indosinian Orogeny. Most Middle to Upper Triassic carbonate successions were deposited on the southwestern margin of the Yangtze Platform and sparsely in the Nanpanjiang Basin. The studied Yize section is located near Changdi town in Luoping city, Yunnan Province (Fig. 1), palaeogeographically belonging to the Upper Yangtze Platform.

Fig. 1. The palaeogeographical and present-day location maps of the study section. (a) Palaeogeographical map during Carnian time (modified from Sun et al. 2019). (b, c) Present-day location maps of the Yize section.

The Yize section was freshly exposed along a hillside quarry. The measured section is 95 m in thickness and consists of the Zhuganpo and the Wayao (also known as Xiaowa) formations. The measured Zhuganpo Fm is ∼90 m in thickness and is dominated by bioturbated carbonates. The unit is characterized by three distinct lithological associations. They are, in ascending order, the fine-laminated limestone member, the cherty limestone member and the nodular limestone member. The overlying Wayao Fm is ∼5 m in thickness, comprising thin- to medium-bedded sandstones and mudstones (Figs 2, 3).

Fig. 2. Field photographs of the study section at Yize, Yunnan Province, South China. (a) Overview of the studied section, showing the outcrop of the Zhuganpo and the Wayao formations. (b) Photo showing the fine-laminated limestones in the lower Zhuganpo Fm. (c, d) Photos showing the cherty limestone member of the Zhuganpo Fm. (e) Photo showing the thin- to medium-bedded sandstones and mudstones of the lower Wayao Fm.

Fig. 3. Log of the Yize section, showing sample positions, conodont occurrences and zonation.

3. Materials and methods

The study section was logged in detail in the field and systematically sampled for conodonts with a resolution of ∼1–3 m. Forty-three carbonate samples, each weighing 7–9 kg, were collected. All samples were crushed into 1–2 cm³ rock chips and dissolved in diluted acetic acid (∼8 vol. %). The undissolved residuals were wet-sieved and dried at room temperature. The heavy fractions were separated using a lithium and sodium heteropolytungstate heavy liquid (density 2.80 g cm⁻³). Conodont elements were picked out under a binocular microscope. A total of 1234 conodont (989 P₁ and 245 ramiform) elements were recovered from 37 samples. Selected specimens were imaged using a scanning electron microscope (SEM) at the China University of Geosciences (Wuhan).

4. Conodont biostratigraphy

Five conodont zones are established and described below in ascending order. Reported conodont taxa and their ranges are shown in Figure 3. Note that some genera and species are revised from their original binomial nomenclature and reassigned in accordance with the latest taxonomic studies and rules of the International Code of Zoological Nomenclature (e.g. Orchard, 2007, 2013; Chen et al. 2015; Kiliç et al. 2015, 2017; Chen & Lukeneder, 2017). For a better description of the stratigraphic context, we use stage and substage nomenclature in the following sections.

4.a. Paragondolella inclinata Zone

Lower limit: not defined.

Upper limit: FO of Q. polygnathiformis.

Associated taxa: P. inclinata, Q. acuminata (= Me. acuminatus in Orchard, 2007) and Q. lobata (= Me. lobatus in Orchard, 2007).

The P. inclinata Zone ranges from the 13.5 m to the 29.4 m level at Yize. This zone represents the last Ladinian conodont zone in South China and British Columbia (Orchard & Tozer, 1997; Orchard, 2007; Lehrmann et al. 2015). The zonal species is cosmopolitan ranging from the lower Longobardian to the upper Julian (Gallet et al. 1998; Orchard, 2007; Rigo et al. 2007; Lein et al. 2012; Chen et al. 2015; Zhang et al. 2017). Morphologically, the species represents a transitional form from P. excelsa and P. foliata (Kovács, 1983) (Fig. 4).

Fig. 4. SEM photographs of conodonts from the Ladinian to Carnian strata at Yize. Scale bar = 200 μm. a – upper view; b – lateral view; c – lower view. 1 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-(-65)-001; 2 – Q. intermedia (Orchard, 2007), YZ-(-65)-014; 3 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-12-1-014; 4 – Q. jiangyouensis (Wang & Dai, 1981), YZ-25-003; 5 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-(-65)-015; 6 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-1-36; 7 – Q. aff. polygnathiformis magna (Igo, 1989), YZ-18-001; 8 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-12-1-013; 9 – Q. aff. polygnathiformis magna (Igo, 1989), YZ-12-2-001; 10 – Q. maantangensis (Dai & Tian in Tian et al. 1983), YZ-25-001; 11 – P. inclinata (Kovács, 1983), YZ-25-002; 12 – P. inclinata (Kovács, 1983), YZ-28-001; 13 – K. praeangustus (Kozur, Miräuta & Mock in Kozur, 1980 b), YZ-42-020; 14 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-42-026; 15 – Q. intermedia (Orchard, 2007), YZ-79-017; 16 – Q. intermedia (Orchard, 2007), YZ-63-005; 17 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-63-007; 18 – Q. intermedia (Orchard, 2007), YZ-79-009; 19 – Q. intermedia (Orchard, 2007), YZ-95-1-003; 20 – Q. sp., YZ-63-008; 21 – Q. jiangyouensis (Wang & Dai, 1981), YZ-79-026; 22 – Q. tadpole (Hayashi, 1968), YZ-95-1-006; 23 – Q. robusta Zhang, Sun & Lai in Zhang et al. 2018 b, YZ-95-2-022; 24 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-95-1-005.

In British Columbia, Q. acuminata and Q. lobata were identified in the P. inclinata Zone and the Q. intermedia Zone, respectively. Both species typically represent the Longobardian to Julian elements (Orchard, 2007). Q. acuminata and Q. lobata are also known in the Zhuganpo Fm from Guizhou and Yunnan in southwestern China as well as in Japan (Zhang et al. 2017, 2018 b, 2019).

4.b. Quadralella polygnathiformis Zone

Lower limit: FO of Q. polygnathiformis.

Upper limit: FO of Q. praelindae.

Associated taxa: P. foliata, P. inclinata, Q. acuminata, Q. intermedia (= Me. intermedius in Orchard, 2007), Q. jiangyouensis, Q. langdaiensis, Q. lobata, Q. maantangensis, Q. aff. polygnathiformis magna, Q. spp., Q. shijiangjunensis, Q. tadpole, Q. wanlanensis and Q. yongyueensis.

The Q. polygnathiformis Zone ranges from the 29.4 m to the 47.1 m level at Yize. This zone was widely accepted as the first Carnian conodont zone (e.g. Orchard & Tozer, 1997; Kozur, 2003; Rigo et al. 2018; Tong et al. 2019). The FAD of Q. polygnathiformis is the secondary marker to define the base of the Carnian (Mietto et al. 2012). Q. polygnathiformis ranges typically from the lower Julian to the lower Tuvalian and may extend to the upper Tuvalian (e.g. Gallet et al. 1994; Rigo et al. 2007, 2018; Muttoni et al. 2014; Chen et al. 2015). Some studies implied that Q. polygnathiformis might appear in older strata below the FAD of Daxatina canadensis (Krystyn et al. 2004; Orchard, 2007, 2010; Mietto et al. 2012). The essential feature of this species is the geniculation(s)/abrupt step(s) at the anterior margins, which differs from Ladinian holdovers. However, Q. polygnathiformis may show large morphological variations in the posterior platform during different ontogenetic stages (e.g. Orchard & Balini, 2007; Chen & Lukeneder, 2017) (Fig. 5).

Fig. 5. SEM photographs of conodonts from Ladinian to Carnian strata at Yize. Scale bar = 200 μm. a – upper view; b – lateral view; c – lower view. 1 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-95-1-029; 2 – K. praeangustus (Kozur, Miräuta & Mock in Kozur, 1980 b), YZ-95-1-030; 3 – Q. jiangyouensis (Wang & Dai, 1981), YZ-95-1-031; 4 – K. praeangustus (Kozur, Miräuta & Mock in Kozur, 1980 b), YZ-95-1-032; 5 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-95-1-033; 6 – Q. intermedia (Orchard, 2007), YZ-95-1-034; 7 – Q. robusta Zhang, Sun & Lai in Zhang et al. 2018 b, YZ-95-1-036; 8 – P. inclinata (Kovács, 1983), YZ-95-1-044; 9 – Q. jiangyouensis (Wang & Dai, 1981), YZ-95-1-017; 10 – Q. tadpole (Hayashi, 1968), YZ-95-2-046; 11 – Q. intermedia (Orchard, 2007), YZ-95-1-061; 12 – Q. aff. zonneveldi (Orchard, 2007), YZ-95-1-066; 13 – P. foliata Budurov, 1975, YZ-95-1-070; 14 – Q. tadpole (Hayashi, 1968), YZ-95-2-019; 15 – Q. praelindae (Kozur, 2003), YZ-95-1-015; 16 – Q. praelindae (Kozur, 2003), YZ-95-1-009; 17 – Q. intermedia (Orchard, 2007), YZ-95-2-012; 18 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-95-2-016; 19 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-95-2-024; 20 – Q. robusta Zhang, Sun & Lai in Zhang et al. 2018 b, YZ-95-2-045; 21 – Q. robusta Zhang, Sun & Lai in Zhang et al. 2018 b, YZ-95-2-047; 22 – Q. jiangyouensis (Wang & Dai, 1981), YZ-95-2-049; 23 – K. praeangustus (Kozur, Miräuta & Mock in Kozur, 1980 b), YZ-95-2-051; 24 – K. praeangustus (Kozur, Miräuta & Mock in Kozur, 1980 b), YZ-95-2-052; 25 – K. praeangustus (Kozur, Miräuta & Mock in Kozur, 1980 b), YZ-95-2-053; 26 – P. inclinata (Kovács, 1983), YZ-99-025; 27 – P. inclinata (Kovács, 1983), YZ-12-2-002.

P. foliata is also a cosmopolitan species and differs from P. inclinata by having a straight basal edge before the basal cavity (Koike, 1982; Kovács, 1983; Muttoni et al. 2014; Zhang et al. 2017). The species ranges from the Longobardian 3 to the Tuvalian 1 (Lein et al. 2012; Muttoni et al. 2014), but is most common in the Julian (Rigo et al. 2007; Zhang et al. 2017).

Q. intermedia was first identified in the Frankites sutherlandi ammonoid zone in British Columbia (Orchard, 2007) and has been reported in Nevada and southwestern China (Orchard, 2007; Orchard & Balini, 2007; Zhang et al. 2017, 2018 b). The FO of Q. intermedia has been used as an effective marker for the base of the Carnian in British Columbia because of its co-occurrence with Daxatina canadensis and its unambiguous evolution lineage (Orchard, 2007). This species is thought to derive from Q. acuminata through further reduction in the anterior platform (Orchard, 2007). Reduction in the anterior platform and a narrowly rounded posterior end are key diagnostic features of Q. intermedia that differentiate the species from Q. polygnathiformis (Orchard, 2007; Orchard & Balini, 2007).

Q. tadpole is characterized by its tadpole-shaped platform and was widely reported in the Tethys, North America and Panthalassa (e.g. Hayashi, 1968; Gallet et al. 1994; Orchard, 2007; Mietto et al. 2012; Zhang et al. 2017, 2019). This species commonly occurred in the Julian and disappeared in the Tuvalian (Rigo et al. 2007; Chen et al. 2015; Kiliç et al. 2017; Tomimatsu et al. 2021) (Fig. 6).

Fig. 6. SEM photographs of conodonts from Ladinian to Carnian strata at Yize. Scale bar = 200 μm. a – upper view; b – lateral view; c – lower view. 1 – Q. intermedia (Orchard, 2007), YZ-95-1-004; 2 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-1-17; 3 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-1-18; 4 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-1-19; 5 – Q. praelindae (Kozur, 2003), YZ-88-1-08; 6 – Q. intermedia (Orchard, 2007), YZ-88-1-01; 7 – Q. tadpole (Hayashi, 1968), YZ-88-1-04; 8 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-1-05; 9 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-1-07; 10 – Q. praelindae (Kozur, 2003), YZ-88-1-12; 11 – Q. tadpole (Hayashi, 1968), YZ-88-1-16; 12 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-1-14; 13 – Q. praelindae (Kozur, 2003), YZ-88-2-03; 14 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-1-15; 15 – Q. intermedia (Orchard, 2007), YZ-88-2-16; 16 – Q. intermedia (Orchard, 2007), YZ-88-2-21; 17 – Q. sp., YZ-95-2-057; 18 – Q. praelindae (Kozur, 2003), YZ-88-1-09; 19 – Q. intermedia (Orchard, 2007), YZ-88-2-22; 20 – Q. acuminata (Orchard, 2007), YZ-(-17)2-001; 21 – P. inclinata (Kovács, 1983), YZ-(-17)2-002; 22 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-(-65)-i001; 23 – Q. acuminata (Orchard, 2007), YZ-(-65)-004; 24 – Q. intermedia (Orchard, 2007), YZ-(-65)-005; 25 – Q. intermedia (Orchard, 2007), YZ-(-65)-i005; 26 – P. foliata Budurov, 1975, YZ-(-65)-012; 27 – P. inclinata (Kovács, 1983), YZ-(-17)2-003.

Q. jiangyouensis, Q. langdaiensis, Q. maantangensis, Q. shijiangjunensis and Q. yongyueensis are only reported from South China to date (Wang & Dai, 1981; Tian et al. 1983; Yang et al. 2002; Sun, 2006; Sun, Y. D. et al. 2016; Zhang et al. 2017, 2018 b). Q. aff. maantangensis was reported in central Japan (Tomimatsu et al. 2021).

Q. polygnathiformis magna was first established in the Carnian in Japan (Igo, 1989). The reported Q. aff. polygnathiformis magna in the Yize section is similar to Q. polygnathiformis magna in its bifurcated keel end but differs in its platform outline (Igo, 1989). Q. aff. polygnathiformis magna is regionally common, also known from the Zhuganpo Fm in Yunnan Province (Zhang et al. 2018 b). Q. wanlanensis was first established in the Julian in southwestern China (Zhang et al. 2017, 2018 b) and was later also recognized in central Japan (Tomimatsu et al. 2021), suggesting the species may be geographically widespread.

4.c. Quadralella praelindae Zone

Lower limit: FO of Q. praelindae.

Upper limit: FO of Q. auriformis.

Associated taxa: G. malayensis, G. tethydis, Kraussodontus (K.) praeangustus, P. inclinata, Q. acuminata, Q. intermedia, Q. jiangyouensis, Q. lobata, Q. polygnathiformis, Q. aff. polygnathiformis magna, Q. shijiangjunensis, Q. spp., Q. tadpole, Q. wanlanensis, Q. wayaoensis, Q. yongyueensis and Q. aff. zonneveldi.

The Q. praelindae Zone ranges from the 47.1 m to the 67 m level. Q. praelindae was first identified in the lower Tuvalian at Silická Brezová, Slovakia (Kozur, 2003; Channell et al. 2003). Later, the species was reported in the Julian strata (Rigo et al. 2007; Sun, Z. Y. et al. 2016; Zhang et al. 2018 b) (Fig. 7).

Fig. 7. SEM photographs of conodonts from Ladinian to Carnian strata at Yize. Scale bar = 200 μm. a – upper view; b – for lateral view; c – for lower view. 1 – Q. sp., YZ-88-1-50; 2 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-79-022; 3 – P. inclinata (Kovács, 1983), YZ-79-023; 4 – Q. tadpole (Hayashi, 1968), YZ-79-003; 5 – Q. sp., YZ-46-008; 6 – Q. lobata (Orchard, 2007), YZ-12-1-020; 7 – Q. praelindae (Kozur, 2003), YZ-74-014; 8 – Q. praelindae (Kozur, 2003), YZ-74-015; 9 – Q. acuminata (Orchard, 2007), YZ-12-1-001; 10 – Q. praelindae (Kozur, 2003), YZ-88-1-58; 11 – Q. tadpole (Hayashi, 1968), YZ-88-1-47; 12 – Q. intermedia (Orchard, 2007), YZ-88-1-42; 13 – P. inclinata (Kovács, 1983), YZ-95-1-020; 14 – K. praeangustus (Kozur, Miräuta & Mock in Kozur, 1980 b), YZ-88-1-39; 15 – Q. praelindae (Kozur, 2003), YZ-95-1-018.

The genus Gladigondolella has a long range from the Spathian to the end of the Julian. A total of five species, including G. arcuata, G. budurovi, G. carinata, G. malayensis and G. tethydis are attributed to this genus (Chen et al. 2015). G. tethydis is known from the Tethys and Panthalassa, ranging from the Aegean to the uppermost Julian (Igo, 1989; Kozur, 1989 a; Gallet et al. 1994; Chen et al. 2015; Kiliç et al. 2017; Zhang et al. 2017). G. malayensis is characterized by its broader platform and posteriorly located basal cavity and ranges from the Longobardian to the uppermost Julian (Kozur, 2003; Chen et al. 2015). G. malayensis and G. tethydis have been reported in Guizhou Province of South China, ranging from the Aegean to the Julian (Lehrmann et al. 2015; Sun, Y. D. et al. 2016; Zhang et al. 2017).

Q. zonneveldi was established in the Julian in British Columbia (Orchard, 2007). The reported Q. aff. zonneveldi shares a similar platform outline with Q. zonneveldi but has a sunken carina (Fig. 8).

Fig. 8. SEM photographs of conodonts from Ladinian to Carnian strata at Yize. Scale bar = 200 μm. a – upper view; b – lateral view; c – lower view. 1 – Q. praelindae (Kozur, 2003), YZ-95-1-025; 2 – Q. aff. zonneveldi (Orchard, 2007), YZ-95-1-049; 3 – Q. lobata (Orchard, 2007), YZ-39-1-038; 4 – Q. praelindae (Kozur, 2003), YZ-95-2-032; 5 – Q. lobata (Orchard, 2007), YZ-88-1-03; 6 – P. inclinata (Kovács, 1983), YZ-15-001; 7 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-39-1-052; 8 – Q. praelindae (Kozur, 2003), YZ-95-2-027; 9 – Q. auriformis (Kovács, 1977), YZ-95-2-038; 10 – Q. tadpole (Hayashi, 1968), YZ-88-1-06; 11 – Q. praelindae (Kozur, 2003), YZ-88-1-24; 12 – Q. lobata (Orchard, 2007), YZ-39-1-009; 13 – P. inclinata (Kovács, 1983), YZ-39-1-055; 14 – Q. auriformis (Kovács, 1977), YZ-99-031; 15 – Q. sp., YZ-12-2-022; 16 – Q. auriformis (Kovács, 1977), YZ-88-1-33; 17 – Q. lobata (Orchard, 2007), YZ-39-1-012.

K. praeangustus is the first reported occurrence in southwestern China. The holotype of K. praeangustus was established from Julian strata in Romania, and the species is also known in southern Turkey (Kozur, 1980 a; Chen & Lukeneder, 2017). Compared to Q. polygnathiformis, K. praeangustus has a slender platform with one/two side constriction(s) in the posterior platform. Q. shijiangjunensis and Q. wayaoensis have not yet been reported outside of southwestern China.

4.d. Quadralella auriformis Zone

Lower limit: FO of Q. auriformis.

Upper limit: FO of Q. robusta.

Associated taxa: K. praeangustus, Ma. baloghi, P. inclinata, Q. intermedia, Q. lobata, Q. polygnathiformis, Q. praelindae, Q. spp., Q. tadpole and Q. aff. zonneveldi.

The Q. auriformis Zone ranges from the 67 m to the 72.2 m level. This zone has been established in southwestern China, the Alps and the Himalayas (Gallet et al. 1994; Krystyn et al. 2004; Sun, 2006; Hornung et al. 2007). The zonal species is characterized by its ear-like platform and differs from Q. tadpole by its expanded middle platform and shorter length (Kovács, 1977; Mastandrea, 1995; Kiliç et al. 2015). It ranges from the lower Julian to the lower Tuvalian and is known in the western Tethys, Japan, the Himalayas, northern Oman and southwestern China (Kovács, 1977; Gallet et al. 1994; Mastandrea, 1995; Hornung et al. 2007; Rigo et al. 2007; Kiliç et al. 2015; Zhang et al. 2018 b; Sun et al. 2019; Tomimatsu et al. 2021). The species could be the direct ancestor of Ma. baloghi and Ma. carnica (Fig. 9).

Fig. 9. SEM photographs of conodonts from Ladinian to Carnian strata at Yize. Scale bar = 200 μm. a – upper view; b – lateral view; c – lower view. 1 – Q. praelindae (Kozur, 2003), YZ-88-2-04; 2 – Q. tadpole (Hayashi, 1968), YZ-88-2-08; 3 – Q. tadpole (Hayashi, 1968), YZ-88-2-46; 4 – K. praeangustus (Kozur, Miräuta & Mock in Kozur, 1980 b), YZ-88-2-42; 5 – Q. intermedia (Orchard, 2007), YZ-88-2-44; 6 – Q. polygnathiformis (Budurov & Stefanov, 1965), YZ-88-3-07; 7 – Q. praelindae (Kozur, 2003), YZ-88-3-16; 8 – Q. auriformis (Kovács, 1977), YZ-88-3-75; 9 – Q. praelindae (Kozur, 2003), YZ-88-3-17; 10 – Q. sp., YZ-88-2-38; 11 – Q. praelindae (Kozur, 2003), YZ-88-1-64; 12 – Q. sp., YZ-88-1-60; 13 – Q. sp., YZ-88-2-43; 14 – Ma. baloghi (Kovács, 1977), YZ-88-1-63; 15 – Q. auriformis (Kovács, 1977), YZ-88-2-26; 16 – Q. sp., YZ-88-2-12.

Ma. baloghi is reported for the first time here in southwestern China. The species shares many common features with Q. auriformis but has distinct nodes on the anterior platform margins (Kovács, 1977; Mastandrea, 1995). It has a short range within the Julian substage and is known in Hungary, Oman and Italy (e.g. Kovács, 1977; Mastandrea, 1995; Rigo et al. 2007; Chen et al. 2015; Kiliç et al. 2015, 2017; Sun et al. 2019) (Fig. 10).

Fig. 10. SEM photographs of conodonts from Ladinian to Carnian strata at Yize. Scale bar = 200 μm. a – upper view; b – lateral view; c – lower view. 1 – Q. praelindae (Kozur, 2003), YZ-88-3-24; 2 – Q. praelindae (Kozur, 2003), YZ-88-3-38; 3 – Q. intermedia (Orchard, 2007), YZ-88-3-32; 4 – Q. sp., YZ-88-3-35; 5 – Q. tadpole (Hayashi, 1968), YZ-88-3-41; 6 – Q. praelindae (Kozur, 2003), YZ-88-3-43; 7 – Q. auriformis (Kovács, 1977), YZ-88-3-44; 8 – Q. praelindae (Kozur, 2003), YZ-88-3-49; 9 – Q. auriformis (Kovács, 1977), YZ-88-3-60; 10 – Q. tadpole (Hayashi, 1968), YZ-88-3-61; 11 – Q. tadpole (Hayashi, 1968), YZ-88-3-63; 12 – Q. tadpole (Hayashi, 1968), YZ-88-3-66; 13 – Q. auriformis (Kovács, 1977), YZ-88-2-54; 14 – Q. sp., YZ-88-2-63; 15 – Q. tadpole (Hayashi, 1968), YZ-88-3-05; 16 – Q. auriformis (Kovács, 1977), YZ-88-3-13.

4.e. Quadralella robusta Zone

Lower limit: FO of Q. robusta.

Upper limit: not defined.

Associated taxa: K. praeangustus, P. foliata, P. inclinata, Q. acuminata, Q. auriformis, Q. intermedia, Q. jiangyouensis, Q. lobata, Q. polygnathiformis, Q. praelindae, Q. spp., Q. tadpole and Q. aff. zonneveldi.

The Q. robusta Zone ranges from the 72.2 m level to the top of the section. This regionally important zone was widely recognized in Julian strata in southwestern China (Zhang et al. 2017, 2018 b). Q. robusta is characterized by having a large platform and a robust main cusp (in mature form). The species is common in the Zhuganpo and Wayao boundary beds and ranges from the uppermost Julian 1 to the lowermost Tuvalian 1? (Sun, Y. D. et al. 2016; Zhang et al. 2017, 2018 b).

5. Discussion

5.a. Conodont palaeogeography and ecology

Early Carnian conodonts show significant provincialism on the globe (Mosher, 1968; Budurov et al. 1985; Lai & Mei, 2000; Klets, 2008; Chen et al. 2015; Martínez-Pérez et al. 2015; Zhang et al. 2018 b). The genera Paragondolella, Gladigondolella, Pseudofurnishius, Quadralella, Mazzaella and Budurovignathus dominated the Julian assemblages in the western Tethys, while Mosherella, Paragondolella and Quadralella are common in North America. Only a few monotonous Budurovignathus and a single Gladigondolella species have been reported in North America (e.g. Mosher, 1968; Orchard et al. 2001; Balini et al. 2007; Orchard & Balini, 2007; Orchard, 2010). Mosherella was rare in the western Tethys and only reported in southern Turkey (e.g. Orchard, 2010; Chen & Lukeneder, 2017). In the Panthalassa and the Himalayas, Paragondolella, Quadralella, Gladigondolella and Budurovignathus were common in the Julian (e.g. Koike et al. 1991; Krystyn et al. 2004; Zhang et al. 2019).

In South China, conodont assemblages are dominated by genera of Paragondolella and Quadralella in the Julian, while Mosherella, Pseudofurnishius, Gladigondolella and Budurovignathus are very rare (Yang et al. 1995, 2002; Wang et al. 2005; Dong & Wang, 2006; Sun, 2006; Sun, Y. D. et al. 2016; Sun, Z. Y. et al. 2016; Zhang et al. 2017, 2018 b). At the species level, indigenous species are more common. However, Julian conodonts from southwestern China still share many common features with those from the western Tethys and North America, laying a biostratigraphic foundation for supra-regional correlation.

Conodont distribution is controlled, in part, by their ecology, i.e. the dwelling habitats of conodont animals (Kozur, 1976; Lai et al. 2001; Chen et al. 2021). This is more conspicuous in the Middle and Late Triassic amongst the genera Gladigondolella and Budurovignathus. Gladigondolella was considered a pelagic conodont element restricted to the Tethys realm, part of Panthalassa and the western margin of North America (Kozur, 1976; Kozur et al. 2009). Two species, namely G. tethydis and G. malayensis, are most common in the western Tethys and Panthalassa but are rather rare in South China (e.g. Koike et al. 1991; Gallet et al. 1994; Balini et al. 2000; Wang et al. 2005; Lein et al. 2012; Zhang et al. 2017; Tomimatsu et al. 2021). They have never occurred in a platform setting, suggesting the taxa may have favoured deeper water habitats (also see Trotter et al. 2015).

Abundant and diverse Budurovignathus species occur in various environments in the western Tethys (e.g. Gallet et al. 1994; Mastandrea et al. 1997; Loriga et al. 1998; Balini et al. 2000; Mietto et al. 2007, 2012; Richoz et al. 2007; Rigo et al. 2007; Lein et al. 2012; Stocker et al. 2013; Muttoni et al. 2014; Karádi et al. 2022). Owing to common occurrences of Budurovignathus species in European sections, the FAD of B. diebeli and the last appearance of B. mungoensis were proposed as markers for the L-CB (Kozur & Mostler, 1971; Kozur, 1989 b). However, these species are rare in other regions, and the FAD of B. diebeli was later proved in the Longobardian (e.g. Krystyn, 1983; Loriga et al. 1998; Karádi et al. 2022). In South China, Budurovignathus species are very rare and by far best known from Guandao, which represents an oxygenated platform margin environment (Wang et al. 2005; Enos et al. 2006; Lehrmann et al. 2015). Budurovignathus specimens are also reported at Guanyinya in northern Sichuan and Dapingzi in Guizhou (Jiang et al. 2018, 2019). However, these elements do not develop nodes on their anterior platform margins or have denticles on the platform and are very unusual. In British Columbia and Nevada, Budurovignathus species are found in diverse environments from slope to basin but were never very diverse (Balini & Jenks, 2007; Balini et al. 2007; Orchard, 2007, 2010; Orchard & Balini, 2007). In the Indian Himalayas, Budurovignathus faunas have been obtained from the Kaga and Chomule formations, which represent a neritic shelf setting below the storm wave base (Bhargava et al. 2004; Krystyn et al. 2004; Sun et al. 2021). Deep-sea sediments in the Panthalassa also contain Budurovignathus faunas (Nakada et al. 2014; Zhang et al. 2019; Tomimatsu et al. 2021). These may indicate that Budurovignathus could thrive in diverse environments. However, why Budurovignathus is so rare in South China remains a mystery.

5.b. The Ladinian–Carnian boundary

Ammonoids and conodonts are favoured as standard biotic indices to define Triassic stage and substage boundaries (Gradstein et al. 2020). Daxatina canadensis is a popular choice for the base of the Carnian in the western Tethys, North America and Himalayas (e.g. Krystyn et al. 2004; Orchard, 2007; Mietto et al. 2012). In South China, ammonoid biostratigraphy is not well established for the Upper Triassic. This is mostly attributed to the paucity of ammonoids commonly found in other regions (e.g. Hsu & Chen, 1944; Wang, 1983; Xu et al. 2003; Balini et al. 2010; Zou et al. 2015; Tong et al. 2019). Some studies implied that the L-CB should be placed in the upper part of the Zhuganpo Fm or even in the lower Wayao Fm (Xu et al. 2003; Li et al. 2013; Zou et al. 2015).

Conodonts are preferred as important markers to define the L-CB. The FAD of several conodont species, including B. diebeli, Mosherella newpassensis and Q. polygnathiformis have been proposed as auxiliary biotic markers for the L-CB (Mosher, 1968; Kozur & Mostler, 1971; Mietto et al. 2007). However, the FAD of B. diebeli is lower than the FAD of Daxatina canadensis, and B. diebeli is not widely distributed (e.g. Krystyn, 1983; Loriga et al. 1998; Krystyn et al. 2004; Mietto et al. 2007, 2012; Orchard, 2010; Plasencia et al. 2018; Karádi et al. 2022). Mosherella newpassensis is known in North America and South China but not in the western Tethys (Orchard, 2010). The FO of Mosherella newpassensis also falls in the Longobardian (Wang et al. 2005; Lehrmann et al. 2015). The FAD of Q. polygnathiformis is the most suitable as the practical marker for the L-CB, owing to its global distribution and nearly co-occurrence with Daxatina canadensis (Krystyn et al. 2004; Sun et al. 2005; Rigo et al. 2007; Mietto et al. 2007, 2012; Lehrmann et al. 2015). However, Orchard (2010) suggested that the occurrence of Q. polygnathiformis could be affected by variations in sedimentation rates.

In southwestern China, the position of the L-CB is a matter of debate owing to the difficulties in defining the FO of Q. polygnathiformis and a lack of other auxiliary markers. Yang et al. (1995) placed the L-CB in the lowermost Zhuganpo Fm based on massive occurrences of Q. polygnathiformis. Sun et al. (2005) argued that the L-CB should be put at the 3.38 m level in the Zhuganpo Fm based on the evolutionary lineage of P. inclinata – Q. polygnathiformis – Q. maantangensis and the FO of Q. polygnathiformis. Zhang et al. (2017) suggested that the L-CB is unlikely lower than the lithological boundary between the Yangliujing and Zhuganpo formations. These minor discrepancies could be due to a lack of robust biostratigraphic control in the L-CB beds and might also point to the diachronous nature of the Zhuganpo Fm.

The Q. intermedia Zone has been proposed as the first Carnian conodont zone in British Columbia. The FO of Q. intermedia is a practical auxiliary biotic marker for the L-CB because it co-occurred with Daxatina canadensis in the Frankites sutherlandi Zone (Balini et al. 2007; Orchard, 2007, 2010; Chen et al. 2015). At Yize, the FOs of Q. polygnathiformis and Q. intermedia occur at the same level, suggesting that the L-CB should be placed in the lower cherty limestone member. However, age-diagnostic conodonts are lacking from the 14 m to the 29 m level at the transition from the fine-laminated member to the cherty limestone member. It is possible that the FAD of polygnathiformis and intermedia, i.e. the true L-CB, is within this interval.

5.c. Strata correlation using conodont zonation

Short-ranged and widely distributed conodont taxa have been favoured as zonal species. At Yize, the P. inclinata Zone is likely the last Ladinian zone or straddles the L-CB, as the species is known worldwide in various environments (e.g. Kovács, 1983; Orchard, 2007; Rigo et al. 2007; Lein et al. 2012; Lehrmann et al. 2015; Tomimatsu et al. 2021). The established Q. polygnathiformis Zone at Yize could correlate to the Q. intermedia Zone in British Columbia and Nevada and the Q. polygnathiformis IZ in the western Tethys and Panthalassa Ocean (Koike, 1979; Orchard, 2007, 2010; Rigo et al. 2018). The Q. auriformis Zone is an important zone that is well correlated to the western Tethys, the Indian Himalayas and the Panthalassa due to its clear evolutionary lineage, widespreadness and short range (e.g. Gallet et al. 1994; Hornung et al. 2007; Tomimatsu et al. 2021). The Q. robusta Zone could be a practical zone for regional correlation in southwestern China. This zone is immediately below the Ma. carnica Zone and correlates well with the widespread Carnian Humid Episode in the Julian 1 to Julian 2 transition (Hornung et al. 2007; Zhang et al. 2018 a,b).

5.d. Implications for conodont evolution

Conodonts were reasonably diverse in much of the Late Triassic period and show tendencies toward (i) shortening of the platform, (ii) increasing ornaments on the platform and (iii) forward-shifting of the basal cavity (Mazza et al. 2010, 2012; Kiliç et al. 2015; Karádi, 2021).

Shortening of the platform is a general trend that has been observed amongst gondolellids since the late Middle Triassic period. Two lineages, P. excelsa – P. inclinata – P. foliata – Q. tadpole and Q. acuminata – Q. intermedia, were recognized by a decreasing anterior platform from the Longobardian to the Julian in Hungary and British Columbia (Kovács, 1983; Orchard, 2007; Orchard & Balini, 2007). Such changes were later widely recognized in the western Tethys and southwestern China (Sun et al. 2005; Kiliç et al. 2015, 2017; Zhang et al. 2017, 2018 b).

The ornamented anterior platform is a most distinctive feature amongst gondolellids in the late Longobardian to the Julian. A transition from smooth to nodose or denticulated platforms is first seen in the B. japonica – B. mungoensis – B. diebeli lineage and in B. mostleri (e.g. Balini et al. 2000; Chen et al. 2015; Plasencia et al. 2018). This evolutionary trend is not yet seen in South China and North America owing to the paucity of Budurovignathus elements. However, a similar evolutionary trend is seen in the Q. auriformis – Ma. baloghi – Ma. carnica lineage in the Julian substage (Kovács, 1977; Mastandrea, 1995; Kiliç et al. 2015; Sun et al. 2019).

The forward-shifting pit was typically seen amongst late Tuvalian conodonts, especially in Metapolygnathus and Carnepigondolella (e.g. Mazza et al. 2010; Kiliç et al. 2017). However, in southwestern China, this change seems to occur earlier in the Julian, evidenced by the appearances of Q. langdaiensis, Q. maantangensis, Q. wanlanensis and Q. polygnathiformis magna, which all have obviously forward-shifted pits (Zhang et al. 2017).

6. Conclusions

Conodont taxonomy and biostratigraphy were investigated in detail at Yize, Yunnan, southwestern China. The Yize conodont fauna represents a typical late Ladinian to early Carnian assemblage with both endemic and cosmopolitan forms. The FOs of Q. polygnathiformis and Q. intermedia are practical markers co-defining the L-CB in the cherty limestone member of the Zhuganpo Fm. Five conodont zones, namely, the P. inclinata Zone, the Q. polygnathiformis Zone, the Q. praelindae Zone, the Q. auriformis Zone and the Q. robusta Zone, are established and represent a practical biostratigraphic framework in the region. The Zhuganpo Fm, one of the most essential Upper Triassic units in South China, could be diachronous and brackets at least the uppermost Ladinian to the lower Carnian.