Early Triassic conodonts from the Liangshan area, Hanzhong, Shaanxi, South China

The Liangshan area in Hanzhong city, Shaanxi Province, China, is in the northwestern part of the Yangtze Platform. Strata across the Permian–Triassic boundary (PTB) are continuous, well developed, and fossiliferous, providing an ideal place for biostratigraphic study. However, there is a dearth of reliable conodont biostratigraphic data from PTB sequences in the Liangshan area. In this study, conodonts are examined at the Zhangkouzi and Chencun sections in the Liangshan area. Three conodont species are documented from the Zhangkouzi section, Hindeodus parvus, H. sosioensis, and H. postparvus, and six conodont species are documented from the Chencun section, Pachycladina multidentata, Pa. costatus, Pa. magnus, Pa. bidentata, Foliella formosa, and Neospathodus concavus. Based on the stratigraphic distribution of conodonts, the Zhangkouzi section is Changhsingian–Griesbachian (early Induan) in age, and the Chencun section is Smithian (early Olenekian) in age. Our data suggest that the genus Foliella evolved from the genus Pachycladina, that F. gardenae evolved from F. formosa, and that the latter evolved from Pa. multidentata. The multi-element apparatus of Pachycladina is reconstructed with 15 elements.

1 Introduction

The end-Permian mass extinction (251.939 ± 0.031 Ma; Burgess et al., 2014; Shen et al., 2019) was the largest biological crises on Earth and had a significant impact on both marine and terrestrial ecosystems (Raup, 1979; Jablonski, 1994; Retallack, 1995; Shen et al., 2011; Song et al., 2014, 2018; Stanley, 2016; Fan et al., 2020; Xu et al., 2021). The Permian–Triassic boundary (PTB) has received widespread attention, as we attempt to understand the causes and consequences of the end-Permian mass extinction.

Conodonts, bivalves, and ammonoids are significant taxa for both stratigraphic correlation and the definition of the PTB (Song et al., 2013; Song and Tong, 2016; Dal Corso et al., 2022). The Global Stratotype Section and Point (GSSP) of the PTB was defined in 2001, by the first appearance datum (FAD) of Hindeodus parvus at Meishan D section, Changxing County, Zhejiang Province, South China (Yin et al., 2001; Zhao et al., 2008). However, H. parvus has not been recorded in the Liangshan area in previous studies (Lu, 1956; Wu, 1957; Kanmera and Nakazawa, 1973; Wang, 1978; Liu, 1982; Rui, 1984; Guo et al., 2016); thus, the precise location of the PTB in this area is unknown.

The Liangshan area in southern Shaanxi, part of the Upper Yangtze Platform (Fig. 1b), is one of the most important areas for the study of Permian–Triassic strata. The Permian–Lower Triassic strata in the Liangshan area are continuous, well developed, and fossiliferous. Studies on the Permian and Triassic strata in the Liangshan area can be dated back to the 1950s (e.g. Lu, 1956; Wu, 1957). Previous research in this area was mainly focused on stratigraphic correlation (Lu, 1956; Kanmera and Nakazawa, 1973; Liu, 1982) and describing the rich fossil assemblages (including conodonts, corals, bivalves and brachiopods) (Wu, 1957; Chen, 1978; Wang, 1978; Chen and Li, 1979; Guo et al., 2016). Conodonts of the Early Triassic, however, are poorly studied, and the index fossil Hindeodus parvus that marks the PTB has never been reported.

Here we report conodont taxa from the Chencun and Zhangkouzi sections in the Liangshan area, Hanzhong city, Shaanxi Province. In addition to conodonts (Figs. 3–9), gastropods and ostracods have been documented (Fig. 10) in an attempt to provide a reliable biostratigraphic framework for the Permian–Triassic strata in this area.

Figure 1 Location of the studied sections in the Liangshan area, Hanzhong, Shaanxi Province. (a) Palaeogeographic reconstruction during the end Permian to Early Triassic (modified after Huang et al., 2018; Dal Corso et al., 2022). SC stands for South China, and NC stands for North China. (b) Map of the Liangshan area. (c) Geological map of the study area. Q stands for Quaternary, N/E represents Neogene and Paleogene, T₁ is the Lower Triassic, P₃ is the Lopingian, and P₁₋₂ is the Cisuralian and Guadalupian.

2 Geological setting

The Liangshan area is located in the northwestern Yangtze block (Fig. 1a, b), bounded to the north by the Mianlüe suture zone (Zhang et al., 2001, 2004; He et al., 2016), and connected to the Qinling orogenic belt (Guo et al., 2016). During the Permian–Triassic transition, the Yangtze block was located near the eastern part of the Palaeo-Tethys Ocean (Fig. 1a), and the PTB strata are relatively complete in the Middle and Upper Yangtze Platform (Feng et al., 2017). The Lopingian strata comprise the Changhsing or Dalong formations, and the Lower Triassic strata consist of the Daye (equivalent to the Feixianguan Formation) and Jialingjiang formations (Zhang and Tong, 2010; Yang et al., 2012; Zhao et al., 2015; Feng et al., 2017; Miao et al., 2021; Shen et al., 2021).

Within the Liangshan area, the strata of Permian–Lower Triassic are well exposed. A major transgression occurred in the Upper Yangtze Platform during the late Lopingian (He et al., 2016), resulting in the deposition of the Wuchiaping and Changhsing formations (He and Luo, 2010). The Daye Formation of the Lower Triassic in the Liangshan area belongs to the transitional facies of Daye and Feixianguan formations. The lower part of the Daye Formation is composed of light grey, light yellow micritic, bioclastic limestone, marl, and shale, and the upper part is composed of thin purple shale and oolitic limestone, indicating deposition in neritic facies (Huo, 1960; Rui et al., 1984; Jian et al., 1992; Liu et al., 1996; Zheng et al., 2021).

The studied Zhangkouzi section (106°54^′00^′′ N, 33°4^′48^′′ E; Figs. 1c, 2) is located about 1 km to the southeast of Chenjiacun Lake, is exposed along a bituminous road with a total thickness of 83 m, and consists of continuous strata from the Changhsing to Daye formations. The Changhsing Formation is mostly limestones and shales, and the Daye Formation is thin-bedded limestone and shale and yields conodonts, gastropods, and ostracods. The Chencun section (106°53^′24^′′ N; 33°5^′24^′′ E; Figs. 1c, 2) is also located along a bituminous road on the southern bank of the Chenjiacun Lake with a total thickness of 18.5 m and is exclusively composed of the Daye Formation, which primarily consists of thin-bedded shale and oolitic limestone with abundant conodonts. Permian–Lower Triassic strata in the Liangshan area are continuous and fossiliferous, providing an ideal opportunity to study conodont evolution and biostratigraphy across the PTB.

Figure 2 Lithology and stratigraphic ranges of taxa through the Zhangkouzi and Chencun sections. Black rectangles indicate productive samples. Blank rectangles indicate barren samples. H. stands for Hindeodus and Pa. stands for Pachycladina.

3 Materials and methods

In total, 84 kg of samples (each sample ranges from 3 to 6 kg) was collected for this study. In the Zhangkouzi section, 17 samples were collected from the Changhsing Formation, while one sample was collected from the Daye Formation. In the Chencun section, three samples were collected from the Daye Formation. All samples were crushed into ca. 5 cm³ fragments and dissolved in plastic buckets with dilute acetic acid (ca. 8 %–10 %; Jeppsson et al., 1985; Jeppsson and Anehus, 1999). The insoluble residues at the bottom of buckets were sieved using 1 mm and 64 µm meshes; undissolved samples were put back into the plastic buckets to continue the dissolution process. Residues in the sieve of 64 µm meshes were collected in a beaker and dried naturally. Conodonts were concentrated in sodium polytungstate and handpicked from dried residues using a wet brush pen under a binocular microscope. A Phenom XL G2 scanning electron microscope was used for photographing the gold-coated conodont, gastropod, and ostracod specimens (Figs. 3–10) at the Department of Geology, Northwest University, Xi'an, China. All of these studied specimens are deposited at the Department of Geology (catalogue nos. 1001–1072).

Figure 3 SEM photos of Foliella formosa (Tian, 1983) P₁ elements from the Daye Formation, Chencun section. (a–c) 1001, (d–f) 1002, (g–i) 1003, (j–l) 1004, (m–o) 1005, and (p–r) 1006. Panels (a)–(i) and (p)–(r) are from sample CC-1, and panels (j)–(o) are from sample CC-3. The scale bar represents 500 µm.

Figure 4 SEM photos of conodonts from the Daye Formation, Chencun section. (a–e) Foliella sp. P₁ elements: (a–c) 1007 and (d–e) 1008. (f–k) Neospathodus concavus Zhao and Orchard, 2007: (f–h) P₁ element 1009 and (i–k) S₀ element 1010. All are from sample CC-1. The scale bar represents 500 µm.

Figure 5 SEM photos of conodonts from the Daye Formation, Chencun section. (a–f) Pachycladina costatus (Tian, 1983) P₁ elements: (a–c) 1011 and (d–f) 1012. (g–l) Pachycladina magnus (Tian, 1983) P₁ elements; (g–i) 1013 and (j–l) 1014. (m–x) Pachycladina bidentata Wang and Cao, 1981 P₁ elements: (m–o) 1015, (p–r) 1016, (s–u) 1017, and (v–x) 1018. Panels (a)–(f) are from sample CC-1, and panels (g)–(x) are from sample CC-3. The scale bar represents 500 µm.

Figure 6 SEM photos of conodonts from the Daye Formation, Chencun section. (a–f) Pachycladina multidentata Wang and Cao, 1981 P₁ elements: (a–c) 1019 and (d–f) 1020. (g–ad) Pachycladina sp. (g–l) S₀ elements (g–i) 1021 and (j–l) 1022, as well as (m–o) S₁ element 1023. Panels (p)–(r) and (v)–(x) show S₂ elements (p–r) 1024 and (v–x) 1025. (s) P₂ element 1026; M elements (t) 1027 and (u) 1028; (y–aa) S₃ element 1029. (ab–ad) S₄ element 1030. Panels (a)–(i), (m)–(o), (s), (v)–(x), and (ab)–(ad) are from sample CC-3, and panels (j)–(l), (p)–(r), (t)–(u), and (y)–(aa) are from sample CC-1. The scale bar represents 500 µm.

Figure 7 SEM photos of Pachycladina sp. from the Daye Formation, Chencun section. Panels (a)–(k) and (t)–(v) are S₀ elements: (a–c) 1031, (d–f) 1032, (g–i) 1033, (j–k) 1034, and (t–v) 1035; (w–y) S₁ element 1036 and (l–n) S₃ element 1037. Panels (o)–(s) and (z)–(ah) are S₂ elements: (o–p) 1038, (q–s) 1039, (z–ab) 1040, (ac–ae) 1041, and (af–ah) 1042. Panels (a)–(c) and (o)–(v) are from sample CC-1, and panels (d)–(n) and (w)–(ah) are from sample CC-3. The scale bar represents 500 µm.

Figure 8 SEM photos of Hindeodus sp. from the Chanshing Formation, Zhangkouzi section. Panels (a)–(c) and (j)–(l) are S₀ elements: (a–c) 1043 and (j–l) 1044. (d–f) M element 1045. (g–i) S₁₋₂ element 1046. Panels (m)–(o) and (p)–(r) are P₂ elements: (m–o) 1047 and (p–r) 1048. Panels (s)–(u) are S₃₋₄ element 1049. All are from sample ZKZ-8. The scale bars represent 200 µm (a–r) and 500 µm (s–u), respectively.

Figure 9 SEM photos of conodonts from the Changhsing and Daye formations, Zhangkouzi section. (a–c) Hindeodus sosioensis Kozur, 1996 P₁ element: 1050. Panels (d)–(f) and (p)–(r) are Hindeodus sp. P₁ elements: (d–f) 1051 and (p–r) 1052. Panels (g)–(i) and (s)–(t) are Hindeodus parvus Kozur, 1976 P₁ elements: (g–i) 1053 and (s–t) 1054. Panels (j)–(o) and (u)–(w) are Hindeodus postparvus Kozur, 1989 P₁ elements: (j–l) 1055, (m–o) 1056, and (u–w) 1057. Panels (a)–(c), (d)–(f), (p)–(r), (g)–(i), (j)–(n), and (u)–(w) are from sample ZKZ-8, and panels (s)–(t) are from sample ZKZ2-8. The scale bar represents 200 µm.

Figure 10 SEM photos of gastropods and ostracods from the Changhsing Formation, Zhangkouzi section. (a–g) Bairdia sp.: (a) 1058, (b) 1059, (c) 1060, (d) 1061, (e) 1062, (f) 1063, and (g) 1064. (h–j) Darwinula sp.: (h) 1065, (i) 1066, and (j) 1067. (k–m) Polygyrina depressa Wang and Xi, 1980: (k) 1068, (l) 1069, and (m) 1070. (n–o) Polygyrina sichuanensis Pan, 1982: (n) 1071 and (o) 1072. Panels (a)–(f) and (o) are from sample ZKZ2-1, panels (g)–(j) are from sample ZKZ2-2, and panels (k)–(n) are from sample ZKZ2-4. The scale bar represents 500 µm.

4 Discussion

4.1 The Permian–Triassic boundary in the Zhangkouzi section

The PTB is typically defined by the FAD of Hindeodus parvus, and in the Zhangkouzi section the lowest occurrence of H. parvus (Fig. 2) is in rock sample ZKZ-8. However, in the Zhangkouzi section the first occurrence of H. parvus co-occurs with H. sosioensis and H. postparvus, two species that typically first occur in slightly younger strata (Kozur, 2003; Tong et al., 2003; Zhao et al., 2007; Jiang et al., 2014). The combination of H. parvus, H. sosioensis, and H. postparvus suggests that sample ZKZ-8 correlates with the H. sosioensis zone (Jiang et al., 2014) that occurs in the basal part of Triassic, which is slightly younger than the PTB. As such, despite sample ZKZ-8 yielding the first occurrence of H. parvus, the PTB in the Zhangkouzi section should be placed below the sampled horizon ZKZ-8.

4.2 The age of the Chencun section

Conodonts obtained from the Chencun section include Pa. costatus, Pa. multidentata, Pa. magnus, Pa. bidentata, F. formosa, and Ns. concavus. Neospathodus concavus was previously reported by Zhao et al. (2007) in the West Pingdingshan section, Chaohu, Anhui Province. In this section (Zhao et al., 2007), Ns. concavus occurs in the Ns. waageni waageni subzone (of the Ns. waageni Zone), which is early Olenekian in age (Maekawa et al., 2016; Lyu et al., 2023). An early Olenekian age is further supported by the presence of the genera Foliella and Pachycladina that have both been documented as occurring in the Smithian (Kolar-Jurkovšek, 1996; Song, 2012; Wu et al., 2021). Based on our conodont data, the Daye Formation at the Chencun section is Smithian (early Olenekian) in age.

4.3 The evolution of Pachycladina and Foliella

Two genera, Foliella and Pachycladina, of the family Ellisoniidae were recognized in the Chencun section (Figs. 3–7). The origin of Foliella has been debated (Orchard, 2007). Orchard (2007) suggested that Foliella may have evolved from the genus Furnishius, but our data provide another possibility. The P₁ element of Foliella is a platform with only two processes; however, the P₁ element of Furnishius normally consists of three processes and has a “Y”-shaped keel. Meanwhile, Foliella differs from Furnishius in bearing regularly arranged and uniformly sized nodes on the oral surface. The morphological differences between Foliella and Furnishius are considerable, so it is unlikely that they have a direct evolutionary relationship.

Foliella and Pachycladina, on the other hand, share numerous morphological similarities. On the oral surface, the carina of both Pachycladina and Foliella are well developed, with a centrally located cusp. On the aboral surface, both Pachycladina and Foliella have a keel without an obvious furrow or basal pit. Stratigraphically, Pachycladina is slightly older, occurring in the Pachycladina–Hadrodontina zone, which is Griesbachian to early Smithian in age (Perri, 1991; Kolar-Jurkovšek and Jurkovšek, 2019; Wu et al., 2019, 2021), while Foliella is Smithian in age (Perri, 1991; Kolar-Jurkovšek and Jurkovšek, 2019; Wu et al., 2019, 2021). Based on their close morphological similarity and stratigraphic relationship, we propose that Foliella evolved from Pachycladina, with an evolutionary lineage from Pachycladina multidentata to Foliella formosa and to Foliella gardenae (Fig. 11).

Figure 11 Evolutionary lineage of Pachycladina and Foliella. Pachycladina multidentata and Foliella formosa are documented from the Chencun section of Liangshan area. Foliella gardenae is taken from Kolar-Jurkovšek and Jurkovšek (1995).

4.4 The multi-element apparatus of Pachycladina

In previous studies, the ellisonid genus Pachycladina was described by seximembrate apparatus (Perri and Andraghetti, 1987; Zhang and Yang, 1991; Kolar-Jurkovšek, 1996; Song, 2012; Wu et al., 2021); however, no natural assemblage or clusters have been documented to support the seximembrate apparatus theory. Sun et al. (2020) reported three well-preserved natural assemblages and confirmed that ellisonid genus Hadrodontina had a multi-element apparatus composed of 15 elements. Koike (2016) described five ellisonid genera (Furnishius, Staeschegnathus, Cornudina, Ellisonia, and Hadrodontina) with septimembrate apparatus containing 15 elements. Because both Hadrodontina aequabilis and Pachycladina belong to the same subfamily Hadrodontinae (Koike, 2016), we recognize Pachycladina as possessing 15 elements (paired P₁, P₂, M, S₁, S₂, S₃, and S₄ elements and unpaired S₀ elements).

Almost all P₁ elements (accounts for 99 % of the total assemblage) from rock samples CC-1 and CC-3 belong to the Pachycladina–Foliella lineage, and thus almost all the P₂, S, and M elements can be assigned to the Pachycladina–Foliella lineage. Because Foliella is a descendent of Pachycladina, their P₂, S, and M elements should be similar. The presence of multiple species from the same sample complicates the assignment of S and M elements to specific species, especially in the absence of natural assemblages. Consequently, we have used open nomenclature (Pachycladina sp.) to deal with these ramiform elements from the CC-1 and CC-3 of Chencun section. Here we reconstruct the apparatus of Pachycladina, tentatively including all ramiform elements from these two rock samples, which may also include ramiform elements of Foliella.

5 Conclusions

In this study, a total of 4 genera and 11 conodont species were recognized in the Zhangkouzi and Chencun sections, Liangshan area, Hanzhong city, Shaanxi Province.

In the Zhangkouzi section, the lowest occurrence of H. parvus is associated with H. sosioensis and H. postparvus, correlating to the H. sosioensis zone, indicating a basal Triassic age. The conodont assemblage of Pa. multidentata, Pa. bidentata, F. formosa, and Ns. concavus suggests that the Chencun section is Smithian (early Olenekian) in age.

Our data support the idea that the genus Foliella evolved from the genus Pachycladina with an evolutionary lineage from Pachycladina multidentata to Foliella formosa and then to Foliella gardenae. The multi-element apparatus of Pachycladina is reconstructed with 15 elements with paired P₁, P₂, M, S₁, S₂, S₃, and S₄ elements and unpaired S₀ elements.

Appendix A: Systematic palaeontology

Systematic descriptions of conodonts herein follow the classification established by Müller (1962), Budurov and Pantić (1973), Tian et al. (1983), Shigeta et al. (2009), Yuan and Shen (2011), Brosse et al. (2017), and Metcalfe and Crowley (2020). The apparatus notation (P₁₋₂, M, S₀₋₄) of Koike (2016) and Sun et al. (2020) is applied to elements of the family Ellisonidae.

•  

  Phylum Chordata Bateson, 1886

•  

  Subphylum Vertebratea Cuvier, 1812

•  

  Class Conodonta Pander, 1856

•  

  Order Prioniodinida Sweet, 1988

•  

  Family Ellisoniidae Clark, 1972

•  

  Genus Foliella Budurov and Pantić, 1973

•  

  Type species. Polygnathus gardenae Staesche, 1964.

•  

  Foliella formosa (Tian, 1983)
  (Fig. 3a–r)

Diagnosis. The specimens are approximately 1–3 mm in size, resembling an oval leaf with a slightly narrower anterior end compared to the posterior end, and the entire unit is slightly arched in lateral view. In oral view, the cusp is in the middle part of the unit and is almost twice the size of other denticles. About 4 to 5 equally sized denticles are developed on each side of the cusp. There is an evenly distributed and separate nodular pattern on the platform. The aboral surface is concave and has pleural and obvious concentric growth lines.

Material. Six specimens

Age. Smithian

Occurrence. Daye Formation, Liangshan, Hanzhong, Shaanxi Province, South China

Remarks. Oral surface specimens, obtained from the Liangshan area, are relatively smooth, and the separate nodular pattern on the platform is obvious, consistent with the specimens of Tian (1983). Tian (1983) described the species as possessing a cusp in a subcentral position, flat denticles that are laterally fused, and an evenly distributed and separate nodular pattern on the oral surface. On the aboral surface there is no obvious basal pit (e.g. Tian 1983, pl. 82, figs. 3–6).

Budurov and Pantić (1973) described the morphological features of the genus Foliella: the overall shape of unit is like an oval leaf, with nodules distributed on platform and a keel through from the front to the back of the unit. The platform is basically symmetrical, with linear denticles distributed on the carina, and the cusp is located near the middle part of the unit that is slightly larger than other denticles.

Tian (1983) assigned this species to the genus Gladigondolella; however, we suggest that this species instead be assigned to the genus Foliella. Tian (1983) described the species “formosa” as possessing a cusp in a subcentral position, with low denticles that are laterally fused and an evenly distributed and separate nodular pattern on the oral surface. On the aboral surface there is no obvious basal pit. Besides, the genus Pachycladina has tall and strong denticles (Perri, 1987; Wu et al., 2021), which are clearly distinguished from genus Foliella. Therefore, through the above description and comparison of the characteristics of the genera Foliella, Pachycladina, and Gladigondolella, “Gladigondolella” formosa (Tian, 1983) is here suggested as belonging to the genus Foliella.

•  

  Genus Pachycladina Staesche, 1964

•  

  Type species Pachycladina obliqua Staesche, 1964

Remarks. Genus Pachycladina was originally established as a mono-element apparatus by Staesche (1964). Sweet (1981) constructed the apparatus of this genus with Pa, Pb, M, Sa, Sb, and Sc elements. Later Perri and Andraghetti (1987), Zhang and Yang (1991), Kolar-Jurkovšek (1996), Song (2012), and Wu et al. (2021) also described Pa. obliqua as seximembrate apparatus.

Diagnosis. The 15-element apparatus of Pachycladina is composed of seven different element types (4S, 1M, 2P), whose morphological description are as follows.

• The P₁ (Pa) element (Figs. 5a–x; 6a–f) is planate and has one or more large denticles of the same size as cusp in the unit. In aboral view, there is a keel without obvious furrow and basal pit.

• The P₂ (Pb) element is asymmetrical and digyrate (Fig. 6s). The element bears four to five laterally and posteriorly inclined discrete denticles.

• The M element (Fig. 6t, u) is digyrate, with two lateral processes of different lengths and a high cusp. There are five to six denticles on the posterior process.

• The S₀ (Sa) element is symmetrical and alate (Figs. 6g–l; 7a–k, t–v). There is an erect cusp on the middle of unit and two short processes with three to five denticles. The S₀ element of Pachycladina is biramous and different from other ellisonids (Koike, 2016), which are normally triramous. Our data confirm previous studies showing that the S₀ element of Pachycladina is biramous and not triramous (Sweet, 1981; Perri and Andraghetti, 1987).

• The S${}_{\mathrm{1}/\mathrm{2}}$ (Sb) elements (Figs. 6m–r, v–x to 7o–s, w–y, z–ah) are asymmetrical and digyrate. There are four to five denticles on the anterior and two to three denticles on the posterior processes. The cusp and denticles tilt posteriorly.

• The S${}_{\mathrm{3}/\mathrm{4}}$ (Sc) elements (Figs. 6y–ad, 7l–n) are asymmetrical. S₃ and S₄ are almost identical. Each element has two processes of different lengths, with three to four denticles on the anterior process and two to three denticles on the posterior process. The cusp is long and very stout, which is 2 or 3 times higher than other denticles.

•  

  Pachycladina costatus (Tian, 1983)
  (Fig. 5a–f)

•  

  1983 Latignathus costatus Tian, p. 363–364, pl. 93, fig. 7.

Diagnosis. The P₁ element is slightly arched and has five to six separate wide denticles. In aboral view, there are concentric growth lines and a keel without a furrow and a basal pit.

Material. Three P₁ elements

Age. Smithian

Occurrence. Daye Formation, Liangshan, Hanzhong, Shaanxi Province, South China

•  

  Pachycladina magnus (Tian, 1983)
  (Fig. 5g–l)

•  

  1983 Latignathus magnus Tian, p. 364, pl. 87, figs. 2, 5.

Diagnosis. The P₁ element is slightly arched and has five to seven separate, strong denticles on the well-developed platform. The first two denticles in the frontier part of the unit are relatively large, and there is a wide gap between these two bigger denticles. Other denticles are relatively small and evenly distributed behind those two large denticles.

Material. Two P₁ elements

Age. Smithian

Occurrence. Daye Formation, Liangshan, Hanzhong, Shaanxi Province, South China

•  

  Pachycladina bidentata Wang and Cao, 1981
  (Fig. 5m–x)

•  

  1981 Pachycladina bidentata Wang and Cao, p. 368, pl. 3, figs. 3–5.

•  

  1993 Pachycladina bidentata Wang and Cao; Wang, p. 262, pl. 58, figs. 8, 10.

Diagnosis. The P₁ element has concentric growth lines in both lower and lateral views. In aboral view, there is a keel without a furrow and basal pit. The anterior process has a denticle that is equal in size to the cusp, and the posterior process has several smaller and shorter denticles compared with the cusp.

Material. Seven P₁ elements

Age. Smithian

Occurrence. Daye Formation, Liangshan, Hanzhong, Shaanxi Province, South China

•  

  Pachycladina multidentata Wang and Cao, 1981
  (fig. 6a–f)

•  

  1981 Pachycladina multidentata Wang and Cao, p. 369, pl. 3, figs. 1–2.

•  

  1983 Latignathus multidentatus Tian, p. 364–365, pl. 87, fig. 3; pl. 89, fig. 2.

•  

  1993 Pachycladina multidentata Wang and Cao; Wang, p. 263, pl. 59, fig. 11.

Diagnosis. The P₁ (Pa) element (Fig. 6a–f) is planate, and the cusp is in the middle part of the unit. The anterior process is straight or curved, with at least three denticles that are almost the same size as the cusp. In aboral view, there is a keel without an obvious furrow and basal pit.

Material. 23 P₁ elements

Age. Smithian

Occurrence. Daye Formation, Liangshan, Hanzhong, Shaanxi Province, South China.