Upper Devonian conodont fauna from Western Hubei, China and its significance

ABSTRACT

Pelagic standard conodont biozonation is less applicable to nearshore shallow-water depositional settings close to the peritidal, where it is needed to establish its own conodont zonation. The Upper Devonian Xiejingsi Formation in South China is an important unit for nearshore conodont biostratigraphy investigation as its deposits in the nearshore argillaceous-carbonate mixed facies which yield numerous marine fossils. 25 conodont species belonging to 6 genera of platform elements were described in four sections of this formation in western Hubei, including 9 new species. Based on the conodont distribution in these sections, six nearshore conodont biozones are introduced in ascending order: Polygnathus brevis – Polynodosus changyangensis sp. nov. Assemblage Zone, Icriodus cornutus – Pelekysgnathus arcuatus sp. nov. – Polygnathus brevilaminus Assemblage Zone, Palmatolepis angulata Zone, Palmatolepis angulata – Palmatolepis quadrantinodosalobata Assemblage Zone, Palmatolepis quadrantinodosalobata Zone and Polygnathus planirostratus Zone. The correlation of these biozones with those of pelagic standard ones indicates an age of the Xiejingsi Formation in western Hubei from at least the Frasnian linguiformis Zone to the Famennian Lower rhomboidea Zone. Through constructing nearshore shallow-water conodont biozones based on the establishment of comprehensive conodont succession, this paper provides a case study for high-resolution conodont biostratigraphy investigation in the shallow-water settings.

KEYWORDS:

• Conodont
• nearshore biostratigraphy
• Upper Devonian
• Xiejingsi Formation
• western Hubei

Introduction

Conodonts are one of the most important marine organisms in the Devonian biostratigraphic correlation (Sandberg and Dreesen 1984; Ziegler and Sandberg 1990; Wang and Wang 2016; Spalletta et al. 2017; Wang 2018; Qie et al. 2019; Zhang et al. 2019). Many studies have revealed that the distribution of conodont elements is controlled by various environmental factors (e.g. depth, temperature, salinity), leading to diverse biofacies in different paleotectonic settings (Seddon and Sweet 1971; Sandberg 1976; Sandberg and Ziegler 1979; Dreesen and Thorez 1980; Sandberg and Dreesen 1984; Klapper and Lane 1985; Matyja 1987; Sandberg et al. 1988; Zhen and Percival 2003). For the Upper Devonian, numerous investigations of pelagic conodont biostratigraphy were carried out and several pelagic standard conodont zonation schemes were proposed, which are established based on the taxa of globally distributed deep-water biofacies (e.g. palmatolepid-polygnathid biofacies) and provide the worldwide high-resolution stratigraphic correlation of the deep-water settings (Sandberg and Ziegler 1973, 1979; Ziegler and Sandberg 1984, 1990; Sandberg et al. 1988, 1989; Ashouri 2006; Over 2007; Mossoni et al. 2012; Bahrami et al. 2013; Navas-Parejo et al. 2015; Huang and Gong 2016; Spalletta et al. 2017). Compared with the pelagic conodont biostratigraphy, nearshore conodont biostratigraphy displays a relatively low-resolution in the stratigraphic correlation due to yielding few diagnostic deep-water taxa for direct correlation with the pelagic standard conodont zones, and lacking continuous favourable lithologic succession suitable for conodont analysis. Although some researchers proposed a few nearshore shallow-water conodont biozones that are roughly related to the pelagic standard ones (e.g. Sandberg and Dreesen 1984; Qin et al. 1988; Ji 1989, 2004; Wang 1995; Ovnatanova and Kononova 2008; Wang et al. 2016), the classification schemes in their studies differ from each other both in the diagnostic taxa and time span, leading to poor application to another location.

In South China, the Upper Devonian rocks are developed in a distinct pattern of basin-platform alternate distribution and generally divided into clastic, argillaceous-carbonate mixed, carbonate platform, and slope-basin facies from nearshore to offshore (Ji 1989). Among them, the pelagic standard zonation is successfully applicable in the carbonate platform margin and slope-basinal facies (Bai et al. 1982, 1994; Ji and Liu 1986; Ji 1989, 2004; Wang 1989; Ji and Ziegler 1993; Ma and Bai 2002; Huang and Gong 2016; Nie et al. 2016; Wang et al. 2016), whereas this zonation is poorly recognised in shallow-water facies because of either the rarity of diagnostic deep-water taxa or recovery of unproductive conodont samples. As a result, biostratigraphic correlation within the shallow-water settings usually exhibits relatively low-resolution since it depends mainly on few diagnostic deep-water conodont taxa or on other fossils such as brachiopods (Shen 1982; Wang and Ziegler 1982; Zhao and Zuo 1983; Ji 1987, 2004; Ma and Bai 2002; Ma and Zong 2010; Gong et al. 2012). Therefore, the need for a high-resolution conodont biostratigraphic framework in the shallow-water settings has become urgent. The Upper Devonian of western Hubei, depositing in nearshore to terrestrial paleo-settings and bearing abundant fossils (e.g. plants, spores, brachiopods) (Feng et al. 1984), includes three formations, namely Huangjiadeng, Xiejingsi and Tizikou formations in ascending order. Among them the Xiejingsi Formation is the only one suitable for nearshore conodont biostratigraphy investigation in western Hubei. Unfortunately, the Upper Devonian conodont biostratigraphy in this area remains poorly studied although Zhang et al. (2002) briefly mentioned a few conodont taxa from the lower part of the Xiejingsi Formation in Jianshi County (Figure 1). The Upper Devonian of western Hubei is well-known for its several sets of oolitic hematite layers (referred to as oolitic/ooidal ironstones in other literatures) (Fu 1959; Hu and Xu 1998; Zhao and Bi 2000; Hui 2014; Wan et al. 2020; Gan et al. 2021), which is the most important sedimentary iron formation in China (known as the ‘Ningxiang’ type) and widely distributed in Hubei, Hunan, Jiangxi and Guangxi of China (Zhao and Bi 2000). In addition to their economic value, the oolitic hematite layers in western Hubei are generally treated as marker beds for stratigraphic correlation (Hu and Xu 1998; Zhao and Bi 2000; Hui 2014). Given that the iron formations occur in the Middle Devonian in Guangxi, Guizhou and Yunnan provinces, while in the Upper Devonian of Hubei, Hunan and Jiangxi provinces (Zhao and Bi 2000), the stratigraphic position of oolitic hematite layers possibly indicates the course of Devonian transgression in South China from the southwest to the northeast. Thus, the investigation of conodont stratigraphy in western Hubei will also provide a high-resolution time frame for the iron formations.

Figure 1. Locality map showing four studied sections (solid triangles) and the Devonian system in western Hubei Province. A, Location of Hubei Province in China; B, Location of studied area in Hubei Province; C, Location of studied sections; D, Devonian system in western Hubei Province.

In this paper, conodonts from four sections in western Hubei are illustrated in detail. By integrating the conodont successions in these sections, we present a comprehensive investigation of the nearshore conodont stratigraphy in this area. Several preliminary nearshore conodont biozones based on the conodont successions are proposed and the stratigraphic correlation between western Hubei and other regions is discussed as well.

Geological setting

Subject to the ‘Guangxi’ (Caledonian) Orogeny at the end of the Silurian, western Hubei did not receive deposits until the late Middle Devonian Givetian, when the Devonian transgression reached this area (Bai et al. 1982; Hou and Wang 1988; Zhong et al. 1992). The Devonian system of western Hubei, unconformably overlying the Middle Silurian while conformably underlying the Lower Carboniferous, is represented by the Yuntaiguan, Huangjiadeng, Xiejingsi and Tizikou formations in ascending order (Figure 1D), with the Yuntaiguan Formation belonging to the Givetian whereas the other three included in the Upper Devonian. The Yuntaiguan Formation is characterised by thick bedded massive quartz sandstone, occasionally with some thin shaly interbeds, yielding plants and spores (Zhu et al. 1983; Feng et al. 1984; Gao 1989; Feng and Zhang 1999). The Huangjiadeng Formation, bearing diverse fossils such as plants, spores, conchostracans and fishes (Pan 1962; Feng et al. 1984; Liu and Gao 1985; Hao and Mei 1987; Geng 1990; Xue and Hao 2007), mainly consists of interbedded quartz sandstone, siltstone and mudstone. This formation may contain two sets of oolitic ironstone or ferritic sandstones in its middle and upper parts respectively in certain area. The two sets of ironstones are usually named as Fe_i and Fe_ii in ascending order for regional stratigraphic correlation and mining activity (Hu and Xu 1998; Zhao and Bi 2000). The Xiejingsi Formation, deposited within the argillaceous-carbonate mixed facies and widely distributed in western Hubei and northwestern Hunan (HNGMRB 1988; HBGMRB 1990), is composed of limestone, dolostone, argillaceous limestone and silty mudstone with numerous marine fossils including bryozoans, brachiopods and conodonts (Yang 1964; Feng et al. 1984; Zhang et al. 2002). This formation may also contain two sets of oolitic ironstone (named as Fe_iii and Fe_iv in ascending order) to occur in the lower and upper parts of the formation respectively. It is worth noting that Fe_iii is mainly made up of hematite and carbonate minerals with abundant bioclasts, such as brachiopods and corals, while Fe_iv is dominated by hematite and quartz grains with few bioclasts. Thus, Fe_iii is easily distinguished from Fe_i, Fe_ii and Fe_iv based on its lithology. According to Hui (2014) and Gan et al. (2021), the ironstone of Fe_iii around Huoshaoping area is represented by three layers of oolitic hematite (named as Fe_iii¹, Fe_iii² and Fe_iii³ in ascending order), separated from each other by siltstone and mudstone in the lower part of the Xiejingsi Formation. Fe_iii¹ is different from Fe_iii² and Fe_iii³ by having numerous coral debris, whereas the latter two layers are dominated by brachiopods instead. As for the Tizikou Formation, it is composed of quartz sandstone, siltstone, shale, mudstone and contains abundant plants and spores (Feng et al. 1984; Wang et al. 2005; Xue et al. 2005).

As the Xiejingsi Formation is the only unit suitable for Devonian conodont analysis in western Hubei, all the studied materials came from four exposures of this formation in Changyang County, Yichang, Hubei Province (Figure 1), namely the Linxiangxi (geographic coordinates: 30° 27′ 0.58″ N, 110° 37′ 56.16″ E), Shaozhuya (30° 31′ 37.13″ N, 110° 42′ 36.07″ E), Sanchuanling (30° 37′ 34.98″ N, 110° 27′ 48.88″ E) and Yazikou (30° 26′ 35.95″ N, 110° 53′ 15.34″ E) sections. Among them, oolitic hematite intercalations are developed in the Shaozhuya, Sanchuanling and Yazikou sections but totally absent in the Linxiangxi section. The Xiejingsi Formation in the Linxiangxi section is well exposed along a roadcut (Figure 2A) situated at about 4.8 km west of Ziqiu Town (Figure 1C), with a thickness more than 39 m. The Shaozhuya section is located about 3 km northwest of Huoshaoping Town (Figure 1C), where the lower part of the Xiejingsi Formation (Beds 1–9, ca. 13 m thick) is well exposed along the highway (S255) and the upper part of the formation and overlying strata are completely destroyed or eroded due to the mining activities around early 2010s (Figure 2C, 2D). Some loose blocks of grey-white thin-bedded argillaceous limestone, probably digging out from the upper part of the section, are scattered around the exposure (herein treated as Bed 10). It is noteworthy that Beds 2, 6 and 8 of the Shaozhuya section herein are respectively equal to the Fe_iii¹, Fe_iii² and Fe_iii³ in Gan et al. (2021) because Bed 2 yields lots of coral debris and Beds 6 and 8 bear numerous brachiopods (Figures 2H, 2I, 3E, 3F). The Sanchuanling section is located about 4 km northwest of Langping Town (Figure 1C). The Xiejingsi Formation of this section is more than 47 m thick, with the top and parts of the middle portion covered by vegetation. Two sets of ironstone (Fe_iii and Fe_iv) are situated at Bed 4 and Bed 9 respectively. The Yazikou section is exposed along a roadcut located at about 4.5 km west of Yazikou Town (Figure 1C), where the lower part of the Xiejingsi Formation (Beds 1–6, ca. 6 m thick) is well exposed and the upper part of the formation is covered by vegetation. The ironstone of Fe_iii in this section can be divided into Fe_iii¹ and Fe_iii².

Figure 2. Outcrops of the Xiejingsi Formation in Changyang County, Hubei Province, China. A, Part outcrop of the Linxiangxi section. A small normal fault can be observed in the middle part of the section; B, Boundary between the Xiejingsi Formation and the overlying Tizikou Formation at the Linxiangxi section; C, D, Part outcrops of the Shaozhuya section. Beds 2, 6 and 8 are oolitic hematite layers (respectively Fe_iii¹, Fe_iii², Fe_iii³). Horizons above Bed 9 are destroyed; E, F, Part outcrops of the Sanchuanling section. Bed 9 is oolitic hematite layer (Fe_iv); G, Close view of Bed 1 of the Linxiangxi section; H, I, Close views of Beds 1 and 8 of the Shaozhuya section respectively showing numerous corals and brachiopods. Note that outcrop of the Yazikou section is not shown here as many horizons are covered by vegetation. Black and white arrows indicate coral and brachiopod respectively. People in A and C are ca. 1.65 m tall. Hummers in B, D-F are 30 cm long. Camera cover in G is ca. 3 cm wide. Coin in I is 2.5 cm wide.

Figure 3. Thin sections of some typical rock types (A-H are under polarising microscope, I is under reflected microscope) of the Xiejingsi Formation in Changyang County, Hubei Province, China. A, Bioclastic grainstone from Bed 4 of the Linxiangxi section; B, Bioclastic packtone from Bed 2 of the Linxiangxi section; C, Bioclastic dolomitic limestone from Bed 8 of the Shaozhuya section; D-F, Oolitic hematite from Beds 2, 6 and 8 of the Shaozhuya section respectively; G-H, Oolitic hematite from Beds 4 and 9 of the Sanchuanling section respectively; I, Same view as H but under reflected microscope. Black, white, red and blue arrows indicate brachiopod, ostracod, hematite ooid and quartz respectively. Scale bars are all 0.5 mm long.

Materials and methods

A total of 40 samples (25 from Linxiangxi, 4 from Shaozhuya, 9 from Sanchuanling, 2 from Yazikou) were collected from the in situ carbonate rocks of the Xiejingsi Formation in the four sections mentioned above for conodont analysis (Tables 1, 2). One sample (Sh10) from loose blocks of Bed 10 of the Xiejingsi Formation in the Shaozhuya section was collected as well. Each sample was at least 3 kg in weight and crushed into ~2 cm sized fragments, then dissolved in 4% formic acid for 48 hours. Subsequently, the residues were washed and dried in air. Conodont specimens were picked out by hand under a stereomicroscope directly from the residue and photographed with JCM-6000Plus SEM. All images were further processed with the software Adobe Photoshop. All illustrated specimens are housed in the Geological Museum of Peking University, Beijing, China (PKUM-02). Abbreviation of conodont genera: I. = Icriodus, Pe. = Pelekysgnathus, Pa. = Palmatolepis, Po. = Polygnathus, Pn. = Polynodosus, Pl. = Polylophodonta.

Table 1. Numerical distribution of conodont species in the Linxiangxi section.

Samples Taxa	L1	L2-1	L2-2	L4-1	L4-2	L4-3	L4-4	L5	L7-1	L7-2	L7-3	L7-4	L8	L9-1	L9-2	L9-3	L12-1	L12-2	L13	L15-1	L15-2	L15-3	L17-1	L17-2	L17-3
I. cornutus	16	15		9				1						1		15	5	12	26				2
I. iowaensis ancylus														2		1	1	4	8
I. i. iowaensis	1	4	2	3	64	25	106										3	16					3
I. multicostatus multicostatus																4	4	3
indeterminate icriodids	28	16		22	5	14	33	1	6		1	1	9	17		10	32	15	2				2
Pe. arcuatus sp. nov.	7		3	21	3	2	5		1				1
Pe. ziqiuensis sp. nov.				7	9	11	84
indeterminate pelekysgnathids			2	10	3	13	9	1	1				2			3	3	3	3				4
Pa. quadrantinodosalobata										4	1	6	27	1
Pa. angulata					123	23	162	12	8		2	7
Po. brevilaminus	25	9	8
Po. dispersus sp. nov.															1	62
Po. peltatus sp. nov.																29	6
Po. sagittiformis sp. nov.																32	2
Po. planirostratus																	27	43
indeterminate polygnathids																212	14
Pn. cf. yazdii		1		1
Pn. nodocostatus	1		1	2			5		3	2				2
indeterminate polynodothids		2														78	6
Pl. concentrica					4	1	2			1			2
Pl. confluens	2	1	1	1	2	1	6					1
Pl. curvata sp. nov.	3
Pl. nodulosa sp. nov.														18
indeterminate polylophodontids																		2	2
Sum	83	48	17	76	213	90	412	15	19	7	4	15	41	41	1	446	103	98	41	0	0	0	11	0	0

Table 2. Numerical distribution of conodont species in the Shaozhuya, Sanchuanling and Yazikou sections.

Sections and samples Taxa	Shaozhuya						Sanchuanling										Yazikou
	Sh3	Sh5	Sh8	Sh9	Sh10		Sa5-1	Sa5-2	Sa5-3	Sa5-4	Sa6-1	Sa6-2	Sa6-3	Sa8-1	Sa8-2		Y6-1	Y6-2
I. cornutus		112	17	140	10		1		1				3	5	2
I. edentatus sp. nov.		57
I. iowaensis ancylus		12	4	4	4										1
I. iowaensis iowaensis		39	18	16	1			1		1	3			1
I. monodozi		21
indeterminate icriodids		192	94	84	18		1	1	1	3	4	5	1		4			6
Pe. arcuatus sp. nov.		8	6	62	6					2
Pe. ziqiuensis sp. nov.					5			3	2	1
indeterminate pelekysgnathids		62	8	13	18					3					1			4
Pa. quadrantinodosalobata												5
Pa. angulata					93		1	1	6	3	7						2	11
Po. brevilaminus		499	28	47	8
Po. brevis	646
Po. mirificus	20
Po. peltatus sp. nov.													1
Po. robustus	92
Po. sagittiformis sp. nov.														3
Po. planirostratus													2	40
indeterminate polygnathids		17	4	16									3	9
Pn. changyangensis sp. nov.	71
Pn. nodocostatus				7	9						1							2
Pn. cf. yazdii				17								1
Pl. concentrica				4	2			1		2
Pl. confluens				8	5
Pl. nodulosa sp. nov.												4
Pl. curvata sp. nov.		158
Sum	829	1177	179	418	179		3	7	10	15	15	15	10	58	8		2	23

In this study, the ‘Late Devonian standard conodont zonation’ of Ziegler and Sandberg (1990) was selected as the referenced conodont zonation because of its wide application in numerous studies (e.g. Ji and Ziegler 1993; Wang and Ziegler 2002; Zhang et al. 2008; Tagarieva 2013; Huang and Gong 2016; Ta et al. 2021; Figure 4). It is noteworthy that the base of the Famennian is positioned at the lower boundary of the Lower triangularis Zone, which was originally defined by the first appearance of Palmatolepis triangularis (Ziegler and Sandberg 1990). Klapper et al. (1993, 2004) and Klapper (2007), however, pointed out that the first appearance of Pa. triangularis in the restricted sense is well above the defined F-F boundary in the GSSP section of Coumiac in the Montagne Noire of France. Based on detailed investigations about the replacement of conodont faunas during the F-F boundary, the lower boundary of the Lower triangularis Zone was redefined by the first appearance of Pa. subperlobata together with the flood occurrence of Pa. ultima (Klapper et al. 2004; Spalletta et al. 2017). Moreover, this redefinition does not change the GSSP for the base of the Famennian (Klapper 2007; Becker et al. 2012, 2020).

Results

More than 4700 conodont specimens (P1/Pa elements) belonging to 25 species of 6 genera (Icriodus, Pelekysgnathus, Palmatolepis, Polygnathus, Polynodosus, Polylophodonta) were obtained from 36 samples in this study, including 9 new species (Tables 1, 2). No conodonts were found in 5 samples from the Linxiangxi section (L15-1 to L15-3, L17-2 and L17-3). The richness of conodont specimens varied significantly in each sample, from less than 10 (L9-2) to more than 1000 (Sh5). More than half of the conodont specimens come from the Shaozhuya section among the studied sections although the sample number for conodont analysis from this section is much less than those from the Linxiangxi and Sanchuanling sections. The Linxiangxi section, on the other hand, contains more diversified conodont species than the rest three sections due to having more complete sedimentary succession and more samples yielding conodonts. Unlike the other contemporary conodont faunas that bear diverse elements of the palmatolepid group in the deep-water and carbonate-dominant facies, the conodont fauna of the Xiejingsi Formation in western Hubei is dominated by elements of the icriodid and polygnathid groups due to the nearshore shallow-water environment (Figure 5; Tables 1, 2). Additionally, in spite of a few species (e.g. Icriodus multicostatus multicostatus, Polygnathus dispersus sp. nov.) that were restricted to one section, most species occurred in two or all the sections, providing a reliable stratigraphic correlation between/among the studied sections (Figure 6).

Figure 4. Correlation of the Upper Devonian pelagic conodont zonation schemes, which proposed by Ziegler and Sandberg (1990) is selected as the reference scheme in this study. Generic abbreviations: Ad.: Ancyrodella, Ag.: Ancyrognathus, M.: Mesotaxis, O.: Ozarkodina, Pa.: Palmatolepis, S.: Scaphignathus.

Figure 5. Conodont composition of the Xiejingsi Formation in the studied sections. A-E, Individual abundance; F, Species number.

Figure 6. Conodont succession of the studied area and correlation between the Shaozhuya, Linxiangxi, Sanchuanling and Yazikou sections. Conodont zones: 1. Polygnathus brevis - Polynodosus changyangensis sp. nov. Assemblage Zone, 2. Icriodus cornutus - Pelekysgnathus arcuatus sp. nov. - Polygnathus brevilaminus Assemblage Zone, 3. Palmatolepis angulata Zone, 4. Palmatolepis angulata - Palmatolepis quadrantinodosalobata Assemblage Zone, 5. Palmatolepis quadrantinodosalobata Zone, 6. Polygnathus planirostratus Zone. Question marks represent unclear and speculated boundaries due to lack of conodont record. Black and white arrows indicate the sampling position with and without conodont specimens respectively.

Discussion

Conodont biostratigraphy of the Xiejingsi Formation

Beds of the studied sections are correlated with each other based on the similarity in conodont composition (Figure 6). Subject to the rarity of index or diagnostic species of the palmatolepid group, the pelagic standard conodont zonation is hardly applicable directly in the Xiejingsi Formation in western Hubei. Those nearshore conodont zones proposed by Sandberg and Dreesen (1984), Qin et al. (1988), Ji (1989), Wang (1995) and Wang et al. (2016) are also inapplicable because they were established on relatively long-ranging taxa or on endemic forms. Thus, the introduction of a local zonation scheme that is correlated with the pelagic standard conodont zonation is required. According to the conodont distribution in the studied sections, a scheme of six conodont biozones in ascending order are proposed for the biostratigraphic correlation of the Xiejingsi Formation in western Hubei (Figure 6):

Polygnathus brevis - Polynodosus changyangensis sp. nov. Assemblage Zone

This lowermost conodont assemblage, consisting of Polygnathus brevis, Po. robustus, Po. mirificus and Polynodosus changyangensis sp. nov., is represented by the sample Sh3 in the lower part of Bed 3 of the Shaozhuya section (Figure 6). This assemblage was not found at the Linxiangxi, Sanchuanling and Yazikou sections due to unfavourable lithology for conodont analysis. Po. brevis, Po. robustus and Pn. changyangensis sp. nov. are dominant in abundance. The lower and upper limits of this zone are defined by the first and last co-occurrence of the nominal species respectively. Among the conodonts of this assemblage, almost all of them are known to appear in the Frasnian, indicating that it is a typical Frasnian conodont fauna. Po. brevis generally occurs in the Lower or Upper rhenana Zones (Klapper and Lane 1985; Savage 1992; Ji and Ziegler 1993; Ovnatanova and Kononova 2008; Ovnatanova et al. 2017). Its upper range can be traced to the linguiformis Zone in America, Poland and South China (Ziegler and Sandberg 1990; Woroncowa-Marcinowska 2006; Huang and Gong 2016). Po. mirificus was firstly reported by Ji and Ziegler (1993) to range from the Upper rhenana to linguiformis Zones in the Lali section of Guangxi, South China. It was documented by Wang and Ziegler (2002) to extend up to the Upper triangularis Zone in the Dongcun section of Guangxi, South China. In Italy, it occurs in the Upper rhenana Zone (Mossoni et al. 2012). Po. robustus, which has been reported only in Poland and Canada so far (Szulczewski 1971; Klapper and Lane 1985), ranges from the punctata Zone to a level no higher than jamieae Zone. As for Pn. changyangensis sp. nov., it was previously recorded as Pn. ilmenensis to appear above the horizons yielding Palmatolepis semichatovae from the Xiejingsi Formation in southeastern Chongqing, South China (Gong et al. 2012). Given that Pa. semichatovae is an accessory index species of the Lower rhenana Zone and being confined to this zone in South China, America, Russian and Canada (Klapper and Lane 1985; Ziegler and Sandberg 1990; Gatovskii et al. 2016; Huang et al. 2018), Pn. changyangensis sp. nov. should enter in a level no lower than the Lower rhenana Zone in southeastern Chongqing. Based on the stratigraphic ranges of above species, it would be logical to infer that the lower limit of the Polygnathus brevis - Polynodosus changyangensis sp. nov. Assemblage Zone is no lower than the Upper rhenana Zone and the upper limit is no higher than the linguiformis Zone (Figure 6), indicating that the Xiejingsi Formation of western Hubei contains part of the late Frasnian deposits. Meanwhile, the presence of Po. robustus within this zone reflects its stratigraphic range up to the Upper rhenana Zone or even to the linguiformis Zone.

Icriodus cornutus – Pelekysgnathus arcuatus sp. nov. – Polygnathus brevilaminus Assemblage Zone

This zone is represented by the samples Sh5 to Sh9 from Bed 5 to Bed 9 of the Shaozhuya section and the samples L1 to L4-1 from the middle part of Bed 1 to lower part of Bed 4 at the Linxiangxi section (Figure 6). Owing to the unfavourable lithology, no samples were collected in the equivalent level of the Sanchuanling and Yazikou sections. This assemblage is characterised by the numerous occurrence of Icriodus cornutus, Pelekysgnathus arcuatus sp. nov. and Polygnathus brevilaminus without Palmatolepis. Other co-existed conodonts include I. iowaensis iowaensis, Polynodosus nodocostatus, Pn. cf. yazdii, Polylophodonta confluens and Pl. curvata sp. nov. in both sections; I. iowaensis ancylus, I. edentatus sp. nov., I. monodozi and Pl. concentrica in the Shaozhuya section and Pe. ziqiuensis sp. nov. in the Lingxiangxi section. Apart from the new species, most taxa (e.g. I. cornutus, Pn. nodocostatus, Pl. concentrica) in this assemblage belong to the Famennian taxa (Table 3), strongly suggesting a Famennian conodont fauna in nature. The lower limit of this zone is defined by the first co-occurrence of the nominal species, while the upper limit is defined by the first occurrence of Palmatolepis angulata. It is known that I. cornutus generally appears in a level no lower than the Middle triangularis Zone according to the records from Europe and South China (e.g. Sandberg and Dreesen 1984; Matyja 1993; Huang et al. 2018; Table 3) and Pa. angulata is restricted in the level from the Lower crepida to the Upper crepida zones (Sandberg and Ziegler 1973; Klapper and Lane 1985; Johnston and Drees 1993; Metzger 1994). Therefore, this assemblage zone is considered here to correspond to a range from the Middle triangularis Zone though the Lower crepida Zone (Figure 6). To place the upper limit of this zone within the Lower crepida Zone is supported by the co-occurrence of Po. brevilaminus and Pa. angulata in Sh10 in the Shaozhuya section because Po. brevilaminus is usually found in a level no higher than the Lower crepida Zone (Walliser et al. 1989; Bai et al. 1994; Ji 1994; Baliński 1995; Wang and Geldsetzer 1995; Wang and Ziegler 2002; Woroncowa-Marcinowska 2006; Cui et al. 2021; Table 3). The occurrence of Pn. nodocostatus, Pl. concentrica and Pl. confluens within this zone might reflect the earlier appearance of them in South China since these species are commonly reported in levels no lower than the Lower crepida Zone (Huddle 1968; Bai et al. 1994; Metzger 1994; Mawson and Talent 1997; Table 3). Similarly, I. monodozi was previously regarded as restricted to the late Frasnian according to Zhao and Zuo (1983), but the discovery of this species within this zone indicates a longer range to the early Famennian.

Table 3. Stratigraphic ranges of some conodonts in the previous investigations. L. = Lower, M. = Middle, U. = Upper, Um. = Uppermost.

Taxa	Stratigraphic range	References and records
I. cornutus	M. triangularis – U. trachytera zones	South China: Wang and Ziegler 1983 (U. triangularis – rhomboidea); Wang and Bai 1988 (U. triangularis – crepida); Ji 1989 (crepida); Wang 1989 (crepida – rhomboidea); Ji and Ziegler 1993 (M. triangularis – M. crepida); Bai et al. 1994 (L. rhomboidea – U. rhomboidea); Huang and Gong 2016 (M. triangularis – L. crepida); Huang et al. 2018 (M. triangularis). Belgium: Sandberg and Dreesen 1984 (M. triangularis – U. marginifera). Poland: Matyja 1993 (M. triangularis). France: Schülke 1999 (U. triangularis – Um. marginifera). Russia: Zhuravlev and Sokiran 2020 (U. triangularis – Um. crepida). Iran: Bahrami et al. 2013 (U. triangularis – L. rhomboidea); Sattari et al. 2021 (U. rhomboidea – U. trachytera). Italy: Corradini 2003 (Um. crepida – U. marginifera).
I. iowaensis ancylus	L. triangularis – L. rhomboidea zones	South China: Bai et al. 1994 (M. triangularis – L. rhomboidea). America and Belgium: Sandberg and Dreesen 1984 (M. triangularis – L. crepida). Canada: Wang and Geldsetzer 1995 (L. triangularis). Russia: Zhuravlev and Sokiran 2020 (U. triangularis – Um. crepida).
I. iowaensis iowaensis	L. rhenana – L. rhomboidea zones	South China: Han 1987 (L. triangularis – M. triangularis); Ji and Ziegler 1993 (M. triangularis – Um. crepida, L. rhomboidea?); Bai et al. 1994 (M. triangularis – L. rhomboidea); Wang and Ziegler 2002 (L. triangularis – L. crepida); Huang and Gong 2016 (L. triangularis); Huang et al. 2018 (L. triangularis); Cui et al. 2021 (linguiformis – L. triangularis). America: Sandberg and Dreesen 1984 (M. triangularis – L. rhomboidea); Ziegler and Sandberg 1990 (linguiformis). Belgium: Sandberg and Dreesen 1984 (M. triangularis – L. rhomboidea). Poland: Matyja 1993 (L. triangularis – M. triangularis); Baliński 1995 (L. triangularis – M. crepida). Canada: Wang and Geldsetzer 1995 (U. rhenana – M. triangularis). Iran: Yazdi 1999 (L. triangularis – L. crepida). Russia: Tagarieva 2013 (linguiformis – M. triangularis); Zhuravlev and Sokiran 2020 (U. triangularis – Um. crepida).
I. monodozi	within U. hassi – linguiformis zones	South China: Zhao and Zuo 1983 (within Ancyrognathus triangularis – gigas = within U. hassi – linguiformis).
I. multicostatus multicostatus	M. triangularis – U. crepida zones	South China: Ji and Ziegler 1993 (M. triangularis – U. crepida); Huang and Gong 2016 (U. triangularis).
Pa.quadrantinodosalobata	L. crepida – L. rhomboidea zones	South China: Bai et al. 1979 (crepida), 1982 (L. crepida), 1994 (U. crepida – L. rhomboidea); Ji and Ziegler 1993 (L. crepida – L. rhomboidea); Ji 1994 (L. crepida – Um. crepida); Gong et al. 2012 (crepida or L. rhomboidea); Nie et al. 2016 (L. crepida – L. rhomboidea). Australia: Druce 1976 (L. crepida – U. crepida). Canada: Klapper and Lane 1985 (L. crepida – M. crepida). Poland: Baliński 1995 (M. crepida – L. rhomboidea); Woroncowa-Marcinowska 2006 (U. crepida). Germany: Sandberg and Ziegler 1973 (L. rhomboidea). America: Sandberg and Ziegler 1973 (M. crepida – U. crepida); Metzger 1989 (crepida or L. rhomboidea), 1994 (M. crepida – L. rhomboidea); Over 2007 (U. crepida); Over and Ruppel 2011 (U. crepida – L. rhomboidea). Russia: Abramova and Artyushkova 2004 (L. crepida); Ovnatanova et al. 2017 (crepida). Italy: Corradini 2003 (L. crepida – L. rhomboidea), 2008 (L. crepida – L. rhomboidea); Mossoni et al. 2012 (U. crepida – Um. crepida); Navas-Parejo et al. 2015 (U. crepida). Iran: Gholamalian et al. 2009 (L. crepida – M. crepida); Bahrami et al. 2013 (L. crepida – possible U. crepida); Sattari et al. 2021 (Um. crepida – L. rhomboidea). Mongolia: Suttner et al. 2020 (L. rhomboidea).
Pa. angulata	L. crepida – U. crepida zones	Canada: Klapper and Lane 1985 (no lower than M. crepida); Johnston and Drees 1993 (L. crepida – lower part of U. crepida); Johnston and Chatterton 2001 (upper part of L. crepida to M. crepida – lower part of U. crepida). America: Sandberg and Ziegler 1973 (M. crepida – U. crepida); Metzger 1994 (U. crepida, may also within M. crepida).
Po. brevilaminus	L. rhenana – Um. crepida zones	South China: Shen 1982 (gigas – crepida = rhenana – crepida); Han 1987 (L. triangularis – M. triangularis); Ji and Ziegler 1993 (L. triangularis – U. triangularis); Bai et al. 1994 (L. triangularis – M. triangularis); Ji 1994 (L. triangularis – L. crepida); Wang and Ziegler 2002 (linguiformis – L. crepida); Huang and Gong 2016 (U. rhenana – U. triangularis); Huang et al. 2018 (U. rhenana – M. triangularis); Cui et al. 2021 (linguiformis – L. triangularis). Germany: Walliser et al. 1989 (L. triangularis – M. triangularis); Ziegler and Sandberg 1990 (U. rhenana – linguiformis). Morocco: Walliser et al. 1989 (L. triangularis – U. triangularis). Australia: Druce 1969 (no lower than U. gigas = no lower than U. rhenana). Poland: Baliński 1995 (L. triangularis – U. triangularis); Woroncowa-Marcinowska 2006 (linguiformis – triangularis). Canada: Wang and Geldsetzer 1995 (U. rhenana – M. triangularis). Iran: Gholamalian et al. 2009 (triangularis – U. crepida); Bahrami et al. 2013 (L. rhenana – possible U. crepida); Sattari et al. 2021 (L. triangularis – U. triangularis). Italy: Corradini 2003 (L. crepida – M. crepida). America: Ziegler and Sandberg 1990 (L. rhenana – linguiformis). Russia: Tagarieva 2013 (linguiformis – L. crepida); Zhuravlev and Sokiran 2020 (U. triangularis- Um. crepida). Mongolia: Suttner et al. 2020 (L. crepida). Vietnam: Ta et al. 2021 (linguiformis – M. triangularis).
Po. brevis	L. rhenana – linguiformis zones	South China: Ji and Ziegler 1993 (U. rhenana); Huang and Gong 2016 (U. rhenana – linguiformis). Poland: Szulczewski 1972 (L. gigas – U. gigas = L. rhenana – U. rhenana); Woroncowa-Marcinowska 2006 (linguiformis). Canada: Klapper and Lane 1985 (U. gigas = U. rhenana). America: Ziegler and Sandberg 1990 (L. rhenana – linguiformis); Savage 1992 (L. rhenana – linguiformis); Klapper and Kirchgasser 2016 (MN12-13b = L. rhenana – linguiformis). Germany: Ziegler et al. 2000 (U. rhenana). Russia: Ovnatanova and Kononova 2008 (U. rhenana); Ovnatanova et al. 2017 (U. rhenana). Vietnam: Ta et al. 2021 (U. rhenana – linguiformis).
Po. mirificus	U. rhenana – U. triangularis zones	South China: Ji and Ziegler 1993 (U. Rhenana – linguiformis); Wang and Ziegler 2002 (linguiformis – U. triangularis). Italy: Mossoni et al. 2012 (U. Rhenana).
Po. robustus	punctata – jamieae zones	Poland: Szulczewski 1971 (possible M. asymmetricus = possible punctata). Canada: Klapper and Lane 1985 (M. asymmetricus – within Ancyrognathus triangularis = punctata – no higher than jamieae).
Po. planirostratus	crepida – L. expansa zones	Northwest China: Zhang et al. 2021 (no lower than L. rhomboidea and no higher than L. marginifera). Poland: Matyja 1974 (crepida – marginifera), 1993 (U. marginifera – L. expansa); Matyja and Zbikowska 1974 (crepida – no higher than trachytera); Baliński 1995 (L. rhomboidea – U. rhomboidea). Australia: Mawson and Talent 1997 (L. marginifera). Belarus: Strel’chenko and Kruchek 2013 (U. rhomboidea). Belgium: Dreesen and Dusar 1974 (rhomboidea – trachytera).
Pn. nodocostatus	L. crepida – M. expansa zones	South China: Zhao and Zuo 1983 (rhomboidea – styriacus = rhomboidea – no lower than L. postera); Qin et al. 1988 (L. marginifera – U. marginifera); Wang and Bai 1988 (crepida); Ji and Ziegler 1993 (L. crepida – L. rhomboidea); Bai et al. 1994 (L. marginifera); Nie et al. 2016 (U. rhomboidea). America: Metzger 1994 (M. crepida – L. rhomboidea); Over 2007 (U. crepida – marginifera); Over and Ruppel 2011 (rhomboidea). Canada: Klapper and Lane 1985 (L. crepida – M. crepida). Poland: Baliński 1995 (U. crepida - M. expansa); Woroncowa-Marcinowska 2006 (U. crepida - rhomboidea). Australia: Mawson and Talent 1997 (L. marginifera). Iran: Gholamalian et al. 2009 (crepida - L. postera); Bahrami et al. 2013 (crepida); Sattari et al. 2021 (Um. marginifera). Italy: Corradini 2003 (M. crepida - postera), 2008 (M. crepida - U. postera); Mossoni et al. 2012 (L. rhomboidea). Mongolia: Suttner et al. 2020 (L. crepida).
Pl. concentrica	rhomboidea – U. marginifera zones	South China: Shen 1982 (rhomboidea). America: Huddle 1968 (rhomboidea - quadrantinodosa = rhomboidea - no higher than U. marginifera)
Pl. confluens	crepida - Um. marginifera zones	South China: Shen 1982 (crepida); Zhao and Zuo 1983 (rhomboidea - quadrantinodosa = rhomboidea - no higher than U. marginifera); Qin et al. 1988 (Um. marginifera). America: Huddle 1968 (quadrantinodosa = L. marginifera - U. marginifera); Over 2007 (rhomboidea - marginifera). Australia: Druce 1976 (rhomboidea - L. quadrantinodosa = rhomboidea - L. marginifera); Mawson and Talent 1997 (L. marginifera).

Palmatolepis angulata Zone

The lower limit of this zone is defined by entry of Pa. angulata and the upper limit is identified by the first co-occurrence of Pa. angulata and Pa. quadrantinodosalobata. This zone is preserved in the Linxiangxi (samples L4-2 to L7-1), Shaozhuya (sample Sh10) and Sanchuanling (samples Sa5-1 to Sa6-1) sections (Figure 6). Besides the nominal species, the associated conodonts within this zone comprise Icriodus cornutus, I. iowaensis ancylus, I. iowaensis iowaensis, Pelekysgnathus arcuatus sp. nov., Pe. ziqiuensis sp. nov., Polygnathus brevilaminus, Polynodosus nodocostatus, Polylophodonta concentrica and Pl. confluens. Although Pa. quadrantinodosalobata is generally treated as an accessory index species of the Lower crepida Zone with its first occurrence at or near the start of the Lower crepida Zone (Ziegler and Sandberg 1990), its first appearance in the studied sections is constantly hysteretic to that of Pa. angulata. This probably reflects the asynchronous occurrence of the two species in the neritic facies of western Hubei accompanied with the transgression during the crepida to Early rhomboidea Zones. Subject to the lack of other index species of deep-water facies, however, Pa. quadrantinodosalobata is inferred to occur in the Upper crepida Zone, whereas Pa. angulata is considered to start with the Lower crepida Zone in the studied sections judging from the thickness of this zone (ca. 4.9 m at the Linxiangxi and ca. 8.3 m at the Sanchuanling sections). Therefore, the upper limit of this zone is positioned within the Upper crepida Zone.

Palmatolepis angulata – Palmatolepis quadrantinodosalobata Assemblage Zone

This zone is characterised by the co-occurrence of Pa. angulata and Pa. quadrantinodosalobata and recognisable only in the Linxiangxi section, represented by 3 samples (L7-2 to L7-4) in Bed 7 with a thickness ca. 1 m (Figure 6). It cannot be recognised in the Sanchuanling section, which may be explained either as the condensed sedimentation in Bed 6 or as failed to discover both of the nominal species due to very low abundance of conodont specimens in the samples from Beds 5 and 6. If this zone is present in the Sanchuanling section, it could be within somewhere between the samples Sa5-4 and Sa6-2. Additional to the nominal species, Polynodosus nodocostatus, Polylophodonta concentrica and Pl. confluens are also found in this zone. Icriodus cornutus, I. iowaensis ancylus, I. iowaensis iowaensis, Pelekysgnathus arcuatus sp. nov. and Pn. cf. yazdii could occur within this zone although they are not recorded in samples of this zone from the Linxiangxi section. The lower and upper limits of this zone are defined by the first and last co-occurrence of the nominal species respectively, with the former placed within the Upper crepida Zone. Given the stratigraphic range of Pa. angulata (Sandberg and Ziegler 1973; Klapper and Lane 1985; Johnston and Drees 1993; Table 3), the upper limit is tentatively drawn at the top of the Upper crepida Zone.

Palmatolepis quadrantinodosalobata Zone

This zone is characterised by the occurrence of Pa. quadrantinodosalobata above the horizons yielding Pa. angulata, represented by samples L8 to L9-1 from Bed 8 to the lower part of Bed 9 of the Linxiangxi section and the sample Sa6-2 from the middle part of Bed 6 of the Sanchuanling section (Figure 6). In the Sanchuanling section, the co-occurrence of Polylophodonta nodulosa sp. nov. and Pa. quadrantinodosalobata in sample Sa6-2 seems to suggest that this zone is condensed in this section as Pl. nodulosa sp. nov. is co-occurred with the nominal species in the last sample (L9-1) of the Linxiangxi section. Besides the nominal species and Pl. nodulosa sp. nov., the accompanied species in this zone includes Icriodus cornutus, I. iowaensis ancylus, Pelekysgnathus arcuatus sp. nov., Polynodosus nodocostatus, Pn. cf. yazdii and Polylophodonta concentrica. I. iowaensis iowaensis, which is not documented in samples of this zone from the Linxiangxi and Sanchuanling sections, could also appear within this zone since it is found in the underlying and overlying horizons. The lower and upper boundary of this zone can be marked by the last occurrence of Pa. angulata and that of Pa. quadrantinodosalobata respectively in the studied sections. Considering the thickness of this zone (ca. 3 m at the Linxiangxi and ca. 0.3 m at the Sanchuanling sections), although Pa. quadrantinodosalobata is known to range up into the Lower rhomboidea Zone (e.g. Sandberg and Ziegler 1973; Bai et al. 1994; Corradini 2008; Sattari et al. 2021), here we tentatively to correlate this zone with the Uppermost crepida Zone.

Polygnathus planirostratus Zone

This is the uppermost conodont assemblage that can be recognised in the studied sections, represented by the samples L12-1 and L12-2 from Bed 12 of the Linxiangxi section (2 m thick) and the samples Sa6-3 to Sa8-1 from the upper part of Bed 6 to the lower part of Bed 8 of the Sanchuanling section (ca. 11 m thick) (Figure 6). This zone is characterised by the occurrence of Polygnathus planirostratus. I. cornutus, I. iowaensis ancylus, I. iowaensis iowaensis, I. multicostatus multicostatus, Po. peltatus sp. nov. and Po. sagittiformis sp. nov. are also discovered within this zone. The lower and upper limits of this zone is defined by the first and last occurrence of the nominal species respectively. It is worth noting that this zone could trace down to Bed 10 in the Linxiangxi section since no samples were carried on conodont analysis due to vegetation cover (Bed 11) and unfavourable lithology (Bed 10) in this section. Po. planirostratus (= Po. szulczewskii Matyja 1974) was firstly reported by Dreesen and Dusar (1974) to range from the rhomboidea to trachytera Zones in Belgium. It was documented to extend up to the Lower expansa Zone in western Pomerania, northwestern Poland (Matyja 1993). In Australia and Belarus, Po. planirostratus respectively occurs in the Lower marginifera Zone (Mawson and Talent 1997) and the Upper rhomboidea Zone (Strel’chenko and Kruchek 2013). This species is also reported in the level no lower than the Lower rhomboidea Zone and no higher than the Lower marginifera Zone in northwestern China (Zhang et al. 2021). Given that I. iowaensis is discovered in the horizons below (L9-3) and above (L13 to L17-1, Sa8-2) this zone, and that I. iowaensis generally occurs in a level no higher than the Lower rhomboidea Zone based on the worldwide records (Table 3), the co-occurrence of I. iowaensis and Po. planirostratus suggests that the Po. planirostratus Zone should be placed within the Lower rhomboidea Zone.

From the above discussion, the Xiejingsi Formation in the studied area comprises at least six conodont zones in ascending order, namely Polygnathus brevis – Polynodosus changyangensis sp. nov. Assemblage Zone, Icriodus cornutus – Pelekysgnathus arcuatus sp. nov. – Polygnathus brevilaminus Assemblage Zone, Palmatolepis angulata Zone, Palmatolepis angulata – Palmatolepis quadrantinodosalobata Assemblage Zone, Palmatolepis quadrantinodosalobata Zone and Polygnathus planirostratus Zone, indicating that the Xiejingsi Formation contains parts of the Frasnian and Famennian deposits, with its lower boundary traced down to at least the linguiformis Zone in the Shaozhuya section and its upper boundary up to Lower rhomboidea Zone in the Linxiangxi and Sanchuanling sections. Those horizons that yield I. iowaensis and are located above Polygnathus planirostratus Zone, namely Bed 13 to the lower part of Bed 17 in the Linxiangxi section (samples L13 to L17-1) and the upper part of Bed 8 in the Sanchuanling section (sample Sa8-2), are dated no higher than Lower rhomboidea Zone based on the worldwide reports of I. iowaensis (Table 3). Although the Frasnian-Famennian boundary in the Xiejingsi Formation cannot be accurately determined as the index species of the deep-water facies are not found, it is likely located between the lower part of Bed 3 (sample Sh3) and the upper part of Bed 5 (sample Sh5) in the Shaozhuya section and the equivalent levels of other sections. Subject to the unfavourable lithology, it is difficult to extract conodonts from the horizons beneath the samples Sh3, L1 and Sa5-1 as well as those above L17. Thus, the conodont successions of the lower and upper limit of the Xiejingsi Formation in the studied area deserve further investigation. Nevertheless, considering the relation between the thickness of horizon and the conodont zones, the lower limit of this formation probably falls within the late Frasnian whereas the upper one is confined to the rhomboidea Zone.

Correlation of the Xiejingsi Formation with other horizons in South China

In addition to western Hubei, the Xiejingsi Formation is also distributed in southeastern Chongqing where the formation contains more carbonate rocks than that in western Hubei. According to SCGMRB (1997), this unit in southeastern Chongqing has been subject to various degrees of erosion at its upper part and unconformably overlain by the Permian Liangshan Formation directly. Gong et al. (2012) carried out a preliminary investigation of conodont fauna of the Xiejingsi Formation in southeastern Chongqing. Based on their records from 8 samples in the Zhuoshui section (Xiejingsi Fm. ca. 53 m) of Qianjiang County, 8 samples in the cement-plant section (Xiejingsi Fm. ca. 29 m) of Qianjiang and 10 samples in the Xinglong section (Xiejingsi Fm. ca. 33 m) of Youyang County, three conodont assemblages can be roughly recognised in ascending order (Figure 7): (1) the first one is marked by the occurrence of Pa. semichatovae associated with Polynodosus cf. sinuosus. This assemblage is reported only at the base of the Xiejingsi Formation (Bed 51) in the Xinglong section and represents the lowest conodont fauna of this unit in southeastern Chongqing; (2) the second one is featured by the occurrence of numerous polygnathids and polynodosids in Bed 53 of the Xinglong section, Beds 5 and 6 of the Zhuoshui section and Beds 5 to 7 of the cement-plant section. Associated conodont species in this assemblage include such as Polygnathus brevis, Po. chongqingensis and Pn. changyangensis sp. nov. (= Pn. ilmenensis in Gong et al. 2012); (3) the third assemblage is characterised by the occurrence of Pa. quadrantinodosalobata. This assemblage is documented only at the top of the Xiejingsi Formation (Bed 7) in the Zhuoshui section and possibly on behalf of the uppermost conodont fauna of this unit in southeastern Chongqing.

Figure 7. Conodont succession of the Zhuoshui, cement-plant and Xinglong sections and correlation between them (based on Gong et al. 2012). A, Beds with Palmatolepis semichatovae. Illustration of the other conodont zone numbers and question marks see Figure 4. Note that arrows indicate the presumed sampling position as the sampling position was not provided by the authors. Besides, indeterminate species in Gong et al. (2012) are not shown in this figure because their records are confused.

Figure 8. Stratigraphic correlation of the Xiejingsi Formation in western Hubei with other contemporary horizons in South China. Data from: Lali (Ji and Ziegler 1993); Yangdi (Huang and Gong 2016); Liujing (Ji 1994), Kuang et al. (1989) noted that the upper part of the Rongxian Formation is covered by the Tertiary; Xikuangshan (Ji 1989; Ma and Bai 2002; Ma and Sun 2008; Ma et al. 2016). Abbreviations: Fm.: Formation, Mb.: Member.

Figure 9. Selected representative conodonts from the Xiejingsi Formation. All are P1 (Pa) elements. A, B, Icriodus cornutus Sannemann, 1955a, upper view of PKUM-02-1101/Sh5 and lateral view of PKUM-02-1102/Sh5; C, D, Icriodus iowaensis ancylus Sandberg and Dreesen, 1984, upper views of PKUM-02-1103/L13 and PKUM-02-1104/L9-3; E, Icriodus iowaensis iowaensis Youngquist and Peterson, 1947, upper view of PKUM-02-1105/L12-2; F, Icriodus multicostatus multicostatus Ji and Ziegler, 1993, upper view of PKUM-02-1106/L9-3; G, Icriodus monodozi Zhao and Zuo, 1983, upper view of PKUM-02-1107/Sh5; H–O, Icriodus edentatus sp. nov. H–J, holotype, upper, lateral, aboral views of PKUM-02-1108/Sh5; K, paratype, lateral view of PKUM-02-1181/Sh5; L–N, paratypes, upper views of PKUM-02-1110/Sh5, PKUM-02-1109/Sh5 and PKUM-02-1111/Sh5; O, paratype, aboral view of PKUM-02-1112/Sh5; P–T, Pelekysgnathus ziqiuensis sp. nov. P–R, holotype, upper, lateral and aboral views of PKUM-02-1113/L4-4; S, paratype, aboral view of PKUM-02-1114/L4-4; T, paratype, lateral view of PKUM-02-1115/L4-4; U–Z, Pelekysgnathus arcuatus sp. nov. U, paratype, upper view of PKUM-02-1116/L4-1; V–X, holotype, upper, lateral and aboral views of PKUM-02-1117/Sh9; Y, Z, paratypes, lateral views of PKUM-02-1118/Sh9 and PKUM-02-1119/Sh9; AA, BB, Palmatolepis quadrantinodosalobata Sannemann, 1955b, upper views of PKUM-02-1121/L7-2 and PKUM-02-1122/L7-3; CC, DD, Palmatolepis angulata Johnston and Chatterton, 2001, upper views of PKUM-02-1119/L4-2 and PKUM-02-1120/L7-2.

Figure 10. Selected representative conodonts from the Xiejingsi Formation. All are P1 (Pa) elements. A, B, Polygnathus brevis Miller and Youngquist, 1947, upper views of PKUM-02-1123/Sh3 and PKUM-02-1124/Sh3; C–E, Polygnathus brevilaminus Branson and Mehl, 1934. C, upper view of PKUM-02-1125/L1; D, lateral view of PKUM-02-1126/L1; E, aboral view of PKUM-02-1127/Sh5; F–M, Polygnathus dispersus sp. nov. F, paratype, lateral view of PKUM-02-1130/L9-3; G–I, holotype, upper, lateral and aboral views of PKUM-02-1131/L9-3; J, L, paratypes, upper views of PKUM-02-1132/L9-3 and PKUM-02-1134/L9-3; K, paratype, oblique view of PKUM-02-1133/L9-3; M, paratype, aboral view of PKUM-02-1186/L9-3; N, O, Polygnathus mirificus Ji and Ziegler, 1993, upper view of PKUM-02-1135/Sh3 and lateral view of PKUM-02-1136/Sh3; P–W, Polygnathus peltatus sp. nov. P, Q, paratypes, upper views of PKUM-02-1143/L9-3 and PKUM-02-1142/L9-3; R–T, holotype, upper, lateral and aboral views of PKUM-02-1141/L9-3; U, V, paratypes, aboral views of PKUM-02-1128/L9-3 and PKUM-02-1145/L9-3; W, paratype, lateral view of PKUM-02-1146/L9-3; X–CC, Polygnathus sagittiformis sp. nov. X–Z, holotype, upper, lateral and aboral views of PKUM-02-1137/L9-3; AA–CC, paratypes, upper views of PKUM-02-1138/L9-3, PKUM-02-1139/L9-3 and PKUM-02-1140/L9-3.

Figure 11. Selected representative conodonts from the Xiejingsi Formation. All are P1 (Pa) elements. A–C, Polygnathus robustus Klapper and Lane, 1985. A, B, upper views of PKUM-02-1147/Sh3 and PKUM-02-1148/Sh3; C, lateral view of PKUM-02-1149/Sh3; D–F, Polygnathus planirostratus Dreesen and Dusar, 1974, upper views of PKUM-02-1150/Sa8-1, PKUM-02-1151/Sa8-1 and PKUM-02-1152/L12-1; G–O, Polynodosus changyangensis sp. nov. G, paratype, upper view of PKUM-02-1153/Sh3; H–J, paratype, upper, lateral and aboral views of PKUM-02-1262/Sh3; K, L, paratypes, lateral views of PKUM-02-1154/Sh3 and PKUM-02-1155/Sh3; M–O, holotype, upper, lateral and aboral views of PKUM-02-1157/Sh3; P, Q, Polynodosus nodocostatus (Branson and Mehl, 1934), upper views of PKUM-02-1158/L4-1 and PKUM-02-1159/Sh9; R–U, Polynodosus cf. yazdii (Gholamalian et al., 2009). R, upper view of PKUM-02-1160/Sh9; S–U, upper, aboral and lateral views of PKUM-02-1161/Sh9; V, W, Polylophodonta concentrica (Ulrich and Bassler, 1926), upper views of PKUM-02-1162/Sh9 and PKUM-02-1163/L4-2; X, Polylophodonta confluens (Ulrich and Bassler, 1926), upper view of PKUM-02-1164/Sh9.

Figure 12. Selected representative conodonts from the Xiejingsi Formation. All are P1 (Pa) elements. A–G, Polylophodonta curvata sp. nov. A–C, paratypes, upper views of PKUM-02-1167/Sh5, PKUM-02-1169/Sh5 and PKUM-02-1168/Sh5; D–F, holotype, upper, lateral and aboral views of PKUM-02-1170/Sh5; G, paratype, aboral view of PKUM-02-1187/Sh5; H–N, Polylophodonta nodulosa sp. nov. H–J, paratypes, upper views of PKUM-02-1171/L9-1, PKUM-02-1172/L9-1 and PKUM-02-1173/L9-1; K–M, holotype, upper, aboral and lateral views of PKUM-02-1174/L9-1; N, paratype, lateral view of PKUM-02-1188/L9-1.

Among the three conodont assemblages above, the third (uppermost) one can be correlated with the Pa. quadrantinodosalobata Zone in this study because it is characterised by the occurrence of Pa. quadrantinodosalobata without other palmatolepids. Thus, the third conodont assemblage in southeastern Chongqing can be dated to the Uppermost crepida Zone. Similarly, the second (middle) conodont assemblage in southeastern Chongqing can be correlated with the Po. brevis – Pn. changyangensis sp. nov. Assemblage Zone for yielding Po. brevis and Pn. changyangensis sp. nov., indicating a stratigraphic position no lower than the Upper rhenana Zone and no higher than the linguiformis Zone for this assemblage. The first (lowermost) conodont assemblage in southeastern Chongqing cannot be correlated with any conodont zones proposed in this study as Pa. semichatovae is not discovered in the studied area. Given that Pa. semichatovae is restricted to the Lower rhenana Zone (Klapper and Lane 1985; Ziegler and Sandberg 1990; Gatovskii et al. 2016; Huang et al. 2018), the lowermost conodont assemblage in southeastern Chongqing can be placed within the Lower rhenana Zone. Although the conodont species from the other conodont zones established in this study are not recorded in southeastern Chongqing which probably results from insufficient sampling or erosion of the formation, the conodont fauna of the Xiejingsi Formation in western Hubei can be found in southeastern Chongqing, providing a reliable stratigraphic correlation of the Xiejingsi Formation between two places. Nonetheless, the absence of Pa. semichatovae conodont assemblage in the studied area and the well-developed carbonate formation in southeastern Chongqing suggest that southeastern Chongqing was submerged earlier than western Hubei during the late Frasnian, which reflects the course of transgression from southeastern Chongqing to western Hubei.

Based on the occurrence of brachiopod Yunnanella – Yunnanellina fauna, which is an index benthic fauna of the Famennian in South China (Grabau 1923–1924, 1931; Tien 1938; Xu 1979; Ma 1995; Chen et al. 2001), the Xiejingsi Formation in western Hubei has been considered confined to the Famennian and correlated with the Xikuangshan Formation in central Hunan in the past few decades (Yang and Mu 1953; Wang et al. 1979; Feng et al. 1984; Hou and Wang 1988). Although the brachiopod fauna of the Xiejingsi Formation in western Hubei is not investigated herein, the established conodont succession and its correlation with the pelagic standard conodont zonation indicate that this unit in western Hubei at least ranges from the Frasnian linguiformis Zone to the Famennian Lower rhomboidea Zone, and provide the more precise correlation between this unit and those contemporary horizons in other regions of South China (Figure 8). On the other hand, the resolution of nearshore conodont biozones proposed in this study can be nearly as accurate as that of the pelagic standard conodont zonation. That is, the upper five conodont biozones (I. cornutus – Pe. arcuatus sp. nov. – Po. brevilaminus Assemblage Zone to Po. planirostratus Zone) can be compared to seven biozones (Middle triangularis Zone to Lower rhomboidea Zone) of the pelagic standard conodont zonation, among which some biozones are nearly equivalent to the pelagic ones (e.g. Po. planirostratus Zone vs. Lower rhomboidea Zone).

Systematic palaeontology

Order Prioniodontida Dzik, 1976

Family Icriodontidae Müller and Müller, 1957

Genus Icriodus Branson and Mehl, 1938

Type species

Icriodus expansus Branson and Mehl, 1938

Icriodus edentatus sp. nov.

(Figure 9H–9O)

Etymology

From Latin, edentatus = edentate, referring to few denticles of the lateral rows.

Holotype

Specimen PKUM-02-1108 (Figure 9H–9J).

Paratypes

Specimens PKUM-02-1109 (Figure 9M), PKUM-02-1110 (Figure 9L), PKUM-02-1111 (Figure 9N), PKUM-02-1112 (Figure 9O), PKUM-02-1181 (Figure 9K).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Famennian, Upper Devonian.

Occurrence

Bed 5 at the Shaozhuya section (Sh5).

Materials

57 specimens.

Diagnosis

Lateral rows composed of one and two denticles respectively, or of two denticles in each; platform straight or slightly arched at posterior part in lateral view. Aboral cavity drop-shaped.

Description

Rows of denticles narrow; medial row consists of six to nine laterally compressed and highly fused denticles, lateral rows composed of only one and two denticles respectively, or of two denticles in each, which usually aligned with the medial row at the middle platform, sometimes connect with them by transverse ridges; cusp inclined posteriorly, higher or equally high to the other denticles; platform straight or slightly arched at posterior part in lateral view, medial and lateral rows of denticles of equal height. Aboral cavity excavates throughout platform, with anterior half narrow while posterior half broadly rounded, exhibiting a drop-shaped outline.

Remarks

I. edentatus sp. nov. is easily distinguished from the other species by having the lateral rows bearing only one and two denticles respectively, or two denticles in each. I. monodozi differs from I. edentatus sp. nov. by possessing only one denticle in both lateral rows.

Genus Pelekysgnathus Thomas, 1949

Type species

Pelekysgnathus inclinatus Thomas, 1949

Pelekysgnathus arcuatus sp. nov.

(Figure 9U–9Z)

Etymology

From Latin, arcuatus = arcuate, referring to the arcuate upper margin in lateral view.

Holotype

Specimen PKUM-02-1117 (Figure 9V–9X).

Paratypes

Specimens PKUM-02-1116 (Figure 9U), PKUM-02-1118 (Figure 9Y), PKUM-02-1119 (Figure 9Z).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Famennian, Upper Devonian.

Occurrence

Beds 1–8 at the Linxiangxi section (L1 to L8); Beds 5–9 and loose blocks at the Shaozhuya section (Sh5 to Sh10); top part of Bed 5 at the Sanchuanling section (Sa5-4).

Materials

127 specimens.

Diagnosis

Denticles longitudinally aligned; cusp distinct, inclined posteriorly, not larger than the one or two denticles in front of it; upper margin arcuate in lateral view, usually with the highest point at the penultimate denticle. Aboral cavity excavates throughout platform, exhibiting a drop-shaped outline.

Description

Denticulation straight or slightly curved at posterior platform; denticles laterally compressed and longitudinally aligned; cusp inclined posteriorly, not larger than the one or two denticles in front of it; upper margin arcuate in lateral view, usually with the highest point at the penultimate denticle; lower margin slightly arched at posterior half. Aboral cavity excavates throughout platform, with narrow anterior half and broadly rounded posterior half, exhibiting a drop-shaped outline.

Remarks

Pe. arcuatus sp. nov. differs from Pe. inclinatus by the much less prominent cusp which is not the highest denticle in the denticulation. Pe. elevatus (Branson and Mehl, 1938) is similar to Pe. arcuatus sp. nov. in lateral view, but the anterior three denticles in the former species project alternately left and right. Pe. arcuatus sp. nov. is distinguished from Pe. planus by the arcuate upper margin in lateral view, and from Pe. guizhouensis Wang and Wang, 1978 by having an aboral cavity that excavates throughout the platform instead of being restricted to the posterior half. In addition, Pe. arcuatus sp. nov. differs from Pe. ziqiuensis sp. nov. by the distinct cusp which is not highly fused with its front denticle.

Pelekysgnathus ziqiuensis sp. nov.

(Figure 9P–9T)

Etymology

Geographic name, after Ziqiu Town, where the present species is abundant.

Holotype

Specimen PKUM-02-1113 (Figure 9P–9R).

Paratypes

Specimens PKUM-02-1114 (Figure 9S), PKUM-02-1115 (Figure 9T).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Famennian, Upper Devonian.

Occurrence

Bed 4 at the Linxiangxi section (L4-1 to L4-4); middle to top part of Bed 5 at the Sanchuanling section (Sa5-2 to Sa5-4).

Materials

122 specimens.

Diagnosis

Denticles longitudinally aligned and highly fused; cusp strongly inclined posteriorly, less prominent, fused with the front denticle; upper margin arcuate in lateral view, with the highest point near the middle. Aboral cavity excavates throughout platform, exhibiting a drop-shaped outline.

Description

Denticulation straight or slightly curved at posterior part; denticles highly fused below their apices, almost equal in sizes except the smallest one at anterior end; cusp relatively large, strongly inclined posteriorly and highly fused with the front denticle; upper margin arcuate in lateral view, usually with the highest point near the middle; lower margin arched at posterior half; aboral cavity excavates throughout platform, with narrow anterior half and broadly rounded posterior half, exhibiting a drop-shaped outline.

Remarks

Pe. ziqiuensis sp. nov. differs from Pe. inclinatus by the much less prominent cusp which is highly fused with the front denticle, and by the highest point of the upper margin that is located near the middle part instead of the cusp. The anterior three denticles project alternately left and right of Pe. elevatus are easily distinguished from those in alignment of Pe. ziqiuensis sp. nov. Pe. ziqiuensis sp. nov. is similar to Pe. planus in having a less prominent cusp, whereas the upper margin is arc-shaped in the former species while nearly straight in the latter species in lateral view. Pe. ziqiuensis sp. nov. differs from Pe. guizhouensis by having an aboral cavity that excavates throughout the platform instead of being restricted to the posterior half. In addition, Pe. ziqiuensis sp. nov. differs from Pe. arcuatus sp. nov. by the highly fused denticulation and strongly inclined cusp which is lower than other denticles in lateral view.

Order Ozarkodinida Dzik, 1976

Family Polygnathidae Bassler, 1925

Genus Polygnathus Hinde, 1879

Type species

Polygnathus dubius Hinde, 1879

Polygnathus dispersus sp. nov.

(Figure 10F–10M)

Etymology

From Latin, dispersus = disperse, referring to the dispersed nodes and short ridges on the platform.

Holotype

Specimen PKUM-02-1131 (Figure 10G–10I).

Paratypes

Specimens PKUM-02-1130 (Figure 10F), PKUM-02-1132 (Figure 10J), PKUM-02-1133 (Figure 10K), PKUM-02-1134 (Figure 10L), PKUM-02-1186 (Figure 10M).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Famennian, Upper Devonian.

Occurrence

Middle and upper part of Bed 9 at the Linxiangxi section (L9-2 and L9-3).

Materials

63 specimens.

Diagnosis

Platform large, with anterior margin perpendicular to blade; carina massive; upper surface of the platform ornamented by few strong transverse ridges at the anterior part and randomly distributed small nodes and short ridges at the rest part. Aboral cavity small and located at the anterior one-third of platform.

Description

Platform asymmetric, large, arched, with anterior margin perpendicular to blade; blade about half of platform in length, composed of six to seven denticles, higher than platform; carina massive and slightly curved, smooth or partly denticulate, reaches the posterior end of platform or dies out before it; adcarinal grooves narrow and shallow, confined to anterior platform; upper surface of the platform ornamented by few strong transverse ridges at the anterior part and randomly distributed small nodes and short ridges at the rest part. Aboral cavity small, located at the anterior one-third of platform; keel reaches the posterior end of platform.

Remarks

Po. dispersus sp. nov. is featured by having a massive carina and the ornamentation which consists of few strong transverse ridges at the anterior platform and randomly distributed small nodes and short ridges at the rest part of the platform. Po. dispersus sp. nov. is similar to Po. fallax Helms and Wolska, 1967 in the ornaments, whereas the anterior margin of the platform is perpendicular to the blade in the former species while obliquely intersects with the blade in the latter species. Besides, the carina may die out before the posterior end of the platform in Po. dispersus sp. nov. whereas always reaches the posterior end of the platform in Po. fallax. Po. dispersus sp. nov. differs from Po. tabasianus Gholamalian, 2007 by having the lateral margins that are neither upturned nor strongly constricted in the anterior platform (e.g. Figure 10K). The upper surface of the platform of Po. tabasianus are mainly ornamented by nodes, which is also different from that of Po. dispersus sp. nov. The Frasnian species Po. reitlingerae Ovnatanova and Kononova, 2008 is similar to Po. dispersus sp. nov. in the ornamentation of the upper surface of the platform, but the anterior platform margin is always distinctly higher on right side than on left side in Po. reitlingerae while equally high on both sides in Po. dispersus sp. nov.

Polygnathus peltatus sp. nov.

(Figure 10P–10W)

Etymology

From Latin, peltatus = peltate, referring to the rectangular or inverted pentagonal platform that looks like a shield.

Holotype

Specimen PKUM-02-1141 (Figure 10R–10T).

Paratypes

Specimens PKUM-02-1128 (Figure 10U), PKUM-02-1142 (Figure 10Q), PKUM-02-1143 (Figure 10P), PKUM-02-1145 (Figure 10V), PKUM-02-1146 (Figure 10W).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Famennian, Upper Devonian.

Occurrence

Upper part of Bed 9 to the lower part of Bed 12 at the Linxiangxi section (L9-3 to L12-1).

Materials

72 specimens.

Diagnosis

Platform small, with anterior margin perpendicular to blade while posterior margin obliquely intersecting or perpendicular to carina, exhibiting a rectangular or inverted pentagonal outline; blade short than platform; carina denticulate and arched downward, posteriorly extends beyond platform; adcarinal grooves reach posterior margin of platform; lateral margins of platform strongly upturned, denticulate; upper surface of platform ornamented by transverse ridges that confined to lateral margins. Aboral cavity large and almost covers entire length of the platform in small specimens, while moderate sized and located at anterior one-third of platform in large specimens.

Description

Platform nearly symmetric, small, arched, with anterior margin perpendicular to blade and posterior margin obliquely intersecting or perpendicular to carina, exhibiting a rectangular or inverted pentagonal outline; blade one-third to one-fourth as long as platform, composed of approximately five denticles, higher than platform; carina straight or slightly curved, denticulate and arched downward, about four denticles posteriorly extend beyond platform; adcarinal grooves deep, reach posterior margin of platform; lateral margins of platform strongly upturned, denticulate; upper surface of platform ornamented by transverse ridges that confined to lateral margins. Aboral cavity large with flared margin and almost covers entire length of the platform in small specimens, while moderate sized and located at anterior one-third of platform in large specimens; keel reaches the posterior end of platform.

Remarks

Po. peltatus sp. nov. is characterised by having a rectangular or inverted pentagonal platform and a carina that posteriorly extends beyond the platform. Po. brevilaminus, also has long carina and upturned platform margins, is similar to Po. peltatus sp. nov. However, the posterior carina of Po. peltatus sp. nov. is arched downward in lateral view (Figure 10S, 10W), while that of Po. brevilaminus is in line with the blade (Figure 10D). Besides, the aboral cavity in small specimens of Po. peltatus sp. nov. almost covers entire length of the platform (Figure 10V), which is also different from that of Po. brevilaminus.

Polygnathus sagittiformis sp. nov.

(Figure 10X–10CC)

Etymology

From Latin, sagittiformis = sagittal form, referring to the sagittal outline of the platform.

Holotype

Specimen PKUM-02-1137 (Figure 10X–10Z).

Paratypes

Specimens PKUM-02-1138 (Figure 10AA), PKUM-02-1139 (Figure 10BB), PKUM-02-1140 (Figure 10CC).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Famennian, Upper Devonian.

Occurrence

Upper part of Bed 9 to the lower part of Bed 12 at the Linxiangxi section (L9-3 to L12-1); lower part of Bed 8 at the Sanchuanling section (Sa8-1).

Materials

37 specimens.

Diagnosis

Platform inverted triangular with anterior margin perpendicular to blade; carina reaches posterior tip of platform; lateral margins of anterior platform strongly upturned, denticulate; upper surface of platform ornamented by transverse ridges. Aboral cavity moderate sized, located at the anterior one-third of platform.

Description

Platform arched and nearly symmetric, gradually narrower towards the posterior to a tip, with anterior margin perpendicular to blade, exhibiting an inverted triangular outline; blade one-third as long as platform, composed of approximately six denticles, higher than platform; carina straight, denticulate, reaches posterior end of platform; adcarinal grooves deep at anterior margin of platform, gradually shallower towards posterior, finally dies out at posterior platform; lateral margins of anterior platform strongly upturned, denticulate; upper surface of platform ornamented by transverse ridges. Aboral cavity moderate sized, located at anterior one-third of platform; keel reaches posterior end of platform.

Remarks

Po. sagittiformis sp. nov. is easily distinguished from the other species by the inverted triangular platform as well as upturned and denticulate lateral margins of the anterior platform. It differs from the most similar species Po. inornatus Branson and Mehl, 1934 by the symmetric platform with equal height of both the inner and outer margin of anterior platform.

Genus Polynodosus Vorontzova, 1993

Type species

Polynodosus nodocostatus (Branson and Mehl, 1934)

Remarks

The genus Polynodosus, with Polynodosus nodocostatus as the type species, was proposed by Vorontzova (1993) to represent those species having longitudinal ornaments on the platform and previously assigned to Polygnathus. This classification scheme is adopted herein. Thus, the taxa with having longitudinal ornaments in this study are included in Polynodosus, namely Pn. changyangensis sp. nov., Pn. nodocostatus and Pn. cf. yazdii (Gholamalian et al., 2009).

Polynodosus changyangensis sp. nov.

(Figure 11G–11O)

2002 Polygnathus germanus germanus Ulrich and Bassler, 1926. Zhang et al.: pl. 4, figs. 15, 18–21.

2012 Polynodosus ilmenensis Zhuravlev, 2003. Gong et al.: 290, pl. 3, figs. 7, 8; pl. 4, figs. 2-5, 7.

Etymology

Geographic name, after Changyang County, where the present species is discovered.

Holotype

Specimen PKUM-02-1157 (Figure 11M–11O).

Paratypes

Specimens PKUM-02-1153 (Figure 11G), PKUM-02-1154 (Figure 11K), PKUM-02-1155 (Figure 11L), PKUM-02-1262 (Figure 11H–11J).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Frasnian, Upper Devonian.

Occurrence

Bed 3 at the Shaozhuya section (Sh3).

Materials

71 specimens.

Diagnosis

Platform lanceolate, with anterior margin obliquely intersecting with blade; carina denticulate through platform length; upper surface of platform ornamented with one row of nodes which usually form longitudinal ridges on both sides of carina; rows of nodes or longitudinal ridges parallel to carina and separated each from carina by long, deep and relatively wide grooves. Aboral cavity small, located at anterior one-third of platform.

Description

Platform lanceolate and arched, with anterior margin obliquely intersecting with blade; blade higher but shorter than platform, composed of about five to six denticles; carina denticulate and reaches posterior tip of platform, commonly sigmoidal; upper surface of platform ornamented with one row of nodes which usually form longitudinal ridges on both sides of carina; rows of nodes or longitudinal ridges parallel to carina and separated each other from carina by long, deep and relatively wide grooves. Aboral cavity small, located at anterior one-third of platform; keel reaches posterior end of platform.

Remarks

Pn. changyangensis sp. nov. had been recorded as either Po. germanus germanus by Zhang et al. (2002) or Pn. ilmenensis by Gong et al. (2012) from the Xiejingsi Formation at Jianshi County of Hubei and Qianjiang and Youyang counties of Chongqing, South China. Pn. changyangensis sp. nov. is characterised by having a denticulate carina and one row of nodes which commonly form longitudinal ridges on both sides of carina. Pn. nodocostatus and Po. germanus germanus differ from Pn. changyangensis sp. nov. in having more rows of nodes that are rarely united to longitudinal ridges. Grooves which separate the rows of nodes are inconspicuous on the posterior platform in Pn. nodocostatus and Po. germanus germanus while obvious in Pn. changyangensis sp. nov. As those specimens in Zhang et al. (2002) possess only one longitudinal ridge on both sides of denticulate carina as well as long and relatively wide grooves that separate these ridges from the carina, these specimens are included in Pn. changyangensis sp. nov. here. Besides, Pn. changyangensis sp. nov. is similar to Pn. ilmenensis in having few longitudinal ridges or rows of nodes on both sides of carina, but the carina is denticulate in the former species while smooth in the latter species (Zhuravlev 1999; Ovnatanova and Kononova 2008). Since those specimens in Gong et al. (2012) are all possess denticulate carina and conform to the morphological characteristic of Pn. changyangensis sp. nov., they are assigned to Pn. changyangensis sp. nov. In addition, Pn. nodocostatoides (Qin et al., 1988) is distinguished from Pn. changyangensis sp. nov. by having more longitudinal ridges (commonly 4–6) and the large aboral cavity which is located at the anterior end of the platform.

Polynodosus cf. yazdii (Gholamalian et al., 2009)

(Figure 11R–11U)

2009 Polygnathus yazdii Gholamalian et al.: 149, pl. 4, figs. 18–23, 25.

Occurrence

Bed 2 to the lower part of Bed 4 at the Linxiangxi section (L2 to L4-1); Bed 9 at the Shaozhuya section (Sh9); middle part of Bed 6 at the Sanchuanling section (Sa6-2).

Materials

20 specimens.

Description

Platform arched, slightly wider at middle part and sharp at posterior end, with anterior margin almost perpendicular to blade; blade about one-third as long as platform, composed of several denticles among which the highest denticle located near the junction to platform; carina curved, denticulate or partly smooth, extends to posterior tip of platform; adcarinal grooves deep at anterior margin of platform, gradually shallower towards the posterior, finally dies out at middle or posterior platform; upper surface of platform ornamented by only one longitudinal row of nodes on both sides of carina. Aboral cavity small, located at anterior one-third of platform.

Remarks

Pn. cf. yazdii is characterised by having only one longitudinal rows of nodes on both sides of carina, which is easily distinguished from the other species with more than one rows of nodes on both sides of carina, such as Pn. nodocostatus. Pn. cf. yazdii is very similar to Pn. yazdii for bearing only one longitudinal rows of nodes, whereas the latter species has a longer blade whose denticles reduce in height towards the posterior.

Genus Polylophodonta Branson and Mehl, 1934

Type species

Polylophodonta gyratilineatus Holmes, 1928

Polylophodonta curvata sp. nov.

(Figure 12A–12G)

Etymology

From Latin, curvata = curvature, referring to the distinct curvature of the carina.

Holotype

Specimen PKUM-02-1170 (Figure 12D–12F).

Paratypes

Specimens PKUM-02-1167 (Figure 12A), PKUM-02-1168 (Figure 12C), PKUM-02-1169 (Figure 12B), PKUM-02-1187 (Figure 12G).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Famennian, Upper Devonian.

Occurrence

Bed 1 at the Linxiangxi section (L1); Bed 5 at the Shaozhuya section (Sh5).

Materials

161 specimens.

Diagnosis

Platform longitudinally elliptical, with anterior margins obliquely intersects with blade, intersection point of outer platform in front of that of inner platform; carina sigmoidal, reaches posterior end of platform or dies out before it; adcarinal grooves confined to anterior inner platform; upper surface of platform ornamented by numerous concentric nodes, some united to short ridges; keel follows the curvature of carina.

Description

Platform longitudinally elliptical, large and arched, with highest point near middle part of platform, anterior platform margins obliquely intersects blade, with intersection point of outer platform in front of that of inner one, a notch usually present at intersection point of inner platform, posterior end of platform blunt or pointed; blade short and higher than platform, composed of several partly fused denticles with the highest one near the junction between blade and platform; carina denticulate and sigmoidal, curved outward anterior to central node while inward posterior to central node, finally reaches posterior end of platform or dies out before it; adcarinal grooves confined to anterior inner platform, gradually shallower towards the posterior; upper surface of platform ornamented by numerous concentric nodes, some united to short ridges. Aboral cavity small, located at anterior one-third of platform, keel sharp and follows the curvature of carina.

Remarks

The most important feature of Pl. curvatus sp. nov. is the sigmoidal carina, which differs from that of the other species. Besides, Pl. curvatus sp. nov. is discriminated from Pl. concentrica by the absence of tongued posterior platform. Pl. confluens, with the horseshoes-like arrangement of nodes and short ridges around the carina, is easily distinguished from Pl. curvatus sp. nov. Pl. gyratilineatus is similar to Pl. curvatus sp. nov. in the platform outline and ornaments, whereas the former species has long continuous concentric ridges with rare nodes both in small and large specimens (Huddle 1968). Also, the carina in Pl. gyratilineatus does not reach the posterior platform. The long and sigmoidal carina of Pl. curvatus sp. nov. is evidently different from that short and straight of Pl. nodulosa sp. nov.

Polylophodonta nodulosa sp. nov.

(Figure 12H–12N)

Etymology

From Latin, nodulosa = nodulose, referring to the platform ornamented by numerous nodes.

Holotype

Specimen PKUM-02-1174 (Figure 12K–12M).

Paratypes

Specimens PKUM-02-1171 (Figure 12H), PKUM-02-1172 (Figure 12I), PKUM-02-1173 (Figure 12J), PKUM-02-1188 (Figure 12N).

Type locality and horizon

Changyang County, west of Hubei Province, southern China; Xiejingsi Formation, Famennian, Upper Devonian.

Occurrence

Lower part of Bed 9 at the Linxiangxi section (L9-1); middle part of Bed 6 at the Sanchuanling section (Sa6-2).

Materials

22 specimens.

Diagnosis

Anterior platform margin perpendicular to blade, posterior platform narrow and tongue-shaped; carina confined to anterior platform; adcarinal grooves converge to carina towards posterior and dies out at anterior platform; upper surface of platform ornamented by numerous concentric nodes and rare ridges, with the nodes at anterior platform relatively larger than those at the posterior. Aboral cavity small, located at anterior one-third of platform.

Description

Platform large and arched, with highest point near middle part of platform; anterior platform wide and its margin perpendicular to blade, while posterior platform narrow and tongued with a pointed posterior end; blade much shorter but higher than platform, composed of several partly fused denticles; carina straight and denticulate, confined to anterior platform; adcarinal grooves restricted to anterior platform, gradually narrower, shallower and converge to carina towards the posterior; upper surface of platform ornamented by numerous concentric nodes, ridges rarely observed; nodes at anterior platform relatively larger than those at posterior. Aboral cavity small, located at anterior one-third of platform, keel reaches posterior end of platform.

Remarks

The above diagnosis and description are mainly concerned the large specimens because the nodes are less in number and randomly distributed or exhibit a weakly concentric pattern in small specimens (Figure 12H), whereas those are more in number and show an obvious concentric pattern in large ones (Figure 12I–12K). Pl. nodulosa sp. nov. is distinguished from the other species by having a platform whose anterior margin is perpendicular to the blade, and a carina which is straight but restricted to the anterior platform. Besides, Pl. nodulosa sp. nov. differs from Pl. confluens by the concentric arrangement of nodes, and from Pl. gyratilineatus by the rare ridges on the platform. The short and straight carina of Pl. nodulosa sp. nov. is different from the long and sigmoidal carina of Pl. curvatus sp. nov.

Conclusion

1. Thousands of conodont specimens belonging to 25 species (9 new species) of 6 genera were discovered in the Xiejingsi Formation in western Hubei, suggesting that conodont elements are also diverse in the very shallow environment.

2. Based on the comprehensive conodont succession of the studied area, six nearshore conodont biozones are introduced in ascending order: Po. brevis – Pn. changyangensis sp. nov. Assemblage Zone, I. cornutus – Pe. arcuatus sp. nov. – Po. brevilaminus Assemblage Zone, Pa. angulata Zone, Pa. angulata – Pa. quadrantinodosalobata Assemblage Zone, Pa. quadrantinodosalobata Zone and Po. planirostratus Zone. The correlation of these biozones with those of pelagic standard ones indicates an age of the Xiejingsi Formation in western Hubei from at least the Frasnian linguiformis Zone to the Famennian Lower rhomboidea Zone, and provides the more precise correlation between the Xiejingsi Formation in western Hubei and other contemporary horizons in South China.

3. The conodont biostratigraphy investigation of the Xiejingsi Formation in western Hubei herein, which proposes several nearshore conodont biozones that are based on the establishment of comprehensive conodont succession and can be compared to those of the pelagic standard conodont zonation (I. cornutus – Pe. arcuatus sp. nov. – Po. brevilaminus Assemblage Zone to Po. planirostratus Zone vs. Middle triangularis Zone to Lower rhomboidea Zone), provides a good case study for the high-resolution conodont biostratigraphy investigation in the shallow-water settings, especially in the nearshore argillaceous-carbonate mixed facies.

Acknowledgments

The authors are grateful to the reviewers for their useful comments that improved the manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [No. 41772015].