The Cambrian brachiopod fauna from the first-trilobite age Shuijingtuo Formation in the Three Gorges area of China

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The Cambrian brachiopod fauna from the first-trilobite age Shuijingtuo Formation in the Three Gorges area of China Zhi-Fei Zhang a,∗ , Zhi-Liang Zhang a , Guo-Xiang Li b , Lars E. Holmer c a

Early Life Institute, State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, 710069, China LPS, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, 210008, China c Uppsala University, Department of Earth Sciences, Palaeobiology, Villavägen 16, SE-752 36 Uppsala, Sweden b

Received 19 March 2015; received in revised form 8 September 2015; accepted 17 October 2015

Abstract The Yangtze platform of South China offers evidence within its Ediacaran–Cambrian geological record of the Cambrian explosion and diversification events in metazoan history. To understand the explosive radiation of animals and the environments in which it took place, the basal Cambrian fauna succession of the Aijiahe section in the Three Gorges area, western Hubei Province, has been studied, revealing the earliest brachiopod fauna (Tsunyidiscus trilobite Zone) in this region, which was dominated numerically by acrotretoids. This is accompanied by abundant skeletal fossils including minute well-preserved phosphatized archaeocyath cups and an assortment of abundant sponge spicules, chancelloriids, mollusks, hyoliths, and bradoriids, retrieved by acid-etching limestone interbeds in the black shale-dominated Shuijingtuo Formation (Series 2). The brachiopods comprise two species of acrotretoids, two types of botsfordiids (Botsfordiidae gen. et sp. indet. A and B), and four species of linguloids. Of the latter, Spinobolus popovi n. gen. n. sp. is strikingly distinctive and typified by spine-like ornamentation seen for the first time in the lower Cambrian; the remaining three linguloid genera, Palaeobolus, Eoobolus, and Lingulellotreta, have a trans-paleocontinental distribution. The Three Gorges Shuijingtuo brachiopod assemblage differs from that of the upper Atdabanian Stage (Cambrian Stage 3) in Siberia and South China, but shows great similarities with those discovered in the Tsanglangpuan (equivalent to Botoman or Stage 4) Stage of eastern Yunnan Province, Siberia, and South Australia, suggesting a much more prolonged sedimentary hiatus in basalmost Shuijingtuo Formation of the Three Gorges area than previously expected. The presence of such unconformities provides a caveat to stable isotope-based correlations that involve a number of discussions of global ocean geochemical changes across the time interval that witnessed Cambrian explosion of metazoans. © 2015 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved. Keywords: Biostratigraphy; Botoman; Brachiopoda; Lingulids; Archaeocyatha; Small shelly fossils (SSFs)

1. Introduction The Cambrian explosive radiation of metazoans (Erwin et al., 2011; Z.F. Zhang et al., 2013; Shu et al., 2014) and the international subdivision of Cambrian strata (Babcock et al., 2005; Peng, 2009; Peng et al., 2011) represent two of the most important paleontological topics in the recent decade (Clausen et al., 2015). The continuous improvement in the

∗

Corresponding author at: Early Life Institute, Geology Department, Northwest University, No. 229, North Taibai Road, Xi’an 710069, China. Tel.: +86 29 88303553; fax: +86 29 88302128. E-mail addresses: [email protected], [email protected] (Z.F. Zhang).

subdivision of Cambrian strata and its international correlation has significant consequences for our understanding of the tempo, pattern, and magnitude of the “Cambrian explosion” of metazoans (Kouchinsky et al., 2012; X.L. Zhang et al., 2014; Z.F. Zhang et al., 2014a, 2015a). However, the chronostratigraphic subdivision of the second series of the Cambrian is still provisional (Peng and Babcock, 2011; Peng et al., 2012; Clausen et al., 2015) and detailed studies of fossil assemblages and faunal successions on a regional or intercontinental scale are necessary contributions for its continuing improvement (Babcock and Peng, 2007; Clausen et al., 2015). Trilobites, brachiopods, and hyoliths are among the most common skeletal fossils in the various facies of Cambrian sediments, and constitute three of the most important

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components of the Cambrian Evolutionary Fauna (Sepkoski, 1984; Stanley, 2007). In particular, trilobites are commonly used as index fossils in the subdivision and global correlation of the Cambrian System (Babcock et al., 2005; Babcock and Peng, 2007; Peng, 2009). Nevertheless, it becomes increasingly clear that the definition of Cambrian Series 2 and Stage 3 and its global correlations, which are based merely on the first appearances of different trilobite genera on separate Cambrian continents, are insufficiently precise by themselves. The distribution, in time and space, of different trilobite genera is largely facies-controlled and the earliest recorded genera in regional or intercontinental areas are commonly endemic. Accordingly, the first appearance datum (FAD) of Cambrian epoch 2 trilobite is not perfectly synchronous among the different paleocontinents (see discussions in e.g., Peng et al., 2012). Moreover, trilobites are essentially lacking in some highly fossiliferous lower Series 2 equivalent deposits of some micro-continents (Landing and Westrop, 2004; Steiner et al., 2007). It is therefore currently considered that the base of Stage 3 should approximate the FAD of trilobites. Meanwhile, other fossil assemblages either prior to or coeval with trilobite-bearing Cambrian successions can provide good alternative index fossils in the regional stratigraphic subdivision and intercontinental correlation of the Cambrian (Li et al., 2011; Peng et al., 2012 and references therein). Owing to the relatively good preservation and wide distribution of outcrops, the Ediacaran–Cambrian (E-C) transition in the Three Gorges area of southern China has become one of the most intensively investigated in China. In this region, the well outcropped E-C transitional sequence encompasses, in ascending order, the Ediacaran Dengying Formation and the traditional “Lower Cambrian” (Terreneuvian–Series 2) Yanjiahe, Shuijingtuo, Shipai, Tianheban formations. It is evident that there are two unconformities in this sequence. One sits at around the E-C boundary between the Dengying and Yanjiahe formations. The other sits just before the FAD of trilobites at the basal Shuijingtuo Formation in this area, which raised a debate regarding the age of these trilobites: can they be correlated with other earliest trilobite occurrence in the world? A Chinese-Japanese joint research project entitled “Coevolution of Early life and environments from Snowball to the Early Palaeozoic Earth records in South China” in 2004 has been aimed at understanding the biological and environmental interactive processes at high-resolution scale, based on a series of pristine on-land drillings in South China (Sawaki et al., 2008a, 2008b; Okada et al., 2014). As a result, ocean geochemical analyses, including those of carbon, oxygen and strontium isotopes, and geochronological data are becoming increasingly available (Ishikawa et al., 2013, 2014; Tahata et al., 2013). They provisionally point to the synchronism or causal link between environmental changes and the rapid diversification of various skeletal metazoans (Ishikawa et al., 2013; Okada et al., 2014). However, investigations of the fauna contained in the first trilobite-bearing Shuijingtuo Formation of the Three Gorges area have never been done in detail due to difficulties in collecting macrofossils hosted in the organic-rich black shale and the overlooked micropaleontological potential of the bioclastic limestone interbeds in this formation.

In this paper, we present the first report of brachiopods and some associated skeletal fossils etched from the banded clastic limestone intercalated within the layers of thin-bedded calcareous black shale of the Shuijingtuo Formation in the Three Gorges area. Apart from abundant brachiopods (see Section 5), the fossil assemblage also yielded hyoliths, hyolithellids, lobopodians, mollusks, sponges, chancelloriids, and well-preserved phosphatized moulds of archaeocyathans. The brachiopods are distinctly dominated in number by acrotretoids (Lingulata, Brachiopoda). Recent studies of the Cambrian of eastern Yunnan Province suggest that the acrotretoids occur in association with the Tsanglangpuan (Botoman, Cambrian Series 2, Stage 4 equivalent) trilobite Malungia (Paterson and Brock, 2007; Yuan et al., 2011), in Malong and Wuding counties in eastern Yunnan Province. These fossils described here present for the first time the brachiopod diversification during the earliest Cambrian eodiscoid trilobite time interval in the Three Gorges area, and thus facilitate the discussion about the definition and global correlation of the as yet undecided stage 3–4 within Cambrian Series 2. 2. Geological background, stratigraphy, and methods China is now located between the Siberian platform in the north and Gondwanaland in the south, comprising the tectonically stable Tarim, Qaidam, North China, and Yangtze platforms and the Cathaysia Block (Fig. 1A). Of these, the Yangtze and North China platforms are of especial importance for studying the evolution of early life in that both terranes have welldeveloped and easily accessible Cambrian sequences (Zhu et al., 2007; X.L. Zhang et al., 2008). In the Yangtze platform, the Terreneuvian and the unnamed Series 2 sedimentary successions finely outcrop in a NE–SW direction (Fig. 1B). In particular, on the western margin of the Yangtze platform (eastern Yunnan Province, Fig. 1B), the lower Cambrian consists of a fairly thick sequence of phosphorite, dolomite limestone or limestone, sandstone, mudstone, and siltstone, generally thought to have been deposited under very shallow-water settings (Luo et al., 1994). The Ediacaran–Cambrian strata of the Three Gorges area, located on the north margin of the Yangtze platform (Fig. 1B and C), has classically been the focus of studies of the Precambrian–Cambrian transition and Ediacaran–Cambrian bio-, sequence and chemostratigraphy (Wang, 1987; Wang et al., 1998; Zhang, 2003; Zhu et al., 2007). In this region, the terminal Neoproterozoic–Cambrian sedimentary successions are markedly well developed and widely outcropped around the southeastern limbs of the Huangling Anticline (Fig. 1C). There, a thick sequence of Late Neoproterozoic carbonate rocks, assigned to the Ediacaran Dengying Formation (ca. 300 m thick), is disconformably overlain by the lowermost Cambrian Yanjiahe (equivalent to Tianzhushan) and Shuijingtuo formations. In some recent publications (Jiang et al., 2010; X.Q. Wang et al., 2012), the Yanjiahe and Shuijingtuo formations are collectively termed the Niutitang Formation, though the former is limestonedominated exclusively with rich pre-trilobite SSFs and the latter is black shale-dominated containing the earliest trilobites in this

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Fig. 1. Simplified geological map, paleogeography, fossil localities, and early Cambrian stratigraphy of the Three Gorges area. (A) Geological map showing the distribution of principal continental blocks, arc terranes and sutures of China, modified from Zhang (2003). (B) A generalized paleogeographic reconstruction of the Yangtze platform during the early Cambrian Period (modified from X.L. Zhang et al., 2008); note the relative positions of eastern Yunnan Province (1), Zhenba County (2), and Yichang (3), marked respectively by a numbered solid triangle; the inset box indicates the location of the Three Gorges area. (C) Geological sketch map showing the distribution of Cambrian strata around the Huanglin anticline in the Three Gorges area; note the three localities of the sections revisited in this study. (D) Field photo of the Wangjiaping (WJP) section showing the bed (arrowed) with concentrations of the first regional trilobite Tsunyidiscus and abundant brachiopods Eohadrotreta and Palaeobolus. (E) Stratigraphic column of the Aijiahe (AJH) section, with ␦13 Ccarb curves and radiometric ages derived from Ishikawa et al. (2008) and Okada et al. (2014); note the levels of samples collected for acid-etching.

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region (Lin et al., 2004; Dai and Zhang, 2011). The Yanjiahe Formation is about 40 m thick and dominated by thin and moderately banded argillaceous limestone, rhythmically intercalated with 1–2 cm-thick thin-bedded muddy siltstone containing finely preserved macroscopic organisms (Guo et al., 2008, 2012, 2014). Disconformably overlaying the Yanjiahe Formation, the 70 m-thick Shuijingtuo Formation is dominated by calcareous black shale, intercalated by variably banded bioclastic and muddy limestones. The Shuijingtuo Formation is typified by basal occurrences of several levels of limestone nodules (lime concretions) up to 1 m across (Fig. 1D). Above the Shuijingtuo Formation there is, in ascending order, the Shipai, Tianheban and Shilongdong formations, usually assigned to the traditional “Lower Cambrian” (equivalent to unnamed Series 2–3) (Wang, 1987). The well-exposed outcrops of the Shuijingtuo Formation containing the rich fossil sample for the current study are located close to the village of Aijiahe, 10 km southwest of Sandouping town of Yichang, western Hubei Province (Fig. 1C and D). The base of the section is located at the base of the uppermost light gray limestone of the Yanjiahe Formation (coordinates of N: 30◦ 44 55.2 , E: 111◦ 03 58.5 ) (Fig. 1E). Our study focuses on the early Cambrian black shale-dominated Shuijingtuo Formation that belongs to the earliest trilobite-bearing horizon in this region (Wang, 1987; Lin et al., 2004; Dai and Zhang, 2011). The Shuijingtuo Formation overlies the gray or light gray limestone of Yanjiahe Formation, and the contact is interpreted as disconformity based on an existing intermediate depositional hiatus at the top of the Yanjiahe Formation (Okada et al., 2014). The Shuijingtuo Formation is divided into three parts (Fig. 1D): the lowermost part is composed of an about 10 mthick organic-rich black shale containing few fossils (samples 1–3 in Fig. 1E), and characterized by bedded black limestone concretions (Guo et al., 2014); above this unit follows a ca. 40 mthick unit with fine alternation of black shale and limestone (weathering to a yellow or grayish-yellow shale), containing fine concentrations of sponge spicules (unillustrated here) and skeletal fossils at approximately around 3 m above the lime nodules (samples 5–7 in Fig. 1E); above this sponge-bearing calcareous shale follows an alteration of banded argillaceous limestone with wavy thin-bedded planes of muddy layers (samples 8 and 9 in Fig. 1E). The formation is overlain conformably by the thin-bedded siltstone of the Shipai Formation (Z.F. Zhang et al., 2015b). Acetic acid maceration of the limestone interbeds and argillaceous limestone in the middle to upper part of the formation (samples 5–9 in Fig. 1E) revealed an assortment of abundant skeletal fossils (Figs. 2–9), with most abundant skeletal and shelly individuals in the level of sample 8 in Fig. 1E. Phosphatized and phosphatic shells and tubes were extracted from buffered acetic acid (5–10%) at the Early Life Institute, Northwest University, Xi’an. Fossils were picked out manually under a binocular microscope from the scattered insoluble residues. Most recovered phosphatic or phosphatized shelly fossils are fragile and grayish or grayish-white in color. Small brachiopod fragments and sponge spicules predominate. In spite of the predominance of scattered shell fragments, some complete

and articulated micromorphic individuals of brachiopods have been found, some with exquisite preservation of the texture and micro-ornaments of the first-formed shell, which allows us to identify them with certainty. Scanning electron microscope (SEM) of uncoated fossils were taken with a PhilipsFei Quanta 400-FEG with 20.2 kV and 70–80 Pa at State Key Laboratory of Continental Dynamics, Northwest University, Xi’an. In addition, some uncoated specimens were taken to Sweden (by Z.F. Zhang), and their SEM documentation was carried out with a Zeiss Supra 35 VP field emission at the Evolutionary Biology Centre of Uppsala University, kindly assisted by Gary Wife. All specimens are presently deposited in the Early Life Institute, Northwest University, Xi’an of China. 3. Previous paleontological work on the Shuijingtuo Formation The earliest work on the lower Cambrian of the Three Gorges area was published in the 1920s, but the lithostratigraphic units, in ascending order, the Shuijingtuo, Shipai, Tianheban and Shilongdong, Qingjiamiao formations, and the Middle Cambrian Sanyoudong Group were first erected by Lu (1962). The first comprehensive investigation of the paleontology and biostratigraphy of the Three Gorges area was carried out in the 1980s by the Yichang Institute of Geology and Stratigraphy (see Wang, 1987); they described the first occurrence of trilobites at 0.8 m above the calcareous concretions that marks the base of the Shuijingtuo Formation (Wang, 1987). The trilobite fauna from this formation (Fig. 2A–E) includes the polymerids Estaingia, Metaredlichia, and Hunanocephalus, but it is dominated by the eodiscoids Tsunyidiscus acutus and Sinodiscus changyangensis (Lin et al., 2004; Dai and Zhang, 2011, 2012). In addition, the brachiopods Palaeobolus liantuoensis (Fig. 2F–I) and Homotreta changyangensis Zeng, 1987 (Fig. 2J and K) were first documented from the Shuijingtuo Formation in the Three Gorges area; however, there were no subsequent studies of the brachiopods and other faunal components from the type locality although Holmer et al. (2001) re-illustrated the type specimens of Palaeobolus liantuoensis in the monographic study of Cambrian–Early Ordovician brachiopods from Tien Shan, Central Asia. Three years later, Li and Holmer (2004) described Palaeobolus liantuoensis and two Acrotretoid species from the Early Cambrian Shuijingtuo Formation in Zhenba County, southern Shaanxi Province, north margin of the Yangtze platform, ca. 360 km northwest of the Aijiahe section studied here. In the Three Gorges area, the classic “Lower Cambrian” stratotype is near Wangjiaping (Fig. 1C), located on the north bank of the Yangtze River, about 15 km northeast of the Aijiahe section, where our additional samples were collected (Fig. 1C). At the classic road-cut exposure (Wangjiaping section) (Fig. 1C), yellowish eodiscoid trilobites can readily be picked up; they are densely aggregated on slabs of black shale (Fig. 2A–E). About 20 cm below the eodiscoid shell bed, a 20 mm-thick bed of argillaceous limestone yields numerous small (around 1 mm long) lingulid brachiopods, most probably

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Fig. 2. Fossils recovered and exposed by crack-out from the black shale in the lower Shuijingtuo Formation at Wangjiaping section on the north bank of the Yangtze River (see location details in Fig. 1C). (A–C) Fragmental and partial segments of the first regional trilobites eodiscoid Tsunyidiscus acutus, ELI-T WJP001, ELI-T WJP002, ELI-T WJP003. (D and E) Complete holaspides (stage 1 and 2) of Tsunyidiscus acutus (courtesy of Tao Dai in Xi’an), NWUJT 20208 and NWUJT 20213, adopted from Dai and Zhang (2011). (F) ELI-BP WJP001, shell valves of the lingulid brachiopod Palaeobolus preserved with a segmental trilobite Tsunyidiscus. (G) ELI-BP WJP006, densely spaced growth lines on the shell surface of Palaeobolus. (H–K) SEM micrographs; (H and I) ELI-BP WJP010, general and detailed views of Palaeobolus; (J and K) ELI-BE WJP004 and ELI-BP WJP008, showing cone-shaped convex ventral valve of the Acrotretoid Eohadrotreta zhenbaensis.

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referable to Palaeobolus liantuoensis (Fig. 2F–I). In addition, two micromorphic poorly preserved Acrotretoid brachiopods were also found from the limestone interbeds; they most probably correspond to the form previously referred to Homotreta changyangensis by Zeng (1987; Fig. 2J and K). The Wangjiaping section is the type locality of both Palaeobolus liantuoensis and Homotreta changyangensis (Zeng, 1987). The topotypic specimens illustrated here (Fig. 2F–K) were collected from the type section in order to make the further comparative study provided in this paper, since the original description was accompanied by poor illustrations. 4. Fossil assemblage from the Shuijingtuo Formation of the Three Gorges area Brachiopods are one of the most abundant and diverse components in the fauna, but with them occur several other skeletal remains.

4.2. Brachiopod taxa Eight species are described (Figs. 4–11, see Section 5 for details), all belonging to the Class Lingulata of the Subphylum Linguliformea (Holmer and Popov, 2000). Acrotretoid brachiopods are the most abundant, and are here referred to two species of Eohadrotreta, i.e., E. zhenbaensis Li and Holmer, 2004 (Fig. 4) and E. zhujiahensis Li and Holmer, 2004 (Fig. 6). The brachiopod fauna also includes five species of linguloids, comprising Spinobolus popovi n. gen. n. sp. (Figs. 7 and 8), Palaeobolus liantuoensis (Fig. 9), Eoobolus aff. viridis (Fig. 10A–E), Lingulellotreta malongensis (Fig. 10F), and two indeterminate botsfordiids (Fig. 11) provisionally labeled as Botsfordiidae gen. et sp. indet. A and B. Most of brachiopod shell valves are represented by disarticulated and broken fragments but some of the juveniles are preserved as conjoined shells. 5. Systematic paleontology

4.1. Occurrence and associated metazoan fossils In addition to the trilobites, bradoriid carapaces are another representative of arthropods, morphologically referred tentatively to Kunmingella douvillei (Fig. 3A–C). Bradoriid arthropod remains are not as common as those of eodiscid trilobites (Tsunyidiscus acutus), but can be arduously collected in the trilobite-bearing layers of sediments. The other ecdysozoan forms are represented by a few micro-sclerites from the lobopod Microdictyon (Fig. 3D and E). Sponge spicules are the most abundant and various skeletal forms (Fig. 3F and G), largely collected by means of the acid-etching technique. Although most are isolated spicules, some are evidently articulated and conjoined (Fig. 3F and G). Even less abundant than sponge spicules, phosphatic molds of coeloscleritophorans (Fig. 3H), mollusks, hyoliths (Fig. 3K and L), and other tubular fossils (Fig. 3M–P) are fairly common. These tubular fossils might be preserved with the original shape, with most infilled by sedimentary matrix (Fig. 3I–O). Originally calcareous fossils, such as archaeocyathids, can be retrieved in the acetic acid-resistant residues because they are replaced by diagenetic phosphate (Bengtson et al., 1990; Wrona, 2004; Skovsted, 2006). Nevertheless, most of the acid-etched archaeocyathan fossils occurred as broken fragments after digestion of the carbonate rock samples (Skovsted, 2006, figs. 7.28, 7.29). In contrast, our collection shows some articulated tubular apatite steinkerns of the archaeocyathids (Fig. 3Q–S). It is reasonable to assume that the calcareous skeletal composition has been dissolved and diagenetic phosphate might then have formed as a thin crust on the surface of the inner and outer walls and parietes, leaving the crust as calcareous skeletal remains. After diagenetic replacement, the secondary phosphatic crusts on the surface of inner and outer walls are composed mainly of calcium phosphate, and the central cavity and intervallum are intruded by surrounding sediments, meanwhile diagenetic phosphate is not eliminated therein (Fig. 3Q–S).

All the figured specimens are deposited in the Early Life Institute (Prefix: ELI), Department of Geology and State Key Laboratory of Continental Dynamics, Northwest University, China. The number and level of the collection samples have been marked in Fig. 1E. Class Lingulata Gorjansky and Popov, 1985 Order Acrotretida Kuhn, 1949 Superfamily Acrotretoidea Schuchert, 1893 Family Acrotretidae Schuchert, 1893 Genus Eohadrotreta Li and Holmer, 2004 Type species. Eohadrotreta zhenbaensis Li and Holmer, 2004. Diagnosis. See Holmer and Popov (2007, pp. 2560–2562). Remarks. Eohadrotreta was erected by Li and Holmer (2004) for a low conical Acrotretoid recovered from the lower part of Shuijingtuo Formation in southern Shaanxi Province (see Fig. 1B). It is similar to Hadrotreta Rowell, 1966, but differs from the latter mainly in having a vestigial apical process and no apical pits in the ventral interior. Recently, the genus Eohadrotreta was documented from the Cambrian Stage 4 of the Tethyan Himalaya (Popov et al., 2015) and Shuijingtuo Formation in Fangxian County of western Hubei Province (Yang et al., 2015), providing a potential link to the contemporaneous linguliformean faunal association with South China. Eohadrotreta zhenbaensis Li and Holmer, 2004 (Figs. 4, 5) 2004 2007

Eohadrotreta zhenbaensis – Li and Holmer, p. 206, figs. 11–13. Eohadrotreta zhenbaensis – Holmer and Popov, p. 2560, figs. 1693, 1694. 2015 Eohadrotreta zhenbaensis – Yang et al., fig. 9E, F.

Examined material. 224 ventral valves, 467 dorsal valves and 30 conjoined shells, from the middle and upper Shuijingtuo

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Fig. 3. SEM micrographs of small shelly fossils acid-etched from the Shuijingtuo Formation of Aijiahe section, Sandouping, Yichang, western Hubei Province, China. (A and B) ELI-AB AJH001 and ELI-AB AJH002, indeterminate bradoriid arthropod carapaces. (C) ELI-AB AJH061, enlarged view of the reticulate ornamentation on the bradoriid carapace surface. (D and E) ELI-LM AJH047, general and detailed view of Microdictyon skeleton. (F and G) ELI-SX AJH047 and ELI-SX AJH124, two articulated sponge spicules. (H) ELI-CH 2015-1 chancelloriids. (I and J) Mollusks; (I) ELI-MO 2015-1, Mellopegma indecora; (J) ELI-MO 2015-2, Oelandiella gobiica. (K and L) ELI-HY 2015-1 and ELI-HY 2015-2, two undetermined hyoliths. (M–P) Tubes of indeterminate Hyolithellus; (M) ELI-HYO 2015-1; (N) ELIHYO 2015-2; (O) ELI-HYO 2015-3; (P) ELI-HYO 2015-4. (Q–S) Phosphate steinkern of archaeocyath cup in lateral (Q), transverse upper (R), and interior (S) views; (Q and R) ELI-AR AJH001; (S) ELI-AR AJH002.

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Fig. 4. The acrotretoid brachiopod Eohadrotreta zhenbaensis acid-dissolved out of the limestone interbeds or nodules of the Shuijingtuo Formation at Aijiahe and Wangjiaping sections in the Three Gorges area, Yichang, western Hubei Province, China. (A–I) Ventral valve; (A) ELI-BE 2015-01, general view, showing the lower cone-shaped morphology; (B) ELI-BE 2015-01, enlarged view of the umbo with a foramen; (C) ELI-BE 2015-02, lateral view of the foramen not enclosed within the larval shell; (D) ELI-BE 2015-02, close-up view of the closely packed flat-based circular pitted imprints in the larval shell (C); (E) ELI-BE 2015-03, lateral view of the ventral posterior umbo; (F) ELI-BE 2015-04, procline ventral pseudointerarea with shallow intertrough; (G) ELI-BE 2015-05, interior view showing the baculate imprints of mantle canals; (H) ELI-BE 2015-06, columnar shell structure of the fragmental shell valve section; (I) ELI-BE 2015-07, interior view of the foramen, concave apical process and no paired apical pits. (J–P) Dorsal valve; (J) ELI-BE 2015-08, general view; (K) ELI-BE 2015-09, dorsal larval ornamentation of pitted structures; (L) ELI-BE 2015-10, dorsal interior; (M) ELI-BE 2015-11, dorsal interior of an adult shell; (N) ELI-BE 2015-12, posterior view of dorsal pseudointerarea and cardinal muscle scars; (O) ELI-BE 2015-13, anterior and lateral view of dorsal interior; (P) ELI-BE 2015-14, lateral view of dorsal interior.

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Fig. 5. Sketch reconstruction of the acrotretoid brachiopod Eohadrotreta zhenbaensis from the Shuijingtuo Formation in the Three Gorges area, Yichang, western Hubei Province, China. (A) Ventral interior; (B) dorsal interior.

Formation at Wangjiaping (Fig. 1C and D) and Aijiahe sections (samples 5–9 in Fig. 1E) in Yichang, and Wangzishi section (Fig. 1C) in Changyang County. Stratigraphy and locality. Shuijingtuo Formation (TsunyidiscusHubeidiscus Zone), Cambrian (Series 2) in southern Shaanxi Province and western Hubei Province, and middle Wulongqing Formation in Wuding County of eastern Yunnan Province. Description. The shell is ventribiconvex, subcircular to transversely oval in commissural outline (Fig. 4A), on average 92% as wide as long; it is finely ornamented with concentric growth lines. The ventral valve is low, conical to gently convex, with a somewhat straight posterior margin bisected by a slightly recessed and shallow intertrough that tapers slightly ventrally (Fig. 4E and F); the ventral apex is moderately convex, situated on a rounded ventral larval valve slightly posterior to the posterior 1/3 of shell length; the ventral larval valve is nearly circular in outline, ca. 150 ␮m in diameter, but perforated by the pedicle foramen, not enclosed within the larval shell (Fig. 4B and C); the larval valve is ornamented by uniform flat-bottomed hemispherical pits ca. 1 ␮m in diameter (Fig. 4D), becoming indistinct towards the margin of the larval valve. The edge of the larval shell is marked by a distinct growth halo; postlarval ornamentation of the concentric fila is seen in the postlarval shell (Fig. 4B and C). The external pedicle foramen is circular to elongate oval, approximately 100 ␮m across, distinctly cutting across the postlarval growth lines for about posterior 2/3 of the length, and pointing posteromedially (Fig. 4C); the ventral pseudointerarea extends from the apsacline to the procline, with shallow to moderately developed intertrough (Fig. 4E and F). The ventral interior is invariably poorly preserved. There is an apical process forming a low boss-shaped projection with a median depression anterior to the foramen; the internal pedicle tube is very short to vestigial; no apical pits are developed on either side of the apical process (Fig. 4I). The four proximal vascula lateralia are impressed as pronounced ridge-like imprints extending anteriorly from the base of the apical process (Fig. 4G). The shell structure is laminar and inserted with sets of columnar textures (Fig. 4H and I). Around the internal pedicle foramen, the epithelial imprints of the interior ventral valve are well preserved (Fig. 4I). Ordinarily, cardinal muscle scars can be seen but are not always strongly marked; they are situated on both sides of the internal pedicle foramen, where they

mark the transition junction of the valve with the posterior valve slope. Immediately anterior to the internal pedicle foramen is a triangular concave depression that represents the position of the apical muscular platform, slightly elevated from the anterior valve slope (Fig. 4I). The dorsal valve is transversely oval in outline with the beak protruding posteriorly beyond the posterior commissural margin (Fig. 4J), moderately convex in lateral profile with the maximum height slightly posterior to the midvalve. The postlarval valve is ornamented with fine growth lines, and the larval shell is defined by a distinct rimmed halo (Fig. 4J) and occupied by pitted ornamentation (Fig. 4K); the dorsal pseudointerarea, small and orthocline–apsacline, is characterized by a narrow subtriangular median groove laterally flanked by two thick-bordered propareas with transverse wrinkles (Fig. 4L–O); the median grooves are seemingly supported by a raised triangular and posteriorly expanding buttress (Fig. 4L and M); the triangular median buttress continues anteriorly as a narrow ridge-like, anteriorly extended median tongue and a pair of submedian ridges. The median tongue is seemingly baculate in young forms (Fig. 4L), becoming spade-like with a lobate extremity (Fig. 4M). The dorsal cardinal muscle region is relatively small and somewhat thickened, extending to 30% of the valve length; the cardinal muscles scars are markedly prominent, widely separated and extend anterolaterally directly anterior to either lateral marginal line of the median groove (Fig. 4M and N), and slightly raised above the valve floor (Fig. 4O and P). Remarks. In the only previous study of the paleontology and biostratigraphy of the Three Gorges area (Wang, 1987), Zeng (1987) described a type of acrotretoid brachiopod (Fig. 2J and K), which was referred to a new species of Homotreta, and named it H. changyangensis Zeng, 1987. Although H. changyangensis was minimally described and poorly illustrated by Zeng (1987), the low conical outline of the ventral valves and their occurrence in the same stratigraphic level as Eohadrotreta zhenbaensis makes it impossible to eliminate the possibility that the former species is a senior synonym of the latter but the poor preservation of the shale-hosted specimens makes it difficult to compare with the specimens extracted by dissolving limestone with weak acids, and more well-preserved material of H. changyangensis is needed in order to address this question.

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Whilst lingulid brachiopods within the Family Acrotretidae have been widely known from the Lower Cambrian limestone or mudstones in the upper Yangtze platform (Zeng, 1987; Zhang and Pratt, 2008; Z.F. Zhang et al., 2014b), Li and Holmer (2004) provided a detailed description of Eohadrotreta zhenbaensis on the basis of a series of measurements of dorsal and ventral valves and statistics of their relative ratios of length, width and height. In view of the most recent study (Yang et al., 2015), the earlier material illustrated by Li and Holmer (2004) was recovered from the Zhenba–Fangxian Block, a possible independent terrane or part of the Yangtze platform. Here, we present additional specimens of E. zhenbaensis recovered for the first time from the limestone interlayer intercalated within the black shale-dominated Shuijingtuo Formation of the Three Gorges area in the northern margin of the Yangtze platform (Fig. 1B). These Three Gorges specimens have strong similarities with those recovered from southern Shaanxi Province in shell size and outline, notably in terms of both lacking any apical pits close to the interior pedicle opening in the ventral interior (Li and Holmer, 2004). In contrast, the specimens presented here have preserved imprints of dorsal and ventral mantle canals and muscular scars (Fig. 4G and L–P), which enable us to provide the new reconstruction in Fig. 5A and B. Eohadrotreta zhujiahensis Li and Holmer, 2004 (Fig. 6) 2004

developed, with a vestigial intertrough (Fig. 6F and G). The external pedicle foramen is circular, not enclosed in the larval shell. The dorsal valve is gently convex in dorsal view (Fig. 6K), with its maximum height posterior to 1/3 of total length; the dorsal pseudointerarea is orthocline, narrow, tripartite, with a shallow median groove, bordered laterally by small propareas; the median buttress is posteriorly overlapped by the median groove, rapidly tapering forward, seemingly aligned with a very low and indistinct median ridge. The drop-shaped cardinal muscle scars are symmetrically disposed at an angle of about 45◦ to the median buttress (Fig. 6L and M). Remarks. Eohadrotreta zhenbaensis has a general similarity in shell shape to E. zhujiahensis, and the two species also have some similarities in the morphology of dorsal interiors, such as having a pseudointerarea with a shallow median groove and buttress and cardinal muscle. The latter species can be distinguished from the former in having a shallow or less conical ventral valve with an open foramen and vestigial intertrough. Nevertheless, it is possible that E. zhujiahensis could be considered as a juvenile stage of E. zhenbaensis. The conjecture remains to be tested by studying the ontogeny of the two species in details. In our collection, conjoined shell valves of E. zhujiahensis are more frequent than those of E. zhenbaensis, possibly because E. zhujiahensis is represented by more abundant juveniles and, in general, juveniles have better chances to be preserved as conjoined shells.

Eohadrotreta zhujiahensis – Li and Holmer, p. 208, figs. 14, 15.

Material. 11 complete shells, 40 ventral and 20 dorsal valves from samples 8 and 9 in Fig. 1E, middle and upper Shuijingtuo Formation near Aijiahe village, Yichang. Stratigraphy and locality. Shuijingtuo Formation (TsunyidiscusSinodiscus Zone), Cambrian (Series 2) in Xiaoyang Town of southern Shaanxi Province, and in Yichang, Changyang and Fangxian regions of western Hubei Province. Revised diagnosis. Shell biconvex or slightly ventribiconvex; ventral valve with vestigial or narrow intertrough; ventral interior with indistinct apical process and apical pits; pedicle foramen open throughout most ontogeny, not enclosed within larval shell; dorsal valve with shallow, subtriangular median groove; cardinal muscles widely bisected by the raised and pronounced triangular buttress. Description. The shell is biconvex or slightly ventribiconvex, the commissural outline typically transversely oval (Fig. 6A); the dorsal larval valve is circular or subcircular outline in dorsal view, ornamented with hemispherical pits (Fig. 6B and J); later growth stages are ornamented with growth lines; the ventral valve is somewhat elongate oval in ventral view, with its maximum width near midlength; the commissural outline has an almost straight posterior margin and strongly arched lateral margins, the anterior margin is gently circular (Fig. 6C); the ventral pseudointerarea is vestigial or weakly developed (Fig. 6D). In lateral profile, the anterior and lateral slopes of the dorsal and ventral valves are both gently convex, but the posterior slope is distinctly steeper in the ventral valve (Fig. 6E and H); there is a catacline or slightly procline ventral pseudointerarea, poorly

Order Lingulida Waagen, 1885 Family Obolidae King, 1846 Subfamily Obolinae King, 1846 Genus Spinobolus n. gen. Zhang and Holmer Etymology. Referring to the prone spine-like protrusion of the outer shell surface, and the obolid-like shape. Type and only species. Spinobolus popovi n. sp. Zhang and Holmer. Diagnosis. Shell thin, biconvex and longitudinally subtriangular in commissural outline, postlarval shell ornamented with distinct prone spinose protrusions along marginal edge of concentric growth lines; larval shell well defined, with faint radius becoming fade outwards; ventral pseudointerarea with variably developed flexure lines, distinctly bisected by broad, deep subtriangular pedicle groove; paired posterolateral muscle scars developed and slightly elevated above valve floor, immediately anterior to flexure lines; dorsal valve interior with small pseudointerarea divided by shallow median groove; dorsal median ridge developed and extending to anterior margin. Remarks. The external ornament of the concentric spines is the most distinctive character of the genus. Spinobolus has a general resemblance to Eoobolus in the shell outline, but the former differs from the latter in lacking pitted larval ornamentation, as well as in possessing ornamentation of prone spine-like pustules along the marginal edge of the growth lines in the postlarval

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Fig. 6. The acrotretoid brachiopod Eohadrotreta zhujiahensis from the Shuijingtuo Formation at Aijiahe section in the Three Gorges area, Yichang, western Hubei Province, China. (A–E) Conjoined shell valves; (A and B) ELI-BEZ 2015-01, dorsal (A) and posterior (B) views; (C–E) ELI-BEZ 2015-02, ventral (C), umbonal (D), and posterior (E) views. (F) ELI-BEZ 2015-03, oblique lateral view of ventral interior. (G) ELI-BEZ 2015-03, posterior view of ventral foramen. (H) ELI-BEZ 2015-04, lateral view of a conjoined shell valves. (I) ELI-BEZ 2015-04, posterodorsal view. (J) ELI-BEZ 2015-04, ornament of larval shell. (K–M) Dorsal valve; (K) ELI-BEZ 2015-05, general view of dorsal exterior; (L) ELI-BEZ 2015-06, dorsal interior; (M) ELI-BEZ 2015-06, detailed view of dorsal interior.

shell valves. The spinose ornamentation of Spinobolus can easily be distinguished from the pustules in the postlarval shell of Eoobolus, but has a general resemblance to the shell ornamentation of spines in the genus Spinilingula, which is well known in the Ordovician of Sweden and Kazakhstan. However, Spinobolus appears to be endemic to the Three Gorges area of the Yangtze platform, and Spinilingula differs from Spinobolus in that the prone spines along the shell growth lines are disposed in a series of lamellose patterns, although the two taxa are presumably related in phylogeny. Spinobolus popovi n. gen. n. sp. Zhang and Holmer (Figs. 7, 8)

Etymology. In honor of Leonid E. Popov (Cardiff), a distinguished and world renowned brachiopod specialist. Holotype. ELI-B YCS 15-001 (samples 6 and 9 in Fig. 1E), housed in Early Life Institute and Department of Geology, Northwest University, Xi’an. Other material. 4 ventral valves and 5 dorsal valves, and some fragments of valves with distinguishable spinose ornaments. Stratigraphy and locality. Shuijingtuo Formation (TsunyidiscusSinodiscus Zone), Cambrian (Series 2). Samples collected from the road cut section of Aijiahe village in Yichang (N: 30◦ 44 55.2 , E: 111◦ 03 58.5 ), western Hubei Province. Diagnosis. As for genus.

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Fig. 7. The linguloid brachiopod Spinobolus popovi n. gen. n. sp. Zhang and Holmer from the Shuijingtuo Formation at Aijiahe section, Sandouping, Yichang, western Hubei Province, China. (A–D) Holotype, ELI-B YCS001; (A) ventral exterior; (B) lateral view; (C and D) details of the spinose ornamentation on the shell surface. (E) ELI-B YCS002, ventral larval shell. (F) ELI-B YCS003, close-up view of posterior ventral valve. (G and H) ELI-B YCS004, general and enlarged views of dorsal exterior. (I and J) ELI-B YCS005, dorsal interior; (I) general view; (J) oblique view of posterior valve. (K–M) ELI-B YCS006, dorsal interior, anterior lateral (K), lateral (L), and anterior (M) views. (N) ELI-B YCS007, shell valve intrastructure.

Description. The shell is thin and biconvex, longitudinally subtriangular in outline, with a maximum width of 1.9 mm, slightly anterior to the midlength (Fig. 7A and B); the ventral valve is gently and evenly convex, about 87% as wide as long, with an acuminated end enclosing an apical angle of 85–95◦ . The postlarval shell is delicately ornamented with concentric growth lines that are marginally occupied externally by equidistantly pronounced and anterior-pointing spines (Fig. 7C and D); the ornamentation of spines is seemingly disposed in sagittal

alignments at intervals of 20–30 ␮m (Fig. 7C); the spinose nodes increase in number with shell growth (Fig. 7D). The ventral larval shell is ca. 180–200 ␮m (Fig. 7E), lacking any ornament of pits or pustules, but faintly occupied by lightly recognizable gentle radial lineations (Fig. 7E). The ventral pseudointerarea is orthocline and triangular, bisected by a shallow, recessed pedicle groove (Fig. 7F) with two lateral propareas slightly raised above the valve floor, and subdivided by oblique flexure lines (Fig. 7F). The ventral interior shows a pair of distinctly impressed muscle

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Fig. 8. Sketch reconstruction of the linguloid brachiopod Spinobolus popovi n. gen. n. sp. Zhang and Holmer from the Shuijingtuo Formation at Aijiahe section, Sandouping, Yichang, western Hubei Province, China. (A) Ventral exterior; (B) ventral interior; (C) dorsal interior.

scars that are obliquely disposed immediately anterolateral to both flexure lines (Fig. 7F). Paired pedicle nerves are impressed as V-shaped lineations on the posteromedian shell interior, just anterior-median to the pedicle groove (Fig. 7F). The dorsal valve is oval or subcircular, with an obtuse posterior margin and the maximum width slightly anterior to the midvalve; the postlarval shell is ornamented with concentric lines with marginal spines; the larval shell is smooth, with no observed ornament of pits or pustules (Fig. 7G and H). The orthocline dorsal pseudointerarea is low, slightly raised above the valve floor, divided by a broad, shallow median groove with an included angle of ca. 50◦ . The dorsal interior with impressed posterolateral muscle scars is bisected by an elongate median ridge that slightly widens and extends anteriorly along the median valve floor (Fig. 7L and M); no impression of mantle canals is clearly detectable in our collection. The shell valve fabric appears to be laminar (Fig. 7N). Remarks. Spinobolus popovi is one of the less abundant brachiopods in the limestones from the Shuijingtuo Formation. So far, no macroscopic specimen of S. popovi has been collected from splitting the carbonaceous shales. In contrast, some specimens of Palaeobolus were collected from the bedding planes of the Shuijingtuo shale. Although these shells show concentric growth lines on the surface, the coarse preservation of specimens in the shale relative to the etched ones from carbonaceous nodules or interlayers precludes detailed examination of the outer shell ornamentation. As a result, it is impossible to determine if some of the shale-hosted specimens of Palaeobolus liantuoensis (Fig. 2F–H) are actually akin to Spinobolus popovi. A preliminary reconstruction of the ventral exterior and interior and dorsal interior of Spinobolus popovi has been made in Fig. 8A–C. Genus Palaeobolus Matthew, 1899 Type species. Palaeobolus bretonensis Matthew, 1899; middle Cambrian (Amgian) Series 3, early Stage 5, Cape Breton, Nova Scotia, eastern Canada. Diagnosis. See Holmer and Popov (2000, p. 50). Remarks. Palaeobolus is characterized by having a subtriangular and inequivalved shell bearing finely lamellose growth fila

disposed peripherally. The ventral interior with raised triangular pseudointerarea, and low dorsal pseudointerarea, is divided by a broad, fairly shallow median groove. Palaeobolus is so far known from the Lower Cambrian of Canada, Kazakhstan, and South China; a detailed description of the genus from Canada and Kazakhstan has been provided by Holmer et al. (2001). In contrast, few details of shell interiors were available although Li and Holmer (2004) showed tens of complete and incomplete specimens of the taxon, referred to P. liantuoensis Zeng, 1987, from the lower Shuijingtuo Formation at Xiaoyang section in Zhenba County, southern Shaanxi Province. Palaeobolus liantuoensis Zeng, 1987 (Fig. 9) 1987 2001 2004

Palaeobolus liantuoensis – Zeng, p. 209, pl. 8, figs. 9–13. Palaeobolus liantuoensis Zeng – Holmer et al., p. 44, pl. 9, figs. 1–13. Palaeobolus liantuoensis Zeng – Li and Holmer, p. 195, figs. 3–5.

Stratigraphy and locality. Shuijingtuo Formation (TsunyidiscusSinodiscus Zone), Cambrian (Series 2). Specimens (samples 4–9 in Fig. 1E) were derived from the road cut section of Aijiahe village (N: 30◦ 44 55.2 , E: 111◦ 03 58.5 ), Wangjiaping village northwest to Yichang City, and the Wangzishi section, 12 km northwest of Changyang County, western Hubei Province. Material. 5 conjoined shell valves, 20 ventral and 13 dorsal valves from the etched residue, most fragmentary, and 30 specimens (e.g., Fig. 1C and D) collected by splitting the black shale slabs in the historic Wangjiaping section. Description. The shell is subtriangular, with maximum shell length up to 6 mm; the ventral valve is evenly convex, with maximum height in the posteromedian 1/3 of the valve length, and acuminate with straight posterolateral margins enclosing an apical angle of 85◦ (Fig. 9A); the ventral pseudointerarea is raised triangular in outline, medially divided by a broad and deep pedicle groove; the lateral margin of the pseudointerarea is straight; the propareas and flexure lines well developed in adults (Fig. 9D and E), but less-developed in non-adult specimens (Fig. 9B and C); the ventral posterolateral muscle scars are oval, teardrop-shaped, anterolateral to the propareas. Impressions of the paired pedicle nerve are occasionally preserved, extending from the pedicle groove. The ventral visceral area is

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Fig. 9. The linguloid brachiopod Palaeobolus liantuoensis Zeng, 1987 from the Shuijingtuo Formation at Aijiahe, Wangjiaping and Wangzishi sections in the Three Gorges area, Yichang, western Hubei Province, China. (A–C) Small ventral valves; (A) ELI-BP AJH-01, ventral exterior; (B and C) ELI-BP AJH-02, ventral internal and close-up view of ventral posterior. (D) ELI-BP AJH-03, ventral pseudointerarea. (E) ELI-BP AJH-04, posterior view of ventral posterior valve, showing and muscular scars (Ms) and the pseudointerarea with well-defined flexure lines (Fl) and propareas (Pr) and faint pedicle nerves (Pn). (F and G) ELI-BP AJH-05, general and detailed view of dorsal valves. (H) ELI-BP AJH-06, showing the dorsal pseudointerarea. (I and J) ELI-BP AJH-07, a conjoined shell valves; (I) dossal view; (J) posterior view. (K) ELI-BP AJH-08, enlarged view of dorsal larval shell.

thickened and ornamented by evenly spaced imprints of pustules (Fig. 9E). The dorsal valve is convex, evenly ovoid or suboval in outline with the posterior margin obtuse to slightly acuminate (Fig. 9F); the postlarval shell is developed with concentric growth lines; the dorsal larval shell is smooth, wide oval in contour, about 270 and 180 ␮m across (Fig. 9G). The dorsal pseudointerarea anacline is slightly elevated above the valve floor, with a broad and long median groove, weakly defined laterally, showing a crescentic anterior projection; no flexure lines are clearly recognizable on the propareas (Fig. 9H). The shell is inequivalved (Fig. 9I), strongly ventribiconvex in posterior view, with a ventral valve slightly larger than the dorsal one (Fig. 9J); the ventral valve has a tapered posterior shell occupied by a subcircular larval shell, approximately 185 ␮m across (Fig. 9K). Remarks. Palaeobolus liantuoensis was first reported by Zeng (1987) based on some specimens of internal molds preserved in

the black shale of the Shuijingtuo Formation in the Three Gorges area. However, Zeng’s (1987) original illustrations lacked satisfactory information on the ornament of the exterior shell and texture of the shell valves, although the earlier study did form the basis for analyzing the distribution of Palaeobolus in China and worldwide (Holmer et al., 2001). Palaeobolus is so far known from the lower Cambrian of Canada, Kazakhstan, Tianshan, the Zhenba–Fangxian Block, and the Yangtze platform in South China (Yang et al., 2015), all of which have been reconstructed paleogeograpically as distributed around low latitudes (Fig. 13). Family Eoobolidae Genus Eoobolus Matthew, 1902 Type species. Obolus triparilis Matthew, 1902; Middle Cambrian Bourinot Group, Cape Breton, Nova Scotia, eastern Canada.

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Fig. 10. The linguloid brachiopods from the Shuijingtuo Formation at Aijiahe, Wangjiaping and Wangzishi sections in the Three Gorges area, Yichang, western Hubei Province, China. (A–E) Eoobolus aff. viridis; (A and B) ELI-BE AJH-01, general and oblique posterior view; (C) ELI-BE AJH-02, shell surface with ornaments; (D and E) ELI-BE AJH-03; (D) posterior umbonal view, the inset box indicating the location of E; (E) detailed view of umbonal pitted ornaments. (F) Lingulellotreta malongensis (Rong, 1974), ELI-BL AJH-01, showing details of the ventral pseudointerarea.

Diagnosis. See Holmer et al. (1996, p. 41). Remarks. Specimens of this genus appear to be quite rare in our dissolved samples of Shuijingtuo Formation from the Three Gorges area in the Yangtze platform. However, it is a more typical and one of the commonest fossils in the underlying Xihaoping Member in the Zhenba–Fangxian Block (Li and Holmer, 2004; Yang et al., 2015). Eoobolus species are characterized by a teardrop-shaped shell outline, a finely pitted larval shell, and a pustulose post-larval shell. Nevertheless, the Cambrian lingulids usually show some great similarities in shell outline, length/width ratios, and the differences in larval and postlarval shell micro-ornamentations can hardly be identified and discerned except by means of the scanning electron microscope (SEM). In addition, the siliciclastic shale-hosted specimens can potentially eliminate the possibility of identification of the larval and postlarval microstructures in shell valves. Accordingly, the lack of detailed shell micro-ornamentations led to considerable confusion in the early identifications of Early–Middle Cambrian lingulids, the majority of which were reluctantly assigned to Lingulella (Ushatinskaya and Korovnikov, 2014). In the light of the most recent study by Ushatinskaya and Korovnikov (2014), the earliest-known lingulids in Siberia, previously referred to Lingulella (Pelman, 1977), are most recently redescribed and revised as four species of Eoobolus, i.e., E. siniellus, E. priscus, E. variabilis, and E. pelmani. The earliest lingulids have been referred to the species E. pelmani with the lowest occurrence in the Judomia Zone, Atdabanian Stage, and the other species

of Eoobolus appear from the Botoman upwards (Ushatinskaya and Korovnikov, 2014). Furthermore, the genus Eoobolus is known from the Lower–Middle Cambrian deposits of Canada, Greenland, Antarctica, the United States, and Australia (Holmer et al., 1996; Ushatinskaya and Holmer, 2001; Skovsted and Holmer, 2005; Skovsted and Peel, 2007, 2010; Balthasar, 2009). Eoobolus aff. viridis (Cobbold, 1921) (Fig. 10A–E) Stratigraphy and locality. Shuijingtuo Formation (TsunyidiscusSinodiscus Zone), Cambrian (Series 2). Specimens were derived from the road cut section of Aijiahe village (N: 30◦ 44 55.2 , E: 111◦ 03 58.5 ). Material. 3 ventral valves and 2 fragmentary dorsal valves from the Shuijingtuo Formation, Aijiahe section. Description. The shell is small and elongate oval or subtriangular in outline (Fig. 10A), slightly biconvex in lateral view, with a maximum width anterior to the midlength. The ventral valve is acuminate, with an apical angle around 85◦ . The pseudointerarea is orthocline, triangular with a conspicuous deep pedicle groove and prominent flexure lines on the ventral valve. The dorsal valve is gently convex, with a subcircular posterior margin enclosing an apical angle of approximate 95◦ . The primary layer is usually preserved in the apical larval shell, which is covered with minute recognizable pits (Fig. 10D and E) about 400 nm across. Thus it is apparent that the postlarval shells are provided with tuberculate microstructures on the surface.

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Remarks. A finely pitted larval and pustulose post-larval shell is characteristic of the genus Eoobolus (Holmer et al., 1996). The specimens from the Three Gorges area are similar to Eoobolus aff. viridis discovered in the Xihaoping Member at the Xiaoyang section in Zhenba County, southern Shaanxi Province (Li and Holmer, 2004). The Chinese specimens exhibit the characteristic features of shell microornamentation, as mentioned above. However, the specimens with weak preservation of the pitted larval shell may be inevitably confused with Palaeobolus and Spinobolus, as discussed above. In addition, the new material of Eoobolus resembles the eoobolid Sukharilingula in possessing several short radial ribs near the ventral umbo (Fig. 10D) (Ushatinskaya, 2012). Family Lingulellotretidae Koneva and Popov, 1983 Genus Lingulellotreta Koneva in Gorjansky and Koneva, 1983 Type and only species. Lingulepis malongensis Rong, 1974 (= Lingulellotreta ergalievi Koneva (in Gorjansky and Koneva, 1983)); type specimen lost. Diagnosis. See Holmer et al. (1997, p. 581).

(Chengjiang Biota) show a striking similarity to those from the Lower Cambrian of Kazakhstan (Holmer et al., 1997), the type specimen of L. malongensis was derived from the muddy siltstone of the Wulongqing Formation (formerly Wulongqing Member) at Siqitian section, Malong County, eastern Yunnan Province (Rong, 1974). Unfortunately, the holotype (NIGP 22154), formerly housed in the Nanjing Institute of Geology and Palaeontology, is lost. Further comparative study of the lectotype specimens of Lingulellotreta malongensis with those from the Chengjiang fauna and the Lower Cambrian of Kazakhstan remains to be done to confirm their monospecific classification and stratigraphic range (Holmer et al., 1996). Recently, our fossil collection in this region (Z.F. Zhang et al., 2015b) revealed that the genus Lingulellotreta occurred in the overlying muddy siltstone of the Shipai Formation in the same section near Wangjiaping village (see Fig. 1C), and thus extended the stratigraphic range from the Cambrian Stage 3 in eastern Yunnan Province to the upper Stage 4 of the Cambrian Series 2 in western Hubei Province. Superfamily Acrotheloidea Walcott and Schuchert, in Walcott, 1908 Family Botsfordiidae Schindewolf, 1955

Lingulellotreta malongensis (Rong, 1974) (Fig. 10F)

Botsfordiidae gen. et sp. indet. A (Fig. 11A–F)

Material. Only one specimen of a fragmentary ventral valve with a well-defined enclosed pedicle tube, just obtained from the middle–upper part of the Shuijingtuo Formation, Aijiahe section (sample 8 in Fig. 1E). Description. The specimen is fragmentary (Fig. 10F), but preserves a nearly complete ventral pseudointerarea. The pseudointerarea exhibits an enclosed homeopseudodeltidium that covers an internal pedicle tube. The internal pedicle tube is posteriorly opened by an elongate oval foramen at the tip of the pseudointerarea (Fig. 10F). Remarks. The genus Lingulellotreta is characterized and readily distinguishable from other Cambrian lingulids by an elongate foramen and internal pedicle tube. It was originally established based on the type and only known species, Lingulellotreta ergalievi from Kazakhstan. Holmer et al. (1997) did a detailed comparative study of the specimens from Kazakhstan and those described earlier by Jin et al. (1993) from the Chengjiang Lagerstätte of eastern Yunnan Province. Therein, Holmer et al. (1997) argued that the materials of “Lingulepis” malongensis Rong, 1974 from the Chengjiang fauna should be referred to Lingulellotreta and regarded L. malongensis as the senior synonym of Lingulellotreta ergalievi. Nevertheless, L. malongensis was originally erected based on the specimens from the Lower Cambrian Tsanglangpu Formation (recently the Wulongqing Formation) containing the Guanshan fauna (Hu et al., 2010) in Malong County, eastern Yunnan Province (Rong, 1974; Holmer et al., 1997). Although the specimens of Lingulellotreta malongensis (formerly Lingulepis malongensis Jin et al., 1993) from the Lower Cambrian Yu’anshan Formation

Material. One nearly complete dorsal valve collected from the mid-upper part of the Shuijingtuo Formation at Aijiahe section, Yichang area (Fig. 11A–F). Description. The dorsal valve is approximately 560 ␮m in length and 700 ␮m in width; the outline is sub-circular (Fig. 11A), about 80% as long as wide with a maximum width at about midvalve (Fig. 11B); the dorsal valve is slightly convex, with the maximum height at around the posterior 1/3 of valve length (Fig. 11B); the dorsal larval shell is rounded with a maximum diameter about 340 ␮m across, roughly occupying 48% of shell width, ornamented by a median apical tubercle flanked by two slightly pronounced lateral nodes (Fig. 11C–E), marginally delineated by a ring-like halo to separate it from the postlarval valve; the larval shell is smooth, without pits or granules; the postlarval valve is ornamented by loosely spaced pustules approximately 4 ␮m across at intervals of 12.5–17.5 ␮m on the shell surface (Fig. 11F). The growth pattern of the dorsal valve is holoperipheral. Remarks. This single specimen may be referred to the Botsfordiidae, because it has a sparsely pustulose post-larval ornamentation (Fig. 11F) and a vestigial dorsal pseudointerarea. However, it differs from known Botsfordiidae both in having a posterior dorsal limbus, and a submarginal beak (Fig. 11A–E), as well as in the single apical tubercle and the two weaker lateral nodes in the dorsal larval shell (Fig. 11D). The growth of the dorsal valve is somewhat similar to that of the enigmatic subfamily Conodiscinae; however, the Botsfordiidae gen. et sp. indet. A has a characteristic pustulose ornamentation that is unknown in all described genera of Conodiscinae.

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Fig. 11. Two indeterminate botsfordiid brachiopods from the Shuijingtuo Formation at Aijiahe section in the Three Gorges area, Yichang, western Hubei Province, China. (A–F) Botsfordiidae gen. et sp. indet. A, ELI-BB AJH15-A01; (A) ventral valve; (B) posterior view of (A); (C) umbonal view; (D) close-up view of ventral larval shell; (E) oblique lateral view of posterior umbo; (F) details of ornaments on the shell surface. (G–K) Botsfordiidae gen. et sp. indet. B; (G and H) ELI-BB AJH15-B01, conjoined shell valves, general view (G), enlargement of posterior umbo (H); (I) shell ornamentation; (J and K) ELI-BB AJH15-B02, an incomplete dorsal valve, posterior oblique view (J), larval shell (K).

Botsfordiidae gen. et sp. indet. B (Fig. 11G–K) Material. One complete articulated shell valve and one nearly complete dorsal valve collected from the mid-upper part of the Shuijingtuo Formation at Aijiahe section, Yichang area. Description. The shell is somewhat large, gently ventrobiconvex (Fig. 11G), with an outline subcircular to slightly transversely suboval; the ventral valve has a small, triangular, apsacline pseudointerarea with undetectable remains of pedicle groove (Fig. 11H); the dorsal valve is approximately 725 ␮m in width and 575 ␮m in length; the dorsal larval shell is subcircular, ornamented without developed tubercles; the larval shell is relatively

smaller, about 235 ␮m across, occupying 32% of shell width; the postlarval shell is ornamented with faint concentric lines, vague radial ribs and fairly sparely distributed pustules (Fig. 11I). Another specimen (Fig. 11J) has great similarity with that illustrated in Fig. 11G in terms of the sparsely disposed pustulose ornament and shell outline and thus is thought to be the same taxon as Botsfordiidae gen. et sp. indet. B. Nevertheless, the larval shell is distinctly ornamented with finely closely spaced pits (Fig. 11J and K). Remarks. This single specimen may also represent a botsfordiid, because it has a very sparsely pustulose postlarval ornamentation (Fig. 11I), as well as a triangular delthyrium and a vestigial ventral pseudointerarea (Fig. 11G and H). Differing from other

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known acrotheloids (Holmer et al., 2001), the larval shells of both valves are finely pitted, but lacking any larval tubercles (Fig. 11G and H). We might attribute the lack of pitted larval ornament in the specimen (Fig. 11H) to poor preservation or over-etching of the larval shell (Fig. 11G and H). The finely pitted ornamentation in the larval shell illustrated in Fig. 11K shows a general resemblance to that in the Acrothelidae, in taxa such as Acrothele. It is therefore suggested that Form B could represent a key intermediate from Botsfordiidae to Acrothelidae. The evolutionary transition from the Botsfordiidae to the Acrothelidae has long been suggested. When compared to Botsfordiidae gen. et sp. indet. A, Botsfordiidae gen. et sp. indet. B. is slightly larger, but bears a distinctly smaller larval shell lacking pronounced apical tubercles and differentiated postlarval ornament. Nevertheless, the two distinct forms of botsfordiid remain questionable and are regarded as tentative species awaiting further new data to confirm them. 6. Regional and global correlations of the Shuijingtuo Formation The black shale Shuijingtuo Formation and similar lithostratigraphic units crop out over a wide area around southeastern Shaanxi Province to northwestern Hubei Province (Qian and Zhang, 1983; Duan, 1984; Steiner et al., 2004, 2007; Li et al., 2012; Yang et al., 2015). In Zhenba County, southern Shaanxi Province and Fangxian County, northwestern Hubei Province (Fig. 1B), the organic-rich black shale Shuijingtuo Formation (Qian and Zhang, 1983; Li and Holmer, 2004) disconformably overlies the uppermost part of Dengying Formation, named here the Xihaoping Member (Qian and Zhang, 1983; Duan, 1984), characterized by light dolomites and dolomitic or phosphatic limestone with abundant skeletal fossils (Fig. 12C). Although the Xihaoping Member was formerly taken to equate chronologically to the pre-trilobite Meishucunian (equivalent to the upper Stage 2 of Terreneuvian Series) Shiyantou Member of eastern Yunnan Province (Duan, 1984; Zhu et al., 2009), acetic acid maceration of the carbonate samples from the member led to the discovery of a great number of librigenal and occipital spines of trilobites that indicate the early or middle Chiungchussuan Stage (Li and Holmer, 2004; Li et al., 2004, 2012). This chronostratigraphical age (Fig. 12C) was also supported by the occurrence of the microfossil assemblage of Microcornus, Cambroclavus, Ninella, all of which are typical faunal components of the late Chiungchussuan and afterwards in Cambrian Series 2 (Li et al., 2012). It was therefore assumed that the overlying Shuijingtuo Formation should be at least not earlier than the late Chiungchussuan Age (Fig. 12C) (Xie, 1988; Shu, 1990; Li and Holmer, 2004). It is worth noting that the brachiopods derived from the Xiaohaoping Member there feature linguloids such as Eoobolus, whereas the Shuijingtuo Formation is characterized by the two Acrotretoids (E. zhenbaensis and E. zhujiahensis) and Palaeobolus (Li and Holmer, 2004) (Fig. 12C). The stratigraphic succession overlaid by the Cambrian Shuijingtuo Formation in the southern bank of the Yangtze River is represented by the ca. 40 m-thick Yanjiahe Formation (Guo

et al., 2008, 2014) (Fig. 12B). In the latest investigation of the Yanjiahe Formation, three SSF assemblage zones have been recognized — the Anabarites trisulcatus-Protohertzina anabarica assemblage zone, the Protohetzina antiqua assemblage zone, and the Aldanella yanjiahensis assemblage zone, in ascending order (Guo et al., 2014). According to the proposal by Li et al. (2011), the FADs of Watsonella crosbyi or Aldanella yanjiahensis have been suggested to define the base of the Cambrian Stage 2 (Guo et al., 2014). If so, the Yanjiahe Formation goes through the earliest Cambrian Fortunian Stage, and then upward to the lowermost part of unnamed Stage 2 (Li et al., 2011). The case might be corroborated by the Yanjiahe ␦13 Ccarb profile, with a negative excursion (N1) in the lower part complying with the E-C boundary, and a positive fluctuation (P2) in the mid-upper part of the Yanjiahe Formation presumably corresponding to the boundary of Fortunian Stage and unnamed stage 2 (Ishikawa et al., 2008; Sawaki et al., 2008b; Jiang et al., 2012; Okada et al., 2014). When compared with the depositional sequences in eastern Yunnan Province (Li and Xiao, 2004; Li et al., 2004; Steiner et al., 2007), the fourth SSF Sinosachites flabelliformis-Tannuolina zhangwentangi assemblage zone is not recognized in the Three Gorges area (Guo et al., 2014) (Fig. 12B). It is possible to infer that there exists a depositional hiatus between the Yanjiahe and the Shuijingtuo formations (Fig. 12B), which can be signified by the depositional gap around the Yanjiahe–Shuijingtuo formational boundary at the Aijiahe section (Okada et al., 2014). Nevertheless, it is not straightforward to determine the magnitude and duration of the deposition hiatus in view of the lack of some index fossils and on account of no absolute bracketed age data. Many authors (Luo et al., 1999; Yuan et al., 2011 and references therein) have traditionally correlated the Shuijingtuo Formation directly with the Yu’anshan Member of the Heilinpu Formation (Chiungchussuan Stage), both because they represent the regional lowermost trilobite-bearing horizons (Luo et al., 1994; Peng, 2009; Okada et al., 2014), and because Shuijingtuo Formation and Yu’anshan Member have a sequence of basal black shales, indicating an oxygen-depleted event (X.Q. Wang et al., 2012; Xu et al., 2012) (Fig. 12A–C). Nonetheless, it is not clear whether a correlation based solely on the first occurrence of trilobites in different lithofacies is accurate (Landing and Westrop, 2004; Landing et al., 2013) and the trilobite multi-taxa contained in the Heilinpu and Shuijingtuo formations are most likely not synchronous. Thus, other fossil assemblages hosted in the Lower Cambrian unequivocally provide additional clues to the intra- and interregional stratigraphic correlation. This note was previously taken in the previous comparative studies of the SSFs from the Lower Cambrian of China between southern Shaanxi Province and western Hubei Province (Li and Holmer, 2004; Li et al., 2004; Topper et al., 2009). Apart from the trilobites, the Shuijingtuo Formation also contains hyoliths, hyolithellids, chancelloriids, sponge spicules, protoconodonts, anabaritiids, monoplacophorans, lobopodian plates, bradoriids, archaeocyathids, notably abundant brachiopods all of which are characteristic of the age-diagnostic fauna. Linguliform brachiopods were important components of early Cambrian skeletal communities, and they can be easily

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Fig. 12. The traditional first trilobite-based regional correlation of the lower Cambrian of Series 2 in the eastern Yunnan Province (A), the Three Gorges area (B), and southern Shaanxi Province (C) (modified from Luo et al., 1994; Yuan et al., 2011), showing the stratigraphical ranges of brachiopods that occur in the three regions; note the potential brachiopod correlation based on the Age 4-diagnostic Palaeobolus-Eohadrotreta brachiopod assemblage that is shadowed in grayish-green block scheme, implying a diachronous development of Series 2 in the Yangtze platform eastward and suggesting a bigger depositional gap for the Chiungchussuan (Stage 3) Stage in southern Shaanxi Province and western Hubei Province than expected before. Note the co-occurrence of eodiscoid trilobite and acrotretoid brachiopods marked in the Three Gorges section (B).

extracted from fine-grained clastic and carbonate lithologies from across the Cambrian paleocontinents (Holmer et al., 2001; Ushatinskaya, 2008; Z.F. Zhang et al., 2008; Popov et al., 2015; Smith et al., 2015). Most linguloid taxa have a wide geographic distribution, which is probably due to their planktonic larval dispersal. However, despite their global distribution, the biostratigraphic importance of Cambrian brachiopods has not yet been fully recognized or demonstrated. The increasing evidence of fossils (Ushatinskaya and Korovnikov, 2014; Popov et al., 2015; Smith et al., 2015; Z.F. Zhang et al., 2015b) demonstrates that first abundant occurrences of the assemblage of lingulids and Acrotretoids constitute key evidence of a Botoman (Stage 4) age across the known Cambrian palaeocontinents and thus facilitate regional correlation with the Tsanglangpuan in eastern Yunnan Province. There is limited evidence for presence of concentrations of lingulid and Acrotretoid assemblages in the Chiungchussuan (late Atdabanian) Stage (Ushatinskaya and Holmer, 2001; Ushatinskaya, 2008; H.Z. Wang et al., 2012; Ushatinskaya and Korovnikov, 2014; Zhang and Holmer, 2015; Z.F. Zhang et al., 2015b). This is the case also in Mongolia, Kazakhstan, North America, Australia and Antarctica (Holmer

et al., 1996; Holmer and Popov, 2000; Skovsted and Holmer, 2005; Jago et al., 2006, 2012; Ushatinskaya, 2008; Smith et al., 2015), as well as in Himalaya that was just documented in Popov et al. (2015). In the Three Gorges area, the oldest-known trilobites (Metaredlichia, Tsunyidiscus acutus, and Sinodiscus changyangensis) have their lowest occurrence in the lower part of the Shuijingtuo Formation, approximately several meters above the basal calcitic concretions (Fig. 1D and E). The occurrence of such trilobites is accompanied by abundant brachiopods (e.g., Fig. 2F–K). The brachiopod fauna is dominated in quantity by the linguloid brachiopod Palaeobolus liantuoensis, and abundant Acrotretoid brachiopods of Eohadrotreta. Palaeobolus is a cosmopolitan linguloid brachiopod from the Lower Cambrian (Botoman) to Middle Cambrian (Amgaian) (Holmer and Popov, 2000; Holmer et al., 2001), and hitherto known from Canada, Kazakhstan, Tianshan and southern China, ranging from Laurentia, Kazakhstan to Gondwana during the early Cambrian time (Fig. 13). Importantly, it is one of the most abundant macroscopic fossils in the Shuijingtuo black shale, but its lowest occurrence corresponds to the FAD of the contemporaneous

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Fig. 13. Paleogeographical reconstruction of the Cambrian (Stage 3) showing the geographical distribution of brachiopod genera Palaeobolus, Eoobolus, Eohadrotreta, Lingulellotreta, Botsfordia, and Kutorgina. The paleogeographic reconstruction is modified from Yang et al. (2015).

trilobite Tsunyidiscus acutus (Dai and Zhang, 2011) (Fig. 1D). The specimens of Palaeobolus liantuoensis are either densely concentrated on surface of the black shale bedding planes, or individual occurrence together with high-density eodiscoid segments or fragments, notably in the classic Wangjiaping section on the north bank of the Yangtze (Fig. 1D) (Wang, 1987). The shell concentrations of P. liantuoensis in the lower–middle Shuijingtuo Formation of the Three Gorges area is reminiscent of the equally numerous accumulations of Palaeobolus in the upper Hongjingshao Formation, which is exposed around the Malong–Yiliang areas of eastern Yunnan Province (Z.F. Zhang et al., 2011). The similar occurrence of a high-density aggregation of Palaeobolus shells in the Hongjingshao sandstones of eastern Yunnan Province and the middle–upper Shuijingtuo bioclastic limestones in the Three Gorges area prompts us to propose that the two deposition sequences are roughly correlated chronostratigraphically (Fig. 12). The similarities between the Acrotretoid-dominated brachiopod faunas in the Shuijingtuo Formations of Zhenba and the Three Gorges area suggest an approximate age of the Tsanglangpuan stage (Duan, 1984; Li and Holmer, 2004; Popov et al., 2015), although the Zhenba–Fangxian region was potentially an independent terrane or part of the Yangtze platform (Fig. 13; also see: Yang et al., 2015). In the meantime, the trilobite Estaingia occurs in the Shuijingtuo Formation as well as in the Botoman Cymbric Valve Formation of western New South Wales, eastern Australia (Paterson and Brock, 2007); this supports the argument for the correlation of the middle–upper Tsunyidiscus-bearing Shuijingtuo Formation with the Botoman Stage (Epoch 2 Stage 4). Accordingly, the first trilobite-bearing strata in the studied area may be younger than previously considered (Yuan et al., 2011) questioning the available regional and international correlation schemes (Peng et al., 2012). It is possible that the pre-trilobitic

interval (the basal concretion-containing shale) of Shuijingtuo Formation represents a condensed section, and is equivalent to end-Terreneuvian (Meishuncunian) to early Series 2 (Fig. 12A and B) depending on the basal existing depositional gap whose duration cannot be assessed. In spite of the repeated attempts in recent studies to correlate the Shuijingtuo Formation within the Chiungchussuan Stage in order to elucidate the synchrony of oceanic environmental changes and the abrupt explosive appearance of various metazoan fossils (Ishikawa et al., 2013, 2014; Tahata et al., 2013; Okada et al., 2014), previous paleontological data are extremely limited and most emanate from decades ago. As discussed above, the fossils presented here substantiate the existence of a more prolongated hiatus at the base of the Shuijingtuo Formation. The presence of such a deposition hiatus within the basal part of the formation provides a caveat to chemostratigraphical correlations made because of the lack of a continuous stratigraphic record. Therefore, the proposed oceanic environment changes reflected in the geochemical signatures of the Three Gorges sections and the presumed environmental impacts on animal diversity and abundance during the Cambrian explosion (internal?) are essentially preliminary and need to be further evaluated (Sawaki et al., 2008b; Tahata et al., 2013; Ishikawa et al., 2014). Thus, the present study of the brachiopods and associated SSF assemblages of the Shuijingtuo Formation in the Three Gorges area represents a stepwise contribution towards a better understanding of the biodiversity and the diversification pattern of metazoans (Li et al., 2007; Kouchinsky et al., 2012), and the new data await to be integrated into the chronostratigraphic, chemicostratigraphic and bio-event framework (Okada et al., 2014). This in turn will further facilitate the correlation studies of the “traditional Lower Cambrian” of South China at both regional and intercontinental scales.

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Acknowledgements This work represents a contribution to the programs funded by the National Natural Science Foundation of China (Grant 41425008, 41372021), the National Basic Research Program of China (973 Project: 2013CB835002), and the Program of Introducing Talents of Discipline to Universities (111 Project). Z.F. Zhang also acknowledges financial support from the Ministry of Education of the People’s Republic of China (FANEDD200936, NCET-11-1046) and the Fok Ying Tung Education Foundation (G121016). Grants from the Swedish Research Council (VR 2009-4395, 2012-1658 to L.E. Holmer) and the Chinese Academy of Sciences (XDB10010101 to G.X. Li) are acknowledged. Sincere thanks are also due to Juan-Ping Zhai and Mei-Rong Cheng (Xi’an) for fossil preparation and laboratory technical assistance in NWU, Xi’an. Susan Turner (Brisbane) assisted with improvement of the English. Thanks are also extended to Christian Skovsted and an anonymous reviewer for their constructive and critical comments, which help improve this final manuscript. References Babcock, L.E., Peng, S.C., 2007. Cambrian chronostratigraphy: current state and future plans. Palaeogeography, Palaeoclimatology, Palaeoecology 254, 62–66. Babcock, L.E., Peng, S.C., Geyer, G., Shergold, J.H., 2005. Changing perspectives on Cambrian chronostratigraphy and progress toward subdivision of the Cambrian System. Geosciences Journal 9, 101–106. Balthasar, U., 2009. The brachiopod Eoobolus from the Early Cambrian Mural Formation (Canadian Rocky Mountains). Paläontologische Zeitschrift 83, 407–418. Bengtson, S., Conway Morris, S., Cooper, B.J., Jell, P.A., Runnegar, B.N., 1990. Early Cambrian fossils from South Australia. Memoir of the Association of Australasian Paleontologists 9, 1–364. Clausen, S., Avaro, J., Devaere, L., Ahlberg, P., Babcock, L.E., 2015. The Cambrian explosion: Its timing and stratigraphic setting. Annales de Paléontologie 101, 153–160. Cobbold, E.S., 1921. The Cambrian horizons of Comley (Shropshire) and their Brachiopods, Pteropoda, Gastropoda etc. Journal of the Geological Society of London 76, 325–386. Dai, T., Zhang, X.L., 2011. Ontogeny of the eodiscoid trilobite Tsunyidiscus acutus from the Lower Cambrian of South China. Palaeontology 54, 1279–1288. Dai, T., Zhang, X.L., 2012. Ontogeny of the trilobite Estaingia sinensis (Chang) from the Lower Cambrian of South China. Bulletin of Geosciences 87, 151–158. Duan, C., 1984. Small shelly fossils from the Lower Cambrian Xihaoping Formation in the Shennongjia District, Hubei Province — hyoliths and fossil skeletons of unknown affinities. Bulletin of the Tianjin Institute of Geology and Mineral Resources 7, 143–188 (in Chinese, with English summary). Erwin, D.H., Laflamme, M., Tweedt, S.M., Sperling, E.A., Pisani, D., Peterson, K.J., 2011. The Cambrian conundrum: early divergence and later ecological success in the early history of animals. Science 334, 1091–1097. Gorjansky, V.J., Koneva, S.P., 1983. Lower Cambrian inarticulate brachiopods of the Malyi Karatau Range (southern Kazakhstan). Trudy Instituta Gologii Geofiziki Sibirskogo Otdeleniya AN SSSR 541, 128–138 (in Russian). Gorjansky, V.Y., Popov, L.E., 1985. Morfologiya, systematicheskoye polozheniye i proiskhozhdeniye bezzamkovykh breakhiopod s karbonatnoy rakovinoy. Paleontologicheskiy Zhurnal 3, 3–14 (in Russian). Guo, J.F., Li, Y., Han, J., Zhang, X.L., Zhang, Z.F., Ou, Q., Liu, J.N., Shu, D.G., Maruyama, S., Komiya, T., 2008. Fossil association from

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