Revisiting the molluscan fauna from the Cambrian (Series 2, stages 3–4) Xinji Formation of North China

A diverse group of molluscs from the Cambrian Series 2, Stages 3–4 Xinji Formation of the North China Block (NCB) is described, based on more than 4500 specimens from three well‐studied sections in Shaanxi and Henan provinces, along the southern and southwestern margin of the NCB. Twenty molluscan species are identified, including one bivalve, three stem group gastropods, and 16 additional helcionelloids. Among these, six helcionelloid species are reported from the NCB for the first time, and one new species, Parailsanella luonanensis sp. nov. is proposed. This diverse molluscan fauna shares a large number of species with contemporaneous faunas of South Australia (15), Antarctica (7), Laurentia (6), Siberia (3) and South China (1). Faunal similarities are even greater on a generic level. The striking similarities of the molluscan faunas of North China, South Australia and Antarctica strongly support the hypothesis that the NCB was situated close to Eastern Gondwana, most likely close to South Australia in the Cambrian Epoch 2. In addition, well‐preserved shell attachment muscle scars were observed in the helcionelloid Figurina figurina, with two pairs of symmetrical, continuous, band‐like muscle scars, which are obviously different from the musculature of both gastropods and monoplacophorans. Because of this unique musculature, these characteristic Cambrian cap‐like molluscs are assigned to the Helcionelloida rather than to the Gastropoda or Monoplacophora.

Abstract: A diverse group of molluscs from the Cambrian Series 2, Stages 3-4 Xinji Formation of the North China Block (NCB) is described, based on more than 4500 specimens from three well-studied sections in Shaanxi and Henan provinces, along the southern and southwestern margin of the NCB. Twenty molluscan species are identified, including one bivalve, three stem group gastropods, and 16 additional helcionelloids. Among these, six helcionelloid species are reported from the NCB for the first time, and one new species, Parailsanella luonanensis sp. nov. is proposed. This diverse molluscan fauna shares a large number of species with contemporaneous faunas of South Australia (15), Antarctica (7), Laurentia (6), Siberia (3) and South China (1). Faunal similarities are even greater on a generic level. The striking similarities of the molluscan faunas of North China, South Australia and Antarctica strongly support the hypothesis that the NCB was situated close to Eastern Gondwana, most likely close to South Australia in the Cambrian Epoch 2. In addition, well-preserved shell attachment muscle scars were observed in the helcionelloid Figurina figurina, with two pairs of symmetrical, continuous, band-like muscle scars, which are obviously different from the musculature of both gastropods and monoplacophorans. Because of this unique musculature, these characteristic Cambrian cap-like molluscs are assigned to the Helcionelloida rather than to the Gastropoda or Monoplacophora. E A R L Y Cambrian skeletal fossil assemblages (small shelly fossils; SSFs) from the Cambrian Series 2 Xinji Formation of the North China Block (NCB) were intensively studied in the 1980s-1990s for the purpose of locating phosphorite mining deposits. This phosphatized skeletal assemblage provides an excellent archive of diverse biomineralizing animal groups, including sponges, brachiopods, molluscs, hyoliths, echinoderms and fossil sclerites of problematic biological function and affinity. Among them, the Mollusca represents a conspicuous element, and thus previously attracted much attention from palaeontologists. A number of papers have been published on the basis of abundant specimens from the Xinji Formation in Henan Province (He et al. 1984;Chen & Wang 1985;He & Pei 1985;Pei 1985Pei , 1995Li & Zhou 1986;Yu & Rong 1991), southern part of Shaanxi Province (Yi 1992), and equivalent strata of the Houjiashan Formation in Anhui Province (Zhou & Xiao 1984). More than 35 nominal molluscan taxa have been described, and many were named based on material from the NCB, e.g. Anhuiconus microtuberus Zhou & Xiao, 1984, Davidonia rostrata (Zhou & Xiao, 1984), Figurina nana (Zhou & Xiao, 1984), Pelagiella madianensis (Zhou & Xiao, 1984), Xinjispira simplex (Zhou & Xiao, 1984), Yochelcionella chinensis Pei, 1985, Bemella xinjiensis (Feng et al., 1994 and Igorellina probosca Feng et al., 1994. The rich molluscan fauna from the NCB has dramatically improved our understanding of the evolution and radiation of early molluscs during the Cambrian explosion. However, the molluscan fauna of the NCB has not been studied during the recent decades, as attention has mostly been focused on the Precambrian-Cambrian transition of South China, due to its well-exposed sections, excellent fossil record and rich phosphorite deposits (Qian & Bengtson 1989;Steiner et al. 2004Steiner et al. , 2007Li et al. 2007;Parkhaev & Demidenko 2010;Guo et al. 2014;Yang et al. 2014Yang et al. , 2015Yang et al. , 2016. Meanwhile, molluscan faunas from Cambrian successions around the world have been intensively studied, and faunas from South Australia, Siberia and Laurentia are now relatively well known (Gravestock et al. 2001;Skovsted 2004;Skovsted & Peel 2007Topper et al. 2009;Kouchinsky et al. 2015Kouchinsky et al. , 2017. More recently, Claybourn et al. (2019) described a diverse molluscan fauna with 12 species from the Shackleton Limestone of the Antarctic Platform. In light of these discoveries, the molluscan species from the Cambrian of North China have been partly re-evaluated (Parkhaev in Gravestock et al. 2001;Skovsted 2004), but insufficient illustrations, low-resolution images and poorly preserved specimens in older publications have made precise species-level identification and comparison difficult.
In recent years, the SSFs on the NCB have received renewed interest by many authors due to the unexpected high diversity and abundance of the assemblage, and the well-preserved shell microstructures of many fossils (Li et al. 2014(Li et al. , 2016(Li et al. , 2017(Li et al. , 2019aPan et al. 2015Pan et al. , 2017Pan et al. , 2018Skovsted et al. 2016;Yun et al. 2016). More than a dozen sections along the southern margin of the NCB have now been investigated, and many taxa have been discovered from the NCB for the first time, including Apistoconcha apheles Conway Morris in Bengtson et al., 1990, Cambroclavus absonus Conway Morris in Bengtson et al., 1990, Paterimitra pyramidalis Laurie, 1986and Microdictyon sp. (Li et al. 2014Pan et al. 2017Pan et al. , 2018. Ongoing work on this skeletal assemblage has yielded a rich molluscan fauna from multiple sections, allowing us to provide a comprehensive taxonomic revision of the Cambrian molluscan fossils of the NCB, and provide the tools for further comparison and biostratigraphical correlation with coeval faunas. In addition, some exceptionally well-preserved specimens provide new information on shell attachment muscle scars, and thus shed new light on Cambrian molluscan functional morphology, systematics and phylogeny.

GEOLOGICAL SETTING AND AGE
The NCB is bounded to the north by the Central Asian Orogenic Belt, to the south by the Qinling-Dabie Belt and the Su-Lu fault against the South China Block, and to the west by the Qilian Orogenic Belt against the Tarim Block (Stern et al. 2018) (Fig. 1A). The lithological sequences and sedimentary patterns through the Precambrian-Cambrian transition are consistent and can be precisely correlated along the southern margin, with sequences in ascending order: Luoquan, Dongpo, Xinji and Zhushadong formations. They are generally in conformable contact except for a distinct disconformity at the base of the Cambrian, with Terreneuvian strata completely absent. The Xinji Formation (equivalent to the Houjiashan Formation in the southeastern part of the NCB) is the oldest Cambrian deposit of the NCB. It is composed mainly of siliciclastic sediments intercalated with carbonate units, which generally rests disconformably on the upper Ediacaran Dongpo Shale, and is conformably overlain by the massive dolostones of the Zhushadong Formation. The carbonate rocks of the Xinji Formation, less than 5 m in thickness, yield an abundant and diverse assemblage of skeletal fossils (Fig. 1B).
The Xinji Formation also contains trilobites important for age constraints and intercontinental biostratigraphic correlation. This low diversity of trilobite assemblage has yielded two described genera: Estaingia and Redlichia (Zhang & Zhu 1979;Zhang et al. 1979). Fragments of Estaingia are very common, and several species have been identified, e.g. E. luonanensis Hsiang in Lu et al., 1965and E. houchiuensis Chang in Hsiang, 1963(see review by Miao 2014. In contrast, Redlichia is extremely rare, but specimens of Redlichia cf. nanjiangensis Zhang & Lin in Lee, 1978 have been reported from the base of the Houjiashan Formation in Anhui Province, North China. The trilobite assemblage has long been correlated to the Drepanuroides trilobite Biozone of the middle Tsanglangpuan Stage (Cambrian Stage 4) on the Yangtze Platform (Zhang & Zhu 1979;Zhang et al. 1979;He et al. 1984;He & Pei 1985;Miao 2014). In recent years, investigations on global palaeogeographical distribution and Cambrian SSF assemblages suggested that the NCB might have been very close to the northern or northeastern margin of Australia within Eastern Gondwana (Brock et al. 2000;Gravestock et al. 2001;Wrona 2003;Yun et al. 2016;Pan et al. 2018). From this perspective, restudy of the trilobite assemblage by Miao (2014) highlighted that the Estaingia trilobite assemblage from the Xinji Formation correlates well with the Pararaia janeae trilobite Biozone of South Australia, and thus confirms the age of the Formation as latest Age 3 or early Age 4 of Cambrian Epoch 2.
Rock samples were treated with buffered, 5% acetic acid to retrieve acid-resistant microfossils. More than 4500 mollusc specimens belonging to 20 species were recovered from the acid-resistant residues. Most of our collections come from the Chaijiawa section, providing the bulk of data for statistical analysis. Specimens were mounted, and sputter-coated with gold for examination with an FEI Quanta 400 FEG scanning electron microscope (SEM) at Northwest University and Hitachi S4300 SEM at the Swedish Museum of Natural History (SMNH). Microfossils described below are deposited at the Shaanxi Key Laboratory of Early Life and Environments (LELE), Northwest University, Xian, China. F I G . 1 . Maps of North China outlining the geology, stratigraphy, localities and outcrops of studied sections. A, geological map showing the Precambrian-Cambrian rocks along the southern margin of the North China Block, marked in dark grey. B, composite stratigraphic column of the Chaijiawa, Shangzhangwan and Sanjianfang section. C, locality of the Sanjianfang section in Henan Province. D, locality of the Shangzhangwan section in Shaanxi Province. E, locality of the Chaijiawa section in Shaanxi Province. F, outcrop of the studied Chaijiawa section. G, outcrop of Zhoujiaqu section, the classic section in Shaanxi Province. Abbreviations: DP, Dongpo; E, Ediacaran; NP, Neoproterozoic; SSF, small shelly fossil; ZSD, Zhushadong. fauna, followed by Davidonia rostrata (13.13%), Anabarella australis Runnegar in Bengtson et al., 1990 (10.5%), Pojetaia runnegari (10.5%), Figurina figurina (3.41%) and Anhuiconus microtuberus (2.63%). The six most common species comprise 91.95% of all specimens, while none of the remaining 14 taxa is represented by more than 100 specimens (0.03-2.23% of the total fauna). Some molluscan species reported here are seemingly endemic to the Cambrian of North China, e.g. Bemella xinjiensis, Igorellina probosca, Parailsanella luonanensis, Ilsanella reticulata, but these account for a very low proportion of the total molluscan fauna in terms of specimens. Otherwise, most species, particularly the more common taxa, generally have a wider distribution among coeval molluscan faunas worldwide. The molluscan fauna of the NCB shares a majority of species (15) with contemporaneous fauna of South Australia, where they occur throughout the upper Abadiella huoi to lower Pararaia janeae trilobite biozones, equivalent to the newly established Dailyatia odyssei Biozone (tommotiid) according to Betts et al. (2016,2017,2018). In particular, a highly comparable molluscan fauna was recovered from the upper Mernmerna Formation in Arrowie Basin and Parara Limestone in Stansbury Basin, South Australia. The close correlation to the Dailyatia odyssei Biozone in South Australia is strengthened by the shared presence of some key taxa, such as Figurina figurina, Stenotheca drepanoida, Horsegullia horsegulliensis, Bemella communis and Humilispira adelocosma (Bengtson et al. 1990;Gravestock et al. 2001;Topper et al. 2009;Betts et al. 2016Betts et al. , 2017Betts et al. , 2018. This pattern is further corroborated by the presence of the comparable Estaingia trilobite assemblage and many other key taxa such as Cambroclavus absonus, Apistoconcha apheles and Paterimitra pyramidalis in both areas (Li et al. 2014(Li et al. , 2016Pan et al. 2018).
In addition to South Australia, the faunal similarity to Antarctica is also notable, given that seven shared species such as Davidonia rostrata, Pojetaia runnegari etc. were recently described by Claybourn et al. (2019) from the Shackleton Limestone of the central Transantarctic Mountains. Among them, it is of great significance to highlight that the new finding of Xinjispira simplex in the Shackleton Limestone represents the first discovery of the species outside North China, and thus strengthen the links (Claybourn et al. 2019). Interestingly, the molluscan fauna of the NCB also shares some species with that of Laurentia, particularly the Cambrian Stage 4 Bastion Formation of North-East Greenland, with nine genera and five species in common, including Davidonia rostrata, Anabarella australis, Anhuiconus microtuberus etc. (Gubanov et al. 2004;Skovsted 2004;Peel & Skovsted 2005). Some shared common taxa were also documented from equivalent strata along the continuous shelf margin of the Laurentian Platform: Davidonia rostrata and A. microtuberus from the Browns Pond Formation of the Taconic Allochthon of New York (Landing & Bartowski 1996); and Yochelcionella chinensis from the Forteau Formation of western Newfoundland and Kinzers Formation of Pennsylvania (Skovsted & Peel 2007Atkins & Peel 2008). In contrast, similar molluscan species from North China are rarely found in Siberia and South China, which were geographically further from the NCB during this interval. Only three shared species (Pojetaia runnegari, Marocella mira and Figurina nana) were described in the Cambrian Series 2 Emyaksin Formation of northern Siberia (Kouchinsky et al. 2015), and one shared species (Truncatoconus? cf. T. yichangensis) was known from the regional Meishucunian (Cambrian Fortunian) Tianzhushan (= Huangshandong) Member of the Dengying Formation and equivalent rocks of South China (Parkhaev & Demidenko 2010). Consequently, the striking similarities of the molluscan fauna from North China, South Australia and Antarctica strongly support the hypothesis that the NCB was situated close to Eastern Gondwana, most likely close to South Australia in the Cambrian (Table 3, Fig. 2).

Muscle scars and their utility for Cambrian molluscan systematics
Although the exceptional fossil record from North China and coeval faunas around the world indicates rapid radiation and speciation of molluscs in the early Cambrian, most of the recovered taxa cannot be easily assigned to any of the modern molluscan clades (Peel 1991;Skovsted 2004;Vendrasco 2012). This conundrum has been a longstanding palaeontological debate, because there is no direct anatomical evidence to infer the orientation of the shell (endogastric/exogastric) and torsion (torted/untorted) from phosphatized internal moulds, which are the most common type of fossils found. Moreover, rare morphological T A B L E 2 . Revision of molluscan species absent in Chaijiawa section and problematic fossils excluded from molluscan affiliation.

Scenella pycna
Cap-like taxon Feng et al. (1994, pl. 1, figs 13-17) features of the external shell are also seemingly insufficient to deduce the specific configuration of soft body tissues in these animals (Skovsted 2004). Owing to these problems, Cambrian molluscs have been variously interpreted as monoplacophorans (Runnegar 1981), paragastropods (Linsley & Kier 1984) or referred to the extinct Class  Helcionelloida Peel, 1991(or more informally as 'helcionelloids', Peel 1991Skovsted 2004). However, see Parkhaev (2007Parkhaev ( , 2017a for an alternative view of these taxa as endogastric and torted gastropods. Notwithstanding this debate, many authors agree that Cambrian helcionelloid molluscs constitute the stem to the Conchifera, a major group of the Mollusca that encompasses modern monoplacophorans, gastropods and other shell-bearing molluscan groups (Smith et al. 2011;Vinther 2015).
Parkhaev (2002, 2004a, 2006, 2014a) argued that regularly distributed polygonal textures on the surface of internal moulds are very likely to be left by shell attachment muscles, and accordingly, he reconstructed the dorsoventral muscular system of several molluscan species. However, other authors proposed an alternative interpretation regarding these delicate cell-like structures on mould surfaces, primarily as casts of prism-like microstructures in the external shells (Vendrasco et al. 2010(Vendrasco et al. , 2011(Vendrasco et al. , 2015Vendrasco & Checa 2015). In our interpretation, the muscle scars of Cambrian helcionelloids would generally be too faint to be observed or preserved on phosphatized steinkerns after diagenesis and acid leaching. However, there is little doubt that shell attachment muscle scars would be the most reliable evidence for better understanding the molluscan body plan, and they may be preserved under exceptional circumstances. The localized delicate polygonal network on the mould surface of Figurina figurina herein, is confidently interpreted as muscle scars in terms of their regularity in distribution, consistent occurrence and distinct band-like pattern. This interpretation does not preclude the less localized polygonal patterns on other parts of mollusc shells representing casts of the shell microstructure.
The new muscle scars in Figurina figurina are almost identical to those illustrated by Parkhaev (2014a) from some Australian specimens of Bemella communis (synonymous to F. figurina, see below), but are more completely preserved. It is clear that F. figurina had a principal pair of elongated, symmetrical and continuous dorsoventral muscle scars and a tiny pair of muscle scars on the subapical wall. The symmetrical, continuous dorsoventral muscle scars are most closely comparable to the horseshoe-shaped musculature of some Palaeozoic limpet-type shells and modern limpet gastropods (Horn y 1963;MacClintock 1963;Runnegar 1981;Harper & Rollins 1982;Peel & Horn y 1999). However, some controversies regarding the phylogenetic significance of these scars remain given that the Palaeozoic limpets resemble monoplacophoran shells in general morphology, although they have continuous bands of shell attachment muscle scars rather than multiple, discrete scars as in living monoplacophorans. As a result, whether the Palaeozoic limpets represent ancestral monoplacophorans or primitive limpet gastropods is difficult to determine (Lindberg 2009). This controversy has lasted for more than half a century, and is far from resolved. The same problem applies to the interpretation of the muscle scars in Figurina figurina herein. In addition to the unique dorsoventral musculature, the function of the tiny pair of muscle scars on the subapical wall is not clear, given that similar muscle scars are rarely present in fossil or living molluscan representatives. As a consequence of these uncertainties, and the fact that the unique musculature of Figurina is obviously different from that of the postulated stem group gastropods discussed below, we assign Figurina figurina and all other examples of the characteristic Cambrian cap-like shells to the Class Helcionelloida Peel, 1991, and confirm the prediction that helcionelloids are not gastropods.
In contrast to the unique musculature of the helcionelloid Figurina, muscle scars in postulated ancestral gastropods such as Pelagiella seem to be less controversial. The anisometrically coiled Pelagiella is known to have a pair of kidney-shaped, asymmetrical muscle scars with a larger right adaxial and smaller left abaxial (Runnegar 1981;Landing et al. 2002). This pattern is considered to reflect the torsion of the animal, and thus Pelagiella is generally regarded as a primitive, torted (at least partly) gastropod. The radial ridges or channels along the circumbilical shoulder of the moulds in Pelagiella (Isakar & Peel 2007), Xinjispira and Protowenella (herein) are interpreted to be formed during ontogeny by spirally migrating muscle scars. These muscle scars are more or less comparable to the columellar muscles of modern gastropods (Price 2003). Based on these observations, we regard all these three species as stem group members of the Gastropoda.

SYSTEMATIC PALAEONTOLOGY
Phylum MOLLUSCA Cuvier, 1797 Class HELCIONELLOIDA Peel, 1991Order HELCIONELLIDA Geyer, 1994Family HELCIONELLIDAE Wenz, 1938Genus TRUNCATOCONUS Yu, 1979 Type species. Truncatoconus yichangensis Yu, 1979. Species included. Type species and Truncatoconus campylurus (Jiang, 1980). Holotype. NIGPAS 54428 from the Cambrian Fortunian Tianzhushan (= Huangshandong) Member of the Remarks. The type species Truncatoconus yichangensis Yu, 1979 was originally described by Yu (1979) from the Cambrian Fortunian Tianzhushan (= Huangshandong) Member of the Dengying Formation of Yichang, South China, and is characterized by a simple conical shell, triangular lateral profile with shallow emargination on the subapical surface. In contrast, Truncatoconus campylurus (Jiang, 1980) from the regional Meishucunian Stage (Cambrian Stage 2) of the Zhujiaqing Formation of South China has a lower shell and distinctive lateral profile, given that its subapical margin is obviously concave, in contrast to the convex margin of the supra-apical field (Parkhaev 2008;Parkhaev & Demidenko 2010). Specimens from North China are clearly comparable to T. yichangensis in general morphology, particularly in terms of the lower triangular lateral profile of the moulds. Similar shells were described from Cambrian Stages 3-4 Bastion Formation of North-East Greenland as Helcionelloid indet. by Skovsted Material. Two specimens (LC03-32-05 and LC03-32-08) from the Cambrian Stages 3-4 Xinji Formation of Chaijiawa section in Shaanxi Province, North China.
Description. The shell is cap-shaped, tall, height roughly equal to width, with subcentral apex, slightly inclined ( Fig. 4A, B). The aperture is planar, circular or subcircular in outline. Subapical field is smooth, slightly concave with middle section drawn out into an apron-like fold (Fig. 4C). Supra-apical field is convex and ornamented by irregularly distributed spines or nodes (Fig. 4D, E).
Remarks. While internal moulds of Emargimantus? cf. E. tunuensis from North China show very few morphological features for precise species identification, they are reminiscent of some specimens of E. tunuensis described by Peel & Skovsted (2005, fig. 3E-L) from the Cambrian Stages 3-4 Bastion Formation of North-East Greenland in terms of the subapical fold and supra-apical spinose ornaments present on internal moulds. The holotype of the species from North-East Greenland has the external shell preserved, and bears prominent spiral cords on each lateral margin of the shell to separate the subapical and supra-apical surfaces (Peel & Skovsted 2005). This feature has not yet been observed in our specimens. The new material available to us is also comparable to some specimens illustrated as Stephaconus trulliformis Jiang, 1980, considered synonymous to Ocruranus trulliformis by Qian & Bengtson (1989) and referred to E. trulliformis by Skovsted et al. (2012), from Terreneuvian strata of South China in terms of shell height and curvature (Qian & Bengtson 1989, fig. 69A, B). However, our specimens differ from E. trulliformis by the presence of the broad fold on the subapical field of the shell.

Occurrence. Xinji Formation in Shaanxi and Henan provinces, North China; Bastion Formation in Albert Helm
Bjerge, North-East Greenland. Cambrian Series 2, Stages 3-4. Bengtson et al., 1990 Type species. Pararaconus staitorum Runnegar in Bengtson et al., 1990. Formation of Chaijiawa section in Shaanxi Province, 14 specimens from the Sanjianfang section in Henan Province, five specimens from the Shangzhangwan section in Shaanxi Province, North China.

Species included. Type species and
Description. The shell is cap-shaped, tall, highly compressed laterally, height/length ratio c. 0.8 ( Fig. 5A-C). The apex is blunt, slightly inclined ( Fig. 5F). Subapical field is almost straight or slightly concave, supra-apical field is convex (Fig. 5C, D). The aperture is elongated, elliptical in outline, and bears a shallow ring-like depression adjacent to the apertural margin ( Fig. 5F). One specimen shows a distinct constriction between the protoconch and the rest of the mould (Fig. 5G).
Remarks. The characters distinguishing the two known species, the type species Pararaconus staitorum and P. paradoxus from South Australia, relate to the presence/ absence of lateral depressions and buttress near the apertural margin (Bengtson et al. 1990;Parkhaev in Gravestock et al. 2001). The specimens from North China are placed in P. paradoxus based on the absence of a characteristic buttress on the lateral field of internal moulds and the general morphology of the material. The only slight difference is that the apertural rim of those South Australian materials of the species is more strongly flaring in contrast to the convex shell wall of North China specimens, but this may represent intraspecific variation or a preservation artefact. In addition, similar specimens assigned to Pararaconus sp. have previously been illustrated from contemporaneous rocks of Antarctica (Wrona 2003)     Diagnosis. Conical shell, laterally compressed with prominent globose protoconch. Aperture narrow, elongated elliptical. External ornamentation of shell consisting of numerous sharp concentric ribs separated by broad grooves that bear reticulate microsculpture.
Description. The shell is cap-shaped, tall, highly compressed laterally, height/length ratio c. 0.6-0.8 (Fig. 6A). Subapical field is concave, supra-apical field convex, lateral surface is straight or slightly concave in early growth stages, but progressively flaring towards the aperture (Fig. 6B-D). Protoconch is globose, large (200 lm in diameter), smooth, inclined, and overhanging the subapical margin. The aperture is narrow, elongated elliptical in outline with length/width ratio c. 2 ( Fig. 6A-D). External shell ornamentation of the largest specimen consists of 7-8 sharp concentric ribs separated by broad grooves (Fig. 6F, G). Some well-preserved specimens express an exquisite reticulate microsculpture within the grooves, formed by intersecting transverse and radiating striations (Fig. 6E).
Remarks. This species resembles to some degree specimens referred to Remarks. Three species of Bemella, including B. obscuricosta Zhou & Xiao, 1984, B. costa Zhou & Xiao, 1984and B. anhuiensis Zhou & Xiao, 1984 were previously described from the Houjiashan Formation in Anhui Province (Zhou & Xiao 1984) and the Xinji Formation in Henan Province (Feng et al. 1994). The latter two were revised as Davidonia rostrata (Zhou & Xiao, 1984)  Description. The shell is cap-shaped, of medium height, moderately compressed laterally with height/length ratio c. 0.6. Subapical field is slightly concave, supra-apical field is well rounded and convex (Fig. 7B, C). The apex is blunt, inclined, and reaching the margin of the aperture (Fig. 7D-F). Aperture is planar, ovoid in outline. Exterior of internal moulds is smooth or sometimes covered by ornaments of fine, densely distributed pits (Fig. 7A).
Remarks. In contrast to the abundant specimens documented by Parkhaev in Gravestock et al. (2001) from the Sellick Hill Formation, Mernmerna Formation and equivalent strata in South Australia, specimens of Bemella communis are relative rare in our molluscan collections from North China. The internal moulds from North China generally have a smooth surface, which differs from the characteristic concentric folds in specimens of South Australia. However, this morphology is comparable to some specimens of B. communis from South Australia, and the difference is probably related to individual variability or taphonomic factors. Two specimens previously described as Bemella communis by Parkhaev (2014a)  Description. The shell is cap-shaped, extremely depressed, moderately compressed laterally, with height/length ratio c. 0.3. In internal moulds subapical field very short, transiting into a parietal train at an acute angle; supra-apical field is convex, longitudinally elongated ( Fig. 8B-E). The apex is acute, recurved, slightly projecting over the margin of the aperture and hooked downward. Aperture is simple, elongated oval in outline with length/width ratio c. 2 (Fig. 8F, G). Surface of internal moulds is generally smooth, but one large specimen bears three to four broad rugae separated by shallow grooves. The rugae appear to increase in amplitude during ontogeny (Fig. 8A).
Remarks. This species was originally described by Feng et al. (1994) as Repenoconus xinjiensis from the Xinji  Holotype. SAMP29017 from UNEL1761, the base of the Cambrian Stages 3-4 Mernmerna Formation, Horse Gully, Yorke Peninsula, South Australia (Bengtson et al. 1990, fig. 164D). Remarks. Specimens preserving phosphatized coatings of the shell are obviously identical to Anabarella australis from South Australia (Bengtson et al. 1990;Gravestock et al. 2001). The shell is small, univalve, rapidly expanding, bilaterally symmetrical and highly compressed laterally. Shells are coiled throughout about one whorl, with the apical part tightly coiled and projecting beyond the apertural margin (Fig. 9B, E-H). The protoconch is hemispherical, clearly separated from the teleoconch by a slightly circumferential ridge and the initiation of growth lines (Fig. 9A). In contrast, associated internal moulds are morphologically very similar to a different helcionelloid genus, Planutenia Elicki, 1994, with the initial part of the mould weakly bent through about half a whorl (Fig. 9D).
Interestingly, the Anabarella-type external coating and Planutenia-shaped internal mould may be observed in the same specimen, with a distinct void chamber in the initial part of the shell (Fig. 9A, C). This observation provides  Material. LC0627-04 and more than 85 additional specimens from the Xinji Formation of Chaijiawa section in Shaanxi Province, SJFH-01-12, SJFH-03-08 and 17 additional specimens from the Sanjianfang section in Henan Province, two specimens (SZW-01-20, SZW-10-19) from the Shangzhangwan section in Shaanxi Province, North China.
Description. The shell is extremely compressed laterally with height/length ratio c. 0.7-0.8. Subapical field is strongly concave, forming a circular or subcircular shape; supra-apical field is evenly rounded. The initial shell is strongly recurved, projecting over the rear margin of the aperture, with pointed apex hooked downwards or backwards. Aperture is extremely narrow and elongated (Fig. 10A). Some specimens preserve polygonal textures on the mould surface, which generally cover the whole surface of the internal moulds (Fig. 10C). In addition, some specimens bear numerous faint comarginal ribs (Fig. 10B).
Description. The shell is cap-shaped, of medium height (average length/height ratio, 1.8) and moderately compressed laterally. The apex is blunt, recurved and projecting over the margin of the aperture. The subapical field is smooth, short and gently concave, forming a sharp angle with the short parietal train from which it is separated by a deep comarginal groove ( Fig. 11A-D). The aperture is ovoid in outline, tapering towards the subapical field (Fig. 11M, N). Surface of internal moulds with distinct, coarse concentric folds, up to 6-7 in large shells, generally present from mid-height of the moulds and towards the aperture (Fig. 11G-L). These folds are generally wider and more prominent on the anterior part of the moulds and taper towards the subapical field ( Fig. 11D-F, K, L).
Description. The shell is cap-shaped, of medium height, expands rapidly and is moderately compressed laterally. Subapical field is very short, strongly concave; supra-apical field is gently convex. The apex is recurved, overhanging the subapical field, and strongly hooked. The aperture is simple, ovoid in outline. Surface of internal moulds is smooth or with faint concentric rugae. In addition, the mould surface is generally covered by a fine micro-ornament of densely distributed narrow, deep pits. Moulds of the apical part of the shells are usually smooth (without pits) and spheroid shaped. The apex of the juvenile shells is recurved, but rarely reaches the margin of the aperture (Fig. 12A-I). In specimens of intermediate size, the shell expands rapidly, is moderately compressed laterally, with elongated elliptical aperture (Fig. 13F-H) and lateral fields slightly convex, flattened, or even concave. The shell of larger specimens is increasingly flaring towards the aperture, which is generally wider ovoid in outline ( Fig. 13A-E).
In some specimens well-preserved muscle scars are easily discernible, usually represented by delicate polygonal textures on the mould surface. Polygons consist of cellular imprints (10 lm in average diameter) delimited by slightly elevated ridges. Two pairs of muscle scars are present. One tiny pair of scars occurs on the subapical surface of the mould, immediately adjacent to the apex and extending downwards towards the apertural margin ( Fig. 14A-D). These scars are narrow, slightly diverging, band-like structures with a width of c. 40-50 lm (Fig. 14E-G). The other pair of scars is larger, forming narrow, continuous bands along the dorsolateral margin of the moulds (Fig. 15D-G). The scars expand anteriorly with terminal ending on the anterolateral fields ( Fig. 15A-C).  (2001, pl. 28, figs 6, 8, pl. 29, figs 3, 5) appear to fit the diagnosis of F. figurina, and were thus referred to this species by Skovsted (2004). The internal moulds of F. figurina from North China are also comparable with F. groenlandica from the Cambrian Stages 3-4 Bastion Formation of North-East Greenland in general morphology (Skovsted 2004), except that the latter shows additional radial ridge-like ornaments on the lateral surface of the moulds, a feature that is absent in specimens from North China and South Australia.
Description. The shell is cap-shaped, of medium height (average length/height ratio, 2.7), expanding rapidly and moderately F I G . 1 6 . Figurina nana (Zhou & Xiao, 1984)  compressed laterally. Subapical field is short, strongly concave, separated by a sharp angle and a deep comarginal groove from the parietal train; supra-apical field is evenly convex, elongated longitudinally. The apex is blunt, recurved, projecting over the margin of the aperture. The aperture is elongated ovoid, tapering towards subapical side with length/width ratio c. 2 (Fig. 16A-E). Exterior of internal moulds bears faint concentric rugae, and is sometimes covered by a micro-ornament of densely distributed pits (Fig. 16F).
Remarks. Internal moulds of Figurina nana are strikingly similar to juvenile shells of Davidonia rostrata described above in general morphology. However, recovered specimens of F. nana, are of the same size as adult shells of D. rostrate (Table 4), and also develop a much wider aperture and distinctive morphology of their internal moulds. This species was originally described as Mellopegma nanum by Zhou & Xiao (1984) fig. 1a-d).
Description. Cyrtoconic shell of medium height, slightly compressed laterally with rapidly expanding whorls. Initial shell recurved, with blunt apex hooked downward and projected beyond the apertural margin. Supra-apical field is well rounded in a distinct circular lateral profile; subapical wall is short, strongly concave. The aperture is subcircular in outline. The largest specimen (Fig. 17H-J; length, 1800 lm; height, 1100 lm; width, 1000 lm) is planispirally coiled, consisting of 0.75 whorls. Surface of internal moulds ornamented by regularly distributed nodules or tubercles aligned in distinct V-shaped rows on the dorsum (Fig. 17A, B, G).  (2004) suspected the two genera to be conspecific, but the insufficient illustration of Anhuiconus microtuberus from North China and South Australia, particularly lacking details of the diagnostic tuberculate ornament, initially made comparison difficult. New collections of A. microtuberus from North China show that the nodes or tubercles are arranged in a typical V pattern on the dorsum, and are effectively identical to Asperconella. In addition, we argue that the size differences of the tubercles are not distinct and are seemingly a consequence of preservational artefacts. Consequently, we consider Asperconella troyensis as a subjective junior synonym of Anhuiconus microtuberus.
Description. The cyrtoconic shell is of medium height and moderately compressed laterally. Initial shell is strongly recurved, with blunt apex hooked downwards, strongly projecting beyond the apertural margin ( Fig. 18A-C). Supra-apical field is well rounded, elongated longitudinally, lateral fields are evenly convex and rapidly flaring towards the aperture, and subapical field is elongated, extremely concave, passing into a very short parietal train at an acute angle ( Fig. 18D-F). Aperture is tear drop shaped, tapering towards the subapical side with length/width ratio c. 1.2 (Fig. 18H). Exterior of internal moulds is covered by a fine micro-ornament of densely distributed flat-bottomed shallow pits, and one large specimen bears faint comarginal rugae separated by wide, shallow grooves (Fig. 18G, I).
Description. Cyrtoconic shell of medium height, moderately compressed laterally, elongated longitudinally with rapidly expanding whorls. Aperture is simple, ovoid in outline, tapering towards the subapical side (Fig. 19F, H). The initial shell is recurved, with apex hooked downwards ( Fig. 19A-D). In larger specimens, the shell wall is flaring rapidly towards the aperture and the initial shell is projecting strongly beyond the apertural margin ( Fig. 19I-K). Protoconch is elongated, bulbous and relatively large (200 lm in diameter), separated from the teleoconch by a distinct circumferential constriction (Fig. 19D-G). Exterior of internal moulds is smooth or ornamented by faint comarginal rugae ( Fig. 19A-D). One large specimen (length, 2000 lm; width, 1000 lm) exhibits narrow, sharp plications separated by wide shallow grooves in later formed parts of the shell (Fig. 20A-C).
Description. The shell is tall, highly compressed laterally with a prominent subapical tubular snorkel that points obliquely upward in adult shells (Fig. 21B, D, E). The initial shell is smooth, rapidly expanding, inclined through about a quarter of one whorl with apex overhanging the subapical field. Subapical field is concave, while supra-apical field is well rounded (Fig. 21A, C). In larger specimens, the shell wall below the snorkel expands slowly and coils in opposite direction to the coiling of early growth stages, and thus results in a distinct sigmodal lateral profile (Fig. 21C, G). Mould ornaments consist of comarginal rugae separated by broad shallow grooves (110 lm in width). In addition, well-preserved specimens are covered with fine micro-ornaments of densely distributed pits (Fig. 21F).
Remarks. This species was originally described by Pei (1985)   Description. The juvenile shell (specimens with maximum length c. 1 mm, 0.5 mm in width) is smooth, moderately compressed laterally, recurved, with truncated apex overhanging the subapical wall, and a simple, elongated ovoid aperture. In larger internal moulds, the shell develops coarse comarginal and radial folds separated by narrow grooves resulting in a coarse cancellate pattern (Fig. 22B, G, H). Supra-apical surface becomes evenly convex, subapical area is initially more concave, but progressively flare towards the aperture. Incomplete internal moulds of the initial teleoconch preserve exquisite replications of the interior compartmental structures of the original shells. Moulds of individual compartments are subquadrate in shape and slightly inclined towards the apex (Fig. 22A, C-E). However, it is obvious that the size and shape of compartments can vary extensively, being bifurcated by secondary partitions along comarginal and radiating directions (Fig. 22F). In addition, well-preserved moulds express very fine pits covering the mould surface (Fig. 22H).
Remarks. Marocella is an enigmatic Cambrian helcionelloid macromollusc, reaching up to 18 mm in length shell followed by the formation of the diagnostic compartmental structures of the adult shell, and hence support the hypothesis that M. australensis is a subjective junior synonym of M. mira.  fig. 14a-d).
Description. Anisometrically coiled shell, expands rapidly from apex to aperture and becomes strongly asymmetrical ( Fig. 23A-C). The aperture of the last whorl is elongated, rectangular or triangular in outline. The peripheral margin is rounded, the lateral margin is convex and a projecting wing or ear is developed at the side of the aperture. The lateral surface bears a spiral carina along the circumbilical shoulder on internal moulds ( Fig. 23D-F). The protoconch is bulb like, 100-120 lm in diameter, and is separated from the teleoconch by a distinct circumferential constriction. The teleoconch is slightly compressed laterally with lateral field depressed in contrast to the wellrounded dorsum (Fig. 23G-I).
The shell exterior is ornamented by distinct microsculptures, although no trace of growth lines was observed. The initial shell is characterized by a smooth surface, which gradually develops numerous pustules (20 lm in diameter), regularly aligned in radial rows (Fig. 24A). These pustules may fuse into ridges with interspace occupied by fine, parallel ribs ( Fig. 24A-C). In mature specimens, the ribs are inclined in relation to the larger ridges with an angle gradually increasing to 40° (Fig. 24D-H).
Remarks. Pelagiella is a common and useful fossil for international biostratigraphic correlation and the species P. subangulata (Tate, 1892) has been suggested as a potential index fossil for defining the base of the Cambrian Series 2, Stage 3 in terms of its narrow stratigraphical range and almost global distribution (Steiner 2007;Zhang et al. 2017). However, Pelagiella exhibit a considerable morphological diversity with up to 30 named species, although not all of these are likely to be valid (see review by Parkhaev in Gravestock et al. 2001). Unfortunately, most reported specimens are internal moulds lacking information on shell ornament etc., and comparative information for species-level classification is rarely preserved. Inevitably, many named species may be synonymous and assemblages consisting of internal moulds without details of shell ornament are generally undiagnostic and should be treated with caution, at least without a detailed analysis of variability (Bengtson et al. 1990;Gravestock et al. 2001;Skovsted 2006).   fig. 3O, P). However, the described specimens are incomplete and the external shell is absent and thus, the affinity to P. madianensis of the Indian specimens is uncertain until more convincing evidence has been presented. Specimens from the Cambrian Stage 4 Emyaksin Formation of Siberia described as Pelagiella sp. 1 by Kouchinsky et al. (2015, fig. 17A-D) exhibit distinctive sculptures of fine radial striations and transverse ribs on the surface of internal moulds, which can easily be discriminated from the ornament of P. madianensis herein.   Material. Fifteen specimens from the Cambrian Stages 3-4 Xinji Formation of Chaijiawa section in Shaanxi Province, SJFH-01-22, SJFH-03-07, SJFH-05-23, SJFH-08-11, SJFH-10-12, SJFH-13-16, SJFH-16-07, SJFH-18-12, SJFH-20-08, SJFH-20-14, and more than 100 additional specimens from the Sanjianfang section in Henan Province, North China.
Description. Internal moulds of planispiral, bilaterally symmetrical, globose and smooth shell. The juvenile shell has a relatively large protoconch (200 lm in diameter) and aperture, but a very short subapical wall ( Fig. 25A-D). In more mature specimens, the shell is openly coiled, reaching one full whorl. Aperture is simple, circular or subcircular and of constant proportions through ontogeny. The circumbilical shoulders bear a pair of prominent helical channels that originate immediately below the apex and extend towards the aperture. As a result, a pair of notches is clearly visible on the subapical side of the aperture (Fig. 25E-J).
Remarks. Specimens assigned to Protowenella are characterized by a planispirally coiled shell with a pair of prominent channels along the circumbilical shoulders. This genus has been widely documented from the Miaolingian, Wuliuan strata of New Zealand (MacKinnon 1985), Australia (Runnegar & Jell 1976;Brock 1998), Bornholm (Berg-Madsen & Peel 1978, Morocco (Geyer 1986), Siberia (Gubanov et al. 2004) and Spain (Wotte 2006). In contrast, material of Cambrian Series 2 has rarely been illustrated but sporadic reports from the Siberian Platform (regional Botomian Stage) and the roughly coeval strata in Maly Karatau of Kazakhstan (Missarzhevsky & Mambetov 1981;Missarzhevsky 1989;Brock 1998), Northern Territories of Australia (Smith et al. 2015) and the Shackelton Limestone of Antarctica (Claybourn et al. 2019) have been published. However, given that almost all known specimens are internal moulds, comparative features for classification are rare, making precise species identification and comparison difficult. As a result, specimens have most often been retained in open nomenclature (Brock 1998;Claybourn et al. 2019). Zhou & Xiao (1984) erected two new species of Protowenella as P. primaria and P. huainanensis from the Cambrian Stages 3-4 Houjiashan Formation of Anhui Province, North China. The stated distinguishable feature of the two species is that P. primaria is less involute, i.e. openly coiled, compared to P. huainanensis (Zhou & Xiao 1984). However, our new collections from North China exhibit a wide range of morphological variation through ontogeny, and suggest that the two forms represent intraspecific variation. Furthermore, in our interpretation, both of the described specimens as well as our new collections are identical to the type species, Genus XINJISPIRA Yu & Rong in Yu, 1987a Type species. Xinjispira simplex Zhou & Xiao, 1984. Species included. The type species only.
Remarks. Xinjispira is an enigmatic mollusc genus known only from North China (Zhou & Xiao 1984;Feng et al. 1994) and Antarctica (Claybourn et al. 2019). The only known species, Xinjispira simplex, is almost identical to the co-occurring Protowenella flemingi in gross appearance, particularly in the fact that both species develop a pair of prominent channels along the circumumbilical shoulders.
The principal distinguishing feature relates to the chirality of the shells, with sinistral and planispiral coiling in Xinjispira and Protowenella, respectively. Interestingly, in the Shackleton Formation of Antarctica, the only known fauna from outside North China where Xinjispira occurs, Protowenella (described as Protowenella? sp.) is also present (Claybourn et al. 2019), perhaps indicating a close phylogenetic relationship between these taxa.
Material. LC0606-07, LC0630-22, LC0633-04, and more than 400 additional specimens from the Cambrian Stages 3-4 Xinji Formation of Chaijiawa section in Shaanxi Province, 12 specimens from the Shangzhangwan section in Shaanxi Province, 12 specimens from the Sanjianfang section in Henan Province, North China. Remarks. Internal moulds of bivalved molluscs are relatively common in our collections from the Xinji Formation of North China. The specimens often show impressions of one or two interlocking teeth and a short ligament (Fig. 26B, D, E) and are indistinguishable from Pojetaia runnegari, which is known to have a worldwide distribution in Cambrian Stages 3 and 4. Specimens of P. runnegari in our collections often preserve delicate replicas of shell microstructures (Fig. 27A, C). The lamellar layer is composed of lath or blade-shaped crystallites that develop straight growth fronts, a typical crystal shape of aragonite. These lath or blade-shaped crystallites of aragonite are assembled into a distinct imbricated pattern with straight growth fronts overlapping each other, and perpendicular to shell margin. This type of shell microstructure, i.e. foliated aragonite, has also been found among other Cambrian molluscs (Vendrasco et al. 2011;Li et al. 2017), which might represent a common and primitive shell microstructural fabric among Cambrian molluscs.
Occurrence. This species is known to have a worldwide distribution in Cambrian Stages 3-4, see review by Elicki & G€ ursu (2009).