Correlation and stratigraphic implications of the lowermost Cambrian small shelly fossils from new sites of South China Open Access

Precambrian–Cambrian deposits are well developed and well exposed in southern Shaanxi, and contain a large number of fossils (Hua et al. 2010; Han et al. 2017; Cai et al. 2019; Zhang et al. 2021a, b). This deposit sequence is primarily found in parts of the Ningqiang, Xixiang, Nanzheng and Zhenba counties (Fu 1983; Liu et al. 2007, 2014, 2016; Yao et al. 2011; Hua et al. 2020). In Ningqiang and most of Xixiang counties, the lowermost Fortunian unit is called the Kuanchuanpu Formation (Ding et al. 1990; Liu et al. 2007; Han et al. 2016); it is composed of arenaceous limestone and phosphorite containing an abundant and diverse suite of SSFs (Liu et al. 2016; Han et al. 2017; Wang et al. 2022), including Olivooides multisulcatus, Olivooides pearlformis, Punctatus emeiensis, Archaeooides granularis, Circotheca subcurvata, Circotheca sp., Turcutheca sp., Hexangulaconularia ningqiangensis, Siphogonuchites triangularis, Lopochites sp., Zhijinites sp., Protohertzina anabarica, Anabarites trisulcatus and Quadrapyrgites sp. This fossil assemblage belongs to the Anabarites trisulcatus–Protohertzina anabarica assemblage zone (Steiner et al. 2007, 2014). The bioclastic limestone of the Yangjiagou Member yields abundant SSFs typical of the Yangjiagou section in the Xixiang region and the Danangou section in the Nanzheng area (Fu 1983; Qin et al. 1988). Key taxa including Protohertzina anabarica, Hyolithellus sp., Chancelloria sp., Zhijinites triangularis, Zhijinites lubricus, Olivooides alveus, Paragloborilus subglobosus, Cambroclavus sp. and Allonnia sp. were recovered from the Yangjiagou Member in the Xixiang and Nanzheng regions, southern Shaanxi (Fu 1983; Qin et al. 1988). The fossil assemblage of the Yangjiagou Member is early Cambrian Meishucunian in age (Fu 1983; Qin et al. 1988).

The samples for this research were collected from a 1.8 m thick bioclastic limestone of the Yangjiagou Member, upper Dengying Formation in the Dayingcun section of the Fucheng area, Hanzhong City, southern Shaanxi (Fig. 1a–c). The lowermost Cambrian (Fortunian) Yangjiagou Member was first discovered and named by Qin and Ding (1988) in the Xixiang region because of the appearance of SSFs. The Yangjiagou Member in the Fucheng area disconformably underlies the yellowish–green shale and bioclastic limestone of the Guojiaba Formation (Fig. 2a) and disconformably overlies the dolostone of the upper Dengying Formation (Fig. 2b). The bioclastic limestone in the Yangjiagou Member of the Dayingcun section is extremely rich in fossils, which can be observed directly on the surface of hand specimens (Fig. 2c).

All palaeontological and lithological samples from the Yangjiagou Member of the Dayingcun section were collected in southern Shaanxi, South China (GPS location 32°28′12″N, 107°14′23.9″E). The bioclastic limestone was extracted by dissolution in acetic acid (5–10%) at the Early Life Institute (ELI), NW University, Xi'an. The fossils were picked using a binocular microscope and archived in microfossil slides. Most recovered phosphatized or phosphatic SSFs are broken and black or greyish. The best preserved and most complete SSFs were selected and prepared for imaging with a Phenom XL G2 scanning electron microscope (SEM) with a resolution of 5 or 10 kV and 0.10 Pa to produce external morphology images. Selected limestone blocks were made into thin sections with a thickness of 0.03 mm by DSM-300, MP-250 and Huiao-1200 and covered with resin. The elemental composition of the bioclastic limestone of the Yangjiagou Member was analysed and mapped using an M4 Tornado Micro X-ray fluorescence (XRF) spectrometer. All imaging and test analyses were performed at the State Key Laboratory of Continental Dynamics. All specimens are housed in the collections (ELI-DYC-YJG) of the Early Life and Environment Laboratory, NW University, Xi'an, China.

The bioclastic limestones have been made into rock thin sections by cutting in different directions. Transverse thin sections (Fig. 3a–d) of the bioclastic limestones that compose the Yangjiagou Member show that fossil bodies are densely distributed in the rock (Fig. 3b and c). XRF analysis shows that the fossils are mainly composed of calcium phosphate, with some preserved iron in the shell wall of a few specimens (Fig. 3d). The surrounding rock matrix is mainly composed of calcite and dolomite. Optical petrological examination of the thin sections reveals that the Yangjiagou Member is a classic biosparite with fossils including hyoliths, hyolithelminths, spherical fossils and other unidentifiable phosphatic steinkerns (Fig. 3b). Longitudinal thin sections (Fig. 3e–h) of bioclastic limestone from the Yangjiagou Member reveal disrupted bedding and rip-up clasts of mud and salty. Other longitudinal thin sections show that stylolites with low amplitude are developed in fine-grained limestone in the Yangjiagou Member of the Fucheng area (Fig. 3i–l). A disconformity representing a hiatus is demonstrated at the lithological contact between the dolostone of the Dengying Formation and the bioclastic limestone of the Yangjiagou Member in this section (Fig. 2b). Through examination of rock thin sections from the Yangjiagou Member of the Fucheng area, we identify the internal structures of several fossils. The internal structure of hyolithelminths varies, ranging from single-layer shells to multi-layer shells (Fig. 4a–e). A cross-sectional view of embryonic fossils shows the black-filled internal structure (Fig. 4f). The Maikhanellidae shows both longitudinal and transverse sections, with shell protrusion and filled interiors visible in the longitudinal section, and multiple annular structures in the transverse section (Fig. 4g and h).

In the Fucheng area, the Yangjiagou Member is composed of many 3D and phosphatized SSFs (Figs 5–10). The recovered fossils belong to the Anabarites trisulcatus–Protohertzina anabarica assemblage zone (see Steiner et al. 2007) and consist of Anabarites trisulcatus Missarzhevsky, 1969 (Fig. 5a), Protohertzina anabarica Missarzhevsky, 1973 (Fig. 5b–d), Protohertzina unguliformis Missarzhevsky, 1973 (Fig. 5e–h), Olivooides pearlformis Li, 1992 (Fig. 5i–k), Olivooides mutisulcatus Qian, 1977 (Fig. 5l–s), Quadrapyrgites quadratacris Li, 2007 (Fig. 6a–n), Archaeooides granulatus Qian, 1977 (Fig. 7a–t), Conotheca mammilata Missarzhevsky, 1969 (Fig. 8a–c), Conotheca subcurvata Yu, 1974 (Fig. 8d–j), Conotheca obesa Qian, 1978 (Fig. 8k and l), Hyolithellus tenuis Missarzhevsky in Rozanov and Missarzhevsky, 1966 (Fig. 8m–r), Coleolella recta Mambetov, 1981 (Fig. 8s and t), Maikhanella multa Zhegallo 1982 (Fig. 9a–d), Maikhanella calvatus Jiang, 1982 (Fig. 9e–h), Maikhanella perelegans Feng, 2001 (Fig. 9i and j) and Mediata xixiangensis Shao, 2015 (Fig. 9k and l). The other species discovered belong to the Paragloborilus subglobosa–Purella squamulosa assemblage zone, including Ramenta cambrina Jiang, 1982 (Fig. 10a–c), Ramenta superatatus Feng, 2001 (Fig. 10d–f), Purella sp. (Fig. 10i–l), Siphogonuchites triangularis Qian, 1977 (Fig. 10m–v) (Qian et al. 2001). The fossils occurring above in the Anabarites trisulcatus–Protohertzina anabarica assemblage zone are dominated by Conotheca subcurvata Yu, 1974, Conotheca mammilata Missarzhevsky, 1969, Olivooides pearlformis Li, 1992, Archaeooides granulatus Qian, 1977 and Maikhanella calvatus Jiang, 1982; only one specimen of Anabarites trisulcatus Missarzhevsky, 1969 has been recovered and the Paragloborilus subglobosa–Purella squamulosa assemblage zone is dominated by Purella sp. and Siphogonuchites triangularis Qian, 1977. These fossil data confirm an early Fortunian age for the Yangjiagou Member in southern Shaanxi.

Terminal Ediacaran–early Cambrian strata are generally widespread in southern Shaanxi, with the Kuanchuanpu Formation of the Ningqiang region in the west (Yao et al. 2011) and the Xihaoping Member of the Zhenba region in the east (Zhang et al. 2021a, b), and the Yangjiagou Member in the Hanzhong and Xixiang regions in the middle of the first two (Luo et al. 1988). From west to east in southern Shaanxi, the thickness of the lower Cambrian SSF-bearing strata decreases, whereas the stratigraphic hiatus between Ediacaran and lower Cambrian strata increases (Steiner et al. 2007). The thickness of the Kuanchuanpu Formation in the Shizhonggou and Dahezigou sections of the Ningqiang region is 65 m (Ding et al. 1983; Wang 2010), where it consists of siliceous and phosphatic limestone, with thick interlayers of limestone, chert and phosphatic sandy limestone (Steiner et al. 2007; Yao et al. 2011). The Guojiaba Formation disconformably overlies the Kuanchuanpu Formation in this area, and the underlying Dengying Formation and the Kuanchuanpu Formation are reported as disconformable (Ding et al. 1983; Steiner et al. 2007). The numerous SSFs recovered from the interbedded phosphatic limestone in the middle and upper Kuanchuanpu Formation belong to the Anabarites trisulcatus–Protohertzina anabarica assemblage zone and the slightly younger, overlying Paragloborilus subglobosa–Purella squamulosa assemblage zones (Steiner et al. 2007). In the Xuanjiangping section in the Ningqiang area, fossils belonging to the Anabarites trisulcatus–Protohertzina anabarica assemblage zone, Paragloborilus subglobosa–Purella squamulosa assemblage zone and Watsonella crosbyi assemblage zone were discovered in the Kuanchuanpu Formation (Luo 1984). In addition to SSFs, many possible algae fossils have been described and reported from the Kuanchuanpu Formation, including Stratufera, Osagia, Oncolithes and Catagraphes (Ding et al. 1983), Spirellus (Qian et al. 2007), Cambricodium (Liu et al. 2014) and Calathophycus irregulatus Tang (Tang et al. 2022). These are interpreted to have affinities with cyanobacteria or multicellular algae (Min et al. 2019; Zheng et al. 2021). Based on the present fossil assemblage, the Yangjiagou Member and Kuanchuanpu Formation can be compared.

The Yangjiagou Member has a maximum thickness of 8.4 m and is laterally distributed along-strike in the Hanzhong region of southern Shaanxi, China (Fu 1983; Qin et al. 1988). The member was first reported by Qin et al. (1988), and later Ding et al. (1990) referred to the small shelly fossil-bearing formation dominated by limestone at the top of the Dengying Formation in the Hannan area as the Yangjiagou Member. The Member is about 1 m thick in the Danangou section of the Liangshan area, located in Hanzhong City of southern Shaanxi. The species reported include Protohertzina anabarica, Chancelloria sp., Hyolithellus tenuis, Olivooides alveus and Zhijinites lubricus. The SSF assemblages from this locality reveal that the Yangjiagou Member in the Liangshan area belongs to the Paragloborilus subglobosa–Purella squamulosa assemblage zone (Fu 1983; Luo et al. 2022). Therefore, the Yangjiagou Member of the Liangshan area is younger than the Yangjiagou Member of the Fucheng area. At Fucheng, another region of southern Shaanxi, the thickness of the Yangjiagou Member further decreases to about 1.5 m thick and it disconformably overlies the Ediacaran dolostones of the Dengying Formation. The SSFs from the lower part of the Yangjiagou Member in this area include Anabarites trisulcatus and Protohertzina anabarica, eponyms for the Anabarites trisulcatus–Protohertzina anabarica assemblage zone; Siphogonuchites triangularis and Purella sp. occur in the uppermost part of the member, suggestive of the Paragloborilus subglobosa–Purella squamulosa assemblage zone. The disconformable contact between the dolostone of the Ediacaran Dengying Formation and the overlying bioclastic limestone of the lowermost Cambrian Yangjiagou Member is distinctly scoured and has an irregular contact, with phosphatic shelly fossils appearing abruptly above the dolostone of the Dengying Formation, indicating a stratigraphic hiatus in this area.

The Xixiang area of southern Shaanxi is about 150 km NE of the Fucheng area. The thickness of the Yangjiagou Member of the Yangjiagou section in the Xixiang area is about 8.5 m, and it is dominated by limestone with various SSFs that include Circotheca sp., Conotheca mammilata, Turcutheca lubrica, Paragloborilus subglobosus, Sachites sp., Olivooides alveus, Archaeooides granulatus, Turcutheca lubrica, Zhijinites sp., Chancelloria sp., Allonnia erromenosa, Onychia tetrathellis, Archiasterella pentactina, Igorella sp., Siphogonuchites triangularis and Cambroclavus fangxianensis (Qin et al. 1988). The assemblage belongs to the Paragloborilus subglobosa–Purella squamulosa assemblage zone. The lack of Anabarites trisulcatus and Protohertzina anabarica and associated taxa shows that the Anabarites trisulcatus–Protohertzina anabarica assemblage zone is not developed or is missing in the Yangjiagou Member of the Xixiang area.

The Zhenba area is the western part of southern Shaanxi, and the earliest Cambrian strata are referred to the Xihaoping Member, which is about 8.4 m thick and widely distributed in Shaanxi, especially in Hubei and other provinces (Qian and Zhang 1983; Zhang et al. 2021a, b). The latest research reveals the presence of the earliest trilobite Parabadiella huoi and brachiopods Eoobolus incipiens, indicating that the Xihaoping Member belongs to the early or middle Chiungchussuan Stage (Zhang et al. 2021a). Other taxa from this section include Parabadiella huoi (Zhang et al. 2021a), Eoobolus incipiens (Zhang et al. 2021a), Cambroclavus fangxianensis (Qian and Zhang 1983), Haoia shaanxiensis (Shu 1990) and sponge spicules. The discovery of the Anabarites trisulcatus–Protohertzina anabarica assemblage zone in the Yangjiagou Member in the Fucheng area supplements the previous absence in this member, and once again supports the existence of sediments equivalent to the Meishucunian Stage in the Yangjiagou Member (Fu 1983).

Ediacaran and Cambrian strata are very well developed with excellent outcrops in the eastern Yunnan area of the Yangtze Platform (Zhu 2001; Yang et al. 2014, 2016; Zhu et al. 2021). In particular, the lower Cambrian stratigraphy of the Kunming area, with its complete stratigraphic successions and abundant fossils, has been the standard for comparative litho- and biostratigraphy of the lower Cambrian in SW China (Luo et al. 1980; Zhu 2001; Yang et al. 2014). The lowermost Cambrian stratigraphy in Yunnan is dominated by the Zhujiaqing Formation (Zhu 2001), which has been divided into three members (Zhu 2010): the Daibu Member, the Zhongyicun Member and the Dahai Member of the Xiaotan section (Yang et al. 2016). The lithology of the Daibu Member is composed of silicified dolostone with chert nodules (Yang et al. 2014). Rugatotheca typica, Ganloudina symmetrica and Olivooides multisulcatus were discovered in the Daibu Member, and Yang et al. (2016) established a new subzone Ganloudina symmetrica–Rugatotheca typica of the earliest SSF zone in eastern Yunnan based on these SSFs. The Zhongyicun Member is dominated by grey dolomitic phosphorite with thin layers of shale (Qian et al. 2001). This member produces abundant and diverse SSFs, and represents the peak period of shelly fossil production (Qian et al. 2001; Feng and Sun 2003) and includes Anabarites trisulcatus, Protohertzina anabarica, P. unguliformis, Conotheca mammilata, Archaeooides granulatus, Hyolithellus sp., Ramenta cambrina, Archaeospira ornata, Lopochites latazonaris, Siphogonuchites triangulatus, Paragloborilus subglobosus, Maikhanella pristinis and Rementa cambriana. The lithology of the Dahai Member consists of well-bedded phosphatic and argillaceous limestone with thin-bedded dolomitic shale. The SSFs recovered from the Dahai Member include Conotheca mammilata, Xianfengella prima, Zhijinites minutus, Bemella bella, Archaeospira ornata, Anabarella plana, Aegitellus places, Securiconus simus, Heraultipegma yunnanensis, Chancelloria irregularis and Latouchella korobkovi. These diverse fossils were the basis for the establishment of four fossil assemblage zones (Steiner et al. 2007; Feng et al. 2022), which have become the standard Terreneuvian biostratigraphic framework for the Yangtze Platform (from oldest to youngest): the Anabarites trisulcatus–Protohertzina anabarica, Paragloborilus subglobosus–Purella squamulosa, Watsonella crosbyi and Sinosachites flabelliformis–Tannuolina zhangwentangi assemblage zones (Luo et al. 1980; Steiner et al. 2007; Yang et al. 2016; Feng et al. 2022). Recently, Yang et al. (2016) redefined the Paragloborilus subglobosus–Purella squamulosa assemblage zone and Watsonella crosbyi assemblage zone based on new data. Two new subzones were added to the Paragloborilus subglobosus–Purella squamulosa assemblage zone, named the Annelitellus yangtzensis–Obtusoconus honorabilis Subzone and Maikhanella cambriana–Oelandiella korobkovi Subzone (Yang et al. 2016). When compared with the eastern Yunnan sections, the fossil assemblage of the Yangjiagou Member in the Fucheng area is similar to SSFs I and SSFs II zones without the trace fossils, and can be correlated to beds 3–7 of the Zhongyicun Member.

The Terreneuvian lithostratigraphy of Sichuan Province is referred to the Maidiping Formation, which consists of 38 m of dolostone intercalated with phosphate-rich rock layers and bioclastic crystalline dolostone (Yang and Steiner 2021). The Anabarites trisulcatus–Protohertzina anabarica assemblage zone was discovered in the lower Maidiping Formation, and the Watsonella crosbyi assemblage zone was found in the upper Maidiping Formation; however, the Paragloborilus subglobosus–Purella squamulosa assemblage zone cannot be recognized in southern Sichuan Province without the occurrence of Purella squamulosa (Steiner et al. 2007; Yang and Steiner 2021; Feng et al. 2022). The Anabarites trisulcatus–Protohertzina anabarica assemblage zone includes Protohertzina anabarica, Anabarites trisulcatus, Rugatotheca typica, Olivooides multisulcatus, Hexaconularia formos and Quadrosiphogonuchites quadratus (Yang and Steiner 2021). The Paragloborilus subglobosus, Oelandiella korobkovi, Eohalobia diandongensis, Ocruranus finial and Lathamella caeca of the Paragloborilus subglobosus–Purella squamulosa assemblage zone co-occur with the Watsonella crosbyi and Zhijinites longistriatus of the Watsonella crosbyi assemblage zone. Two hypotheses are proposed regarding this phenomenon: one is that the upper stratigraphical range of Paracarinachites, Eohalobia, Ocruranus, Annelitellus yangtzensis, Obtusoconus rostriptuetus and Phrygula nana is higher than previously reported in Yunnan (Yang et al. 2014); the other is that the fossils from the upper Maidiping Formation represent the reworking of older strata and a space–time mixture with SSFs from Zone III (Yang and Steiner 2021). Moreover, Feng et al. (2022) first reported the Watsonella crosbyi assemblage zone from the upper Maidiping Formation of the Fandian section, southern Sichuan. The finding of Watsonella crosbyi provides new fossil evidence for defining the base of Cambrian Stage 2 in South China and expanding the distribution of this fossil. Therefore, the Yangjiagou Member in the Fucheng area can be correlated with the Maidiping Formation of the Maidiping and Fandian sections in Sichuan Province.

The Yanjiahe Formation in the Three Gorges area of Hubei Province was formed during the Fortunian to Stage 2 of the early Cambrian Terreneuvian Series (Guo et al. 2020a, b). It has excellent outcrops in the field, continuous stratigraphy and many SSFs (Guo et al. 2020b). The Yanjiahe Formation is currently divided into five lithological members. Microplants and the acritarch Micrhystridium regulare were recovered from the siliceous rock of Member 1 (Guo et al. 2014). The Anabarites trisulcatus–Protohertzina anabarica assemblage zone is represented the Member 2, with Anabarites trisulcatus, Circotheca sp., Conotheca sp., Hyolithellus tenuis, Protohertzina unguliformis, P. robusta, P. anabarica and Spinulitheca billingsi (Chen 1984; Guo 2010). The Purella antiqua–Siphogonuchites triangularis assemblage zone was discovered in Member 3, with Purella antiqua, Anabarites trisulcatus, Conotheca sp., Protohertzina unguliformis, P. anabarica and abundant tubular microfossils (Guo et al. 2014; Pan and Feng 2021). The Aldanella yanjiaheensis–Zhijinites lingistriatus assemblage zone was recovered in Member 5, with taxa including Aldanella attleborensis, Anabarella plana, Zhijinites longistriatus, Purella antiqua, Scenella jijiapoensis, Igorella sp., Xianfengella ovata, Anabarites cf. trisulcatus, Conotheca subcurvata, Cupittheca mira, Turcutheca lubrica, Lophotheca costellata, Sacciconus yanjiaheensis and Yunnanodus sp. typically present (Pan et al. 2018; Guo et al. 2020b; Qiang et al. 2023a, b). Qiang et al. (2023a) further conducted a systematic study of Anabarella and found that Anabarella is mostly similar to Wasonella in microstructure and shell form, and proposed that Anabarella is possibly the ancestor of Wasonella. Another species, Aldanella attleborensis, belongs to the gastropods, based on the muscle scar similarity, the laminar microstructure on the shell, the protoconch and regular septa, and the asymmetrically turbospiral shell (Qiang et al. 2023b). Comparison of fossil assemblages from the Yangjiagou Member in the Fucheng area suggests correlation with the second and third members of the Yanjiahe Formation in Hubei Province, equivalent to the Fortunian of the early Cambrian Terreneuvian.

The Siberian Platform has an extensive Ediacaran–Cambrian shallow-marine platform (Zhu et al. 2017; Bowyer et al. 2023). Wood et al. (2017) proposed that the late Ediacaran to early Cambrian early diagenetic setting underwent step changes coincident with the rise of skeletal metazoans, based on the petrography, cathodoluminescence (CL), and electron microprobe element analysis for the widely distributed carbonate minerals and macrofossils on the Siberian Platform. Zhu et al. (2017) established a high-resolution carbon isotopic chemostratigraphy and biostratigraphy sequences for the Ediacaran–Cambrian succession in Siberia and discovered a rich Cambrian skeletal biota that overlaps with the Ediacaran. Thus, there is a gradual transitional evolution between the Cambrian and Ediacaran biotas in the Siberian Platform (Zhu et al. 2017). The skeletal fossils are mainly preserved in the Ust’–Yudoma Formation of the Yudoma Group, including cloudinids, anabaritids, hyoliths, protoconodonts, halkieriids, siphogonuchitids, chancelloriids, hyolithelminthes and other fossils (Zhuravlev et al. 2012; Wood et al. 2017; Zhu et al. 2017). As above, the terminal Ediacaran Cloudina and Anabarites co-occur in this formation (Bowyer et al. 2023). According to the new data, there are four fossil assemblage zones in the Kyra–Ytyga section of eastern Siberia, the Cloudina–Namacalathus–Sinotubulites assemblage zone (Zhu et al. 2017), Anabarites trisulcatus–Protohertzina anabarica assemblage zone, Purella antiqua assemblage zone and Watsonella crosbyi assemblage zone (Steiner et al. 2007; Khomentovsky and Karlova 2009; Yang et al. 2016). In the Bol'shaya Kuonamka River sections, the Anabarites trisulcatus assemblage zone and the Purella antiqua assemblage zone are developed (Kouchinsky et al. 2017). In the Kotuj River sections, the Anabarites trisulcatus assemblage zone does not appear, but the Purella antiqua assemblage zone is widely developed (Kouchinsky et al. 2017). The species occurrence of Protohertzina anabarica, Anabarites trisulcatus and Siphogonuchites triangularis of the Yangjiagou Member in the Fucheng area compares closely with the Nemakit–Daldynian and the Medvezhya formations of the Siberian Platform (Fig. 6).

The Anabarites trisulcatus–Protohertzina anabarica assemblage zone and Paragloborilus subglobosus–Purella squamulosa assemblage zone have not been recovered from Australia mainly because of the presence of unsuitable marginal marine siliciclastic facies in the lower Cambrian (Jacquet et al. 2017). However, the lower boundary of the Watsonella crosbyi assemblage zone has been recovered from the upper part of the Mount Terrible Formation of the Normanville Group in the eastern Stansbury Basin of South Australia (Jacquet et al. 2017). Except for Watsonella crosbyi, Aldanella cf. golubevi, Parailsanella cf. murenica, Bemella sp., the hyoliths Cupitheca sp. and Hyolithellus micans, the chancelloriid Eremactis mawsoni, Eremactis mawsoni and Australohalkieria sp. represent the oldest shelly fossil assemblage in Australia (Jacquet et al. 2017). The faunal assemblage is equivalent to the Watsonella crosbyi assemblage zone of South China and Siberia and correlates with early Stage 2 of the Cambrian Terreneuvian Series (Jacquet et al. 2017). Betts (2017) also reported a large number of SSFs of the Dailyatia odyssei Zone from the Andamooka Limestone, the Wirrapowie Limestone, the Wilkawillina Limestone and the Mernmerna Formation of Australia. The SSFs consist of tommotiids, cambroclaves, brachiopods, molluscs and bradoriids (Betts et al. 2017, 2019). The brachiopod fossils have brought the distance between South China and South Australia closer together (Steiner et al. 2007; Zhang et al. 2016). The Dailyatia odyssei Zone of South Australia is equivalent to the Wutingaspis–Eoredlichia trilobite zone of South China (Betts et al. 2017, fig. 21). The overlying Kulparina rostrata and Micrina etheridgei zones based on endemic tommotiid species in the Arrowie Basin of South Asustralia can broadly be correlated with the Sinosachites flabelliformis–Tannuolina zhangwentangi assemblage zone of the Meishucunian Stage in South China (Betts et al. 2016).

The stratigraphic distribution and fossil production on the intercontinental scale may be summed up as follows. The presence of Hyolithellus sp., Chancelloria sp. and Zhijinites triangularis in the Yangjiagou Member of the Liangshan area suggests that it is younger than the Yangjiagou Member of the Dayingcun section in the Hanzhong region. The Yangjiagou Member in the Hanzong region is equivalent to the Zhongyicun Member of the Zhujiaqing Formation of eastern Yunnan (Fig. 11), and members 2–3 of the Yanjiahe Formation in the eastern Three Gorge region (Fig. 11), the upper Maidiping Formation of northern Sichuan (Fig. 11), the phosphorite layer of the upper Kuanchuanpu Formation of the Ningqiang region (Fig. 11), the microsparitic limestone and phosphatic limestone of the Kuanchuanpu Formation in the Xixiang region (Fig. 11) and the Nemakit–Daldynian stages of the Siberian Platform (Fig. 12). The SSFs described from the Yangjiagou Member herein indicate that this package belongs to the Fortunian of the early Cambrian Terreneuvian.

Systemic palaeontology

Phylum and class uncertain

Order Angustiochreida Val'kov and Sysoev, 1970 

Family Anabaritidae Missarzhevsky, 1974 

Genus Anabarites Missarzhevsky, 1969 in Voronova & Missarzhevsky, 1969

Type species. Anabarites trisulcatus Missarzhevsky, 1969 in Voronova & Missarzhevsky, 1969.

Anabarites trisulcatus Missarzhevsky, 1969

Figure 5a 

Material. One specimen from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. Tube slender, in the shape of a curved tube with rounded triangular cross-section (Fig. 5a), length 4.5 mm, cross-section width 0.6 mm; divergence angle 8°, slightly expanding toward the aperture; three uniformly wide longitudinal sulci on surface conforming to the growth direction of the shell; surface ornamented with growth lines parallel to the apertural margin.

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Zhongyicun Member of eastern Yunnan, Yanjiahe Formation of eastern Hubei, Maidiping Formation of Sichuan, Yurtus Formation of Xinjiang, China; Laurentia; West Gondwana; Avalonia; western Mongolia; Kazakhstan; Siberia.

Remarks. Anabarites species with slowly expanding tubes, the internal moulds of which express three rounded lobes separated by uniform, shallow grooves. The growth lines in the grooves often curve towards the aperture (Kouchinsky et al. 2009). The uncertainty about the biological affinity and phylogeny of the anabaritids is due to the lack of soft parts in anabaritids, along with the diversity and variability of morphological features (Kouchinsky et al. 2009).

Phylum Chaetognatha Leukart, 1854

Class ProtoconodontaLanding, 1995 

Order and Family uncertain

Genus Protohertzina Missarzhevsky, 1973 

Type species. Protohertzina anabarica Missarzhevsky, 1973.

Protohertzina anabarica Missarzhevsky, 1973 

Figure 5b–d 

Material. Approximately 14 specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are small and slender, bilaterally symmetrical, uniformly curved along symmetry plane, generally 1–3 mm in length (Fig. 5b–d). Anterior and posterior margins are different in morphology, delimited by lateral ridge (Fig. 5c). Anterior margin semielliptical in cross-section (Fig. 5d), with a narrow, slender longitudinal furrow present at centre of anterior side and disappearing at apical end of cusp; cross-section of apical end becoming subcircular, and the cusp becoming almost equal in width and length.

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Zhongyicun Member of eastern Yunnan, Maidiping Formation of central Sichuan, South China; western Mongolia; Siberia; Kazakhstan; northern Iran and India; Canada.

Remarks. Protohertzina anabarica has a close affinity to P. unguliformis in that it has a flattened shape and weak lateral rib development (Novozhilova and Korovnikov 2019). The cross-section of the cusp in the former is nearly equidistant with the weak posterior median ridge and the lateral ridge developing strongly; in the latter, the cusp is higher than wide, the cross-section is unequidistant with the posterior median ridge appearing strongly raised and the lateral ridge weakly defined. Protohertzina anabarica is stout, but P. unguliformis is slender and elongated (Qian 1989). Both co-occur in the Yangjiagou Member and other formations.

Protohertzina unguliformis Missarzhevsky, 1973 

Figure 5e–h 

Material. A total of 15 specimens from the Yangjiagou Member of the Dayingcun section of Fucheng area, southern Shaanxi, China.

Description. The shell is clearly narrow and elongated, claw-like, bilaterally symmetrical, curved along the symmetry face (Fig. 5e–h), and has obvious curvature in the apical and basal parts (Fig. 5f). The spine at the tip termination is pointed and slender. The anterior and posterior margins are separated by low and obscure lateral ridges on both sides of the cusp (Fig. 5g and h). The posterior ridge has a strikingly high and pointed posterior median ridge. The posterolateral furrow is narrow and shallow.

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, lower Zhongyincun Member of eastern Yunnan, Maidiping Formation of Sichuan, China; south Kazakhstan.

Phylum CnidariaHatschek, 1888 

Subphylum MedusozoaPetersen, 1979 

Order uncertain

Family Olivooidae Steiner et al., 2014

Genus Olivooides Qian, 1977 

Type species. Olivooides mutisulcatus Qian, 1977.

Olivooides pearlformis Li, 1992

Figure 5i–k 

Material. More than 30 specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are spherical or subspherical, approximately 0.49–0.71 mm in diameter (Fig. 5i–k), with the surface of the sphere closed and smooth and undecorated. Some specimens have a few surface pits and breaks (Fig. 5i and j) or scattered adhering mineral grains. The pits indicate that the spheres may have been somewhat pliable. The spheres have walls approximately 0.04–0.07 mm thick and consist of phosphate minerals, with the interior of the spheres hollow, solid or partially filled (Fig. 5k), the filling being predominantly calcium phosphate particles. The interior of the partially filled spheres is reticulated and supports the surface of the spheres (Fig. 5k).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Yanjiahe Formation of eastern Three Gorges of Hubei Province, Niutitang Formation of Guizhou Province, China.

Remarks. Olivooides pearlformis Li, 1992 is distinguished from other species by a smooth spherical surface without grooves, wrinkles or nodules (Li 2002). The fossils of this species found in the present study lack internal structural and biological features relevant for identification. Therefore, the genus is used only for a brief description and classification.

Olivooides mutisulcatus Qian, 1977 

Figure 5l–s 

Material. About 56 specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are broadly oval with a spherical diameter of 0.49–0.69 mm (Fig. 5l–s). The shell wall is thicker, between 0.04 and 0.07 mm (Fig. 5m). The shell is hollow or filled with secondary calcite. The surface of the shell is smooth and unadorned (Fig. 5q–s). Under the outer wall is a complex of spines that extended to the outside of the sphere (Fig. 5o). These spines form dense stellate protrusions connecting the outer wall and inner wall (Fig. 5m and p).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Yanjiahe Formation of eastern Hubei, Daibu Member of eastern Yunnan, China; south Kazakhstan.

Remarks. Olivooides mutisulcatus Qian, 1977 was first reported in the Kuanchuanpu Formation from the Ningqiang area (Qian 1977). The fossil was interpreted as an embryo and Punctatus was the hatching stage of Olivooides (Bengtson and Yue 1997 ). Further progress has been made in the entire developmental life history of the Olivooides (Bengtson and Yue 1997; Yue and Bengtson 1999), including the embryo, hatched larvae, juveniles and adults (Chen and Dong 2008). The exquisite external and inner structure of livooides has been reported many times (Hua et al. 2004; Steiner et al. 2004; Liu et al. 2006; Chen and Dong 2008). The affinity of Olivooides has been controversial, but we agree with Yue and Bengtson (Bengtson and Yue 1997; Yue and Bengtson 1999) that it is most probably related to the Cnidaria (Steiner et al. 2004; Liu et al. 2006; Dong et al. 2016).

GenusQuadrapyrgites Li et al., 2007 

Type species. Quadrapyrgites ningqiangensis Li et al., 2007.

Quadrapyrgites quadratacris Li 1984

Figure 6a–n 

Material. Fifty specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The fossils are more than 1 mm in length and are made up of two stages (Fig. 6a–n). The embryonic stage is pagoda-shaped with a smooth surface and has a tetraradially symmetrical apex, including a square cone (Fig. 6e and i) or a pointed and square cone (Fig. 6g). The cross-section of the apex is squared. The post-embryonic stage is trumpet-shaped (Fig. 6d, h, k and m) and widens at an angle of about 39° towards the oral end. Most specimens have been compressed, causing the cross-section to be flattened (Fig. 6f). The cone is characterized by 5–16 ribbed crests (Fig. 6a, d, h, m and n). Distal crests are wider than the underlying ones. There are longitudinal folded stripes between the crests (Fig. 6j and l), and the surface is smooth, curved and uneven, with apparent compression. The oral end is complete (Fig. 6b) or broken without any terminal lobes (Fig. 6k).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, China.

Remarks. Quadrapyrgites from the lower Cambrian Kuanchuanpu Formation consists of a quadrate apical portion and a reversed pagoda-like, corrugated abapical portion bearing 12 V-shaped apertural lobes (Guo et al. 2020b). A species similar to Quadrapyrgites quadratacris is Octapyrgites elongatus from Bed 5 of the Yanjiahe Formation, Hubei area (Guo et al. 2020a, b). Quadrapyrgites was postulated to be a coronate scyphozoan (Liu et al. 2017) or the sister group of Olivooides (Steiner 2014), or might be an early representative of the cycloneuralians (Shao et al. 2018). The flat termination shared by all specimens of Quadrapyrgites may have served as an adaptation for a sessile mode of life by increasing the contact area of this structure with the seafloor or other substrate (Guo et al. 2020b). Li et al. (2007) established the genus Quadrapyrgites based on the four-spoke symmetrical square tower characteristics in specimens from the Kuanchuanpu Formation in the Ningqiang area of Shaanxi (Li et al. 2007). Later, the interior structure of the fossil was thoroughly examined by using computed tomography (Chen and Dong 2008). Quadrapyrgites undulatuscostalis was also found in the second unit of the Kuanchuanpu Formation of the Zhangjiagou section, Xixiang County. The growth pattern of the square pyramid-shaped shell was discussed and the evolutionary sequence of Quadrapyrgites was established (Liu et al. 2009, 2013). It demonstrated the growth mode of the terminal lobes and annuli. The annuli of Quadrapyrgitesi post-embryonic tissue, which make up the post-embryonic tissue evolutionary sequence, vary in number from one to at least 18 (Liu et al. 2014; Steiner 2014; Shao et al. 2018). The different species of Quadrapyrgites and Olivooides represent abundant cnidarians in the Fortunian Stage (Shao et al. 2018). The adult length of Quadrapyrgites is up to 1 cm with more than 50 annuli on the post-embryonic body (Shao et al. 2018). The absence of internal anatomy necessitates additional evidence of the affinity of the Quadrapyrgites.

Phylum, Order, Class and Family uncertain

Genus Archaeooides Qian, 1977 

Type species. Archaeooides granulatus Qian, 1977.

Archaeooides granulatus Qian, 1977 

Figure 7a–t 

Material. About 51 specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are round and spherical with a diameter of 0.66–1.46 mm (Fig. 7a–t). The wall is thick, phosphatic, solid or hollow inside the fossil (Fig. 7i–l and p). The entire fossil surface has a dense, regular arrangement of nodules (Fig. 7e, h and p); the mean diameter of the nodules is 0.03–0.05 mm (Fig. 7j, l and o). Occasionally, small holes are seen on the nodules. The nodules are prominently and conspicuously papillate, with a small pore in the centre of the papillary nodules (Fig. 7o).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Maidiping Formation of northern Sichuan, Huangshandong Formation of eastern Three Gorges of Hubei Province, Zhongyicun Member and Dahai Member of eastern Yunnan, Gezhongwu Formation of Guizhou Province, Yurtus Formation of Xinjiang, China; western Mongolia; Siberia; India; Iran; Germany.

Remarks. The genus Archaeooides has three species, including Archaeooides granulatus Qian, 1977, A. kuanchuanpuensis Qian, 1977 and A. acuspinatus Qian, 1977, and is distinguished by the size of fossils and the morphology of nodules on their surface (Qian et al. 1977). The nodules on the surface of the spheres are rounded, bulbous protrusions. The infilled material can be seen on the broken surface of the sphere (Qian 1989). However, recent research suggests that Archaeooides granulatus may be the only valid species of Archaeooides (Xian et al. 2024). The varying morphology and size between Archaeooides may be the result of intraspecific variation (Xian et al. 2024). In terms of fossil affinities, it has always been worthy of discussion. It was previously reported that this species could be an animal embryo (Pyle et al. 2006; Yin et al. 2018). By comparing the nodules of Archaeooides with extant algal vesicles, it was considered that this species is more closely related to algae (Xian et al. 2024). In this paper, Archaeooides is described here only for stratigraphic correlation.

Phylum uncertain

Class Hyolitha Marek, 1963 

Order Orthothecida Marek, 1966 

Family uncertain

Genus Conotheca Missarzhevsky in Rozanov et al., 1969 

Type species. Conotheca mammilata Missarzhevsky in Rozanov et al., 1969.

Conotheca mammilata Missarzhevsky in Rozanov et al., 1969 

Figure 8a–c 

Material. Dozens of phosphatic specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. Tubes straight or gently dorsoventrally curved (Fig. 8a–c), with circular cross-section. Juvenile shell blunt and bulbous (Fig. 8c). The shell has a bluntly round embryonic shell, divergence angle of 5–9°, thin initial part and thick apertural part, without a uniformly upward expanding apertural end. Abapical conch is always broken (Fig. 8a and b).

Occurrence. Yangjiagou Member of southern Shaanxi, DaHai Member of Huize County of eastern Yunnan, China; Siberia.

Remarks. Conotheca mammilata Missarzhevsky in Rozanov et al., 1969 occurs in the Tommotian and Atdabanian Stages of Siberia. In the type material, the apical angle ranges from 8 to 20° and the presence of both robust and gracile forms was indicated in the original description of C. mammilata by Missarzhevsky (Rozanov et al. 1969). The microstructure of some of the other hyoliths described herein is similar to those of specimens with better-preserved phosphatic walls (Kouchinsky 2000 ).

Conotheca subcurvata Yu, 1974 

Figure 8d–j 

Material. Over 20 phosphatic specimens were obtained from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. Tubes are small, thin and long straight or curved conoid (Fig. 8d–j). Shell round in cross-section. The ends of the shell are usually broken (Fig. 8i and j). Shell extending from the apex to aperture; divergence angle is 4–13°. The outer surface of the shell is ornamented with thin and dense growth lines (Fig. 8e). Wall thin, phosphatic and homogeneous in structure.

Occurrence. Yangjiagou Member of southern Shaanxi, Zhongyicun Member of eastern Yunnan, Maidiping Formation of Shatan and Xinli regions of northern Sichuan Province, China; Siberia.

Remarks. This species is more curved and slimmer than the type species C. mammilata (Kouchinsky et al. 2022). The former differs from the latter in that it has a round apex, an apertural end that expands uniformly upward, and a small divergence angle, whereas the latter has a bluntly rounded embryonic shell, the divergence angle of the middle part of the tube is higher than the initial or terminal parts, and there is a thin initial part and a thick apertural part, and an apertural end that does not expand uniformly upward (Qian 1989).

Conotheca obesa Qian, 1978 

Figure 8k and l 

Material. About 23 phosphatic specimens were preserved in three dimensions and obtained from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. Tubes stout, straight conical. Shell mouth straight, rounded in cross-section (Fig. 8k and l). The tube tip is rounded, expanding faster towards the mouth of the shell, growth angle of 12–13°. The tube surface is smooth, with faint growth lines (Fig. 8k), parallel to mouth margin.

Occurrence. Yangjiagou Member of southern Shaanxi, Zhongyicun Member of eastern Yunnan, China.

Remarks. Conotheca maidipinensis Yu and C. longiconica are similar to this species, but the shells of the two former species are elongated and have a smaller growth angle of 6–7°, with the C. maidipinensis shell being slightly curved at the tip, which distinguishes it from this species (Qian et al. 1978).

Phylum and class uncertain

Order Hyolithelmintida Fisher, 1962 

Family Hyolithellidae Walcott, 1886 

Genus Hyolithellus Billings, 1871

Type species. Hyolithellus micans Billings, 1871.

Hyolithellus tenuis Missarzhevsky in Rozanov and Missarzhevsky, 1966

Figure 8m–r 

Material. More than 100 specimens preserved in three dimensions from the Yangjiagou Member of the Dayingcun section of Fucheng area, southern Shaanxi, China.

Description. Tubes small, elongated, slightly curved and almost tube-shaped (Fig. 8m–r). The shell surface is smooth, with slightly transverse growth lines on the surface less regularly distributed (Fig. 8m). Neither end structure of the shell is preserved (Fig. 8o and p). Shell length is 0.6–1.2 mm, growth angle is 2–3°, transverse section rounded.

Occurrence. Yangjiagou Member of southern Shaanxi, Yanjiahe Formation of Hubei Province, Maidiping Formation of northern Sichuan, China; Siberia; North America.

Phylum, class and order uncertain

Family Coleolidae Fisher, 1962 

Genus Coleolella Missarzhevsky in Rozanov et al., 1969 

Type species. Coleolella billingsi Sysoev, 1962.

Coleolella recta Mambetov, 1981 in Missarzhevsky and Mambetov, 1981 

Figure 8s and t 

Material. About six tubular specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. Tubes small, narrowly tubular from the apex to aperture; aperture margin flat, circular in cross-section; apex of ribs obtusely round; transverse furrow between ribs much narrower and crack-like in appearance; the external surface is smooth or ornamented with growth lines, which are arranged densely and fasciculated in appearance (Fig. 8s and t); wall thin, phosphatic, with no septa on the longitudinal section of shell.

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Maidiping Member of Emei County of Sichuan Province, Yanjiahe Formation of eastern Hubei, Zhongyicun Member of eastern Yunnan, China.

Remarks. The tubular specimens described here can be attributed to Coleolella recta Mambetov, 1981 based on their resemblance to each other in the straight and narrow cone, the flat apertural margin, the circular cross-section and the dense transverse ribs (Qian 1989).

Phylum Mollusca Cuvier, 1797 

Class Monoplacophora Wenz in Knight, 1952 

Order uncertain

Family Maikhanellidae Missarzhevsky, 1989 

Genus Maikhanella Zhegallo, 1982 

Type species. Maikhanella multa Zhegallo 1982.

Maikhanella multa Zhegallo, 1982 

Figure 9a–d and m 

Material. Three specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The fossil is cap-shaped or low hat-shaped, with a rounded shell apex, located on the anterior side, near the anterior margin (Fig. 9a–d). The top of the shell is bare and undecorated, whereas the rest of the shell surface is decorated with elongate tuberculate sculpture or lumpy protrusions (Fig. 9m), which are arranged concentrically and parallel to the mouth edge (Fig. 9b and d). Partial erosion of shell protrusion is hollow and appears as rhombic or oblong–ovate outlines, which are closely connected.

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, China; northern Iran.

Remarks. The type species Maikhanella multa is unique in the size and position of the shell apex and the morphology of the shell ornamentation (Zhang 2019). The shell apex of Maikhanella multa is near the anterior margin and the exposed area of the shell apex is very small. In addition, the posterior shell ornamentation of this species is not developed, forming a longitudinal sparse band, which is also distinctly different from the common posterior shell ornamentation of other genera, which protrudes to form a dorsal ridge (Qin et al. 2019).

Maikhanella calvatus Jiang, 1982

Figure 9e–h and n 

Material. Eight specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. Fossils small, ovoid or oblong–ovoid in top view, bow–conical in lateral view. Shell height moderate, the fossil anterior side narrower, posterior side broadly rounded (Fig. 9e and f). The apex of the fossil is located in the centre or near the front side of the centre, the apex of the fossil is smooth, the exposed area is large, and the boundary between the exposed area of the apex of the fossil and the protrusions area is obvious (Fig. 9g and h). The fossil surface is decorated with oblong or irregular protrusions (Fig. 9n), the direction of the protrusions is perpendicular to the fossil surface and the decoration is arranged in a laminar pattern, parallel to the mouth edge (Fig. 9g).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, China; Mongolia.

Remarks. Compared with the type species Maikhanella multa, the exposed area on the top of this species is large (Zhang 2019). An ovate or ellipsoid shell, and shell ornamentation with oblong protuberances oriented perpendicular to the shell surface; the latter has a very small exposed area at the top of the shell, a wider shell and protuberances on both sides of the shell that are reversed in inclination (Zhang 2019).

Maikhanella perelegans Feng et al., 2001 

Figure 9i, j and o 

Material. Five specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are small and round or oblique in the apical view. The apex is bare and smooth, situated between the front margin and the middle part, and distinct from the surrounding area of ornamentation (Fig. 9i and j). Shell surface sculptured with circular platform (Fig. 9i), closely arranged mammilla-shaped protrusions (Fig. 9o). The front side of the shell is steep and straight, at an angle to the lateral sides of the shell.

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Zhongyicun Member of eastern Yunnan, China; Iran; western Mongolia.

Remarks. Maikhanella perelegans was established by Feng et al. (2001) and is characterized by a steep and straight front side of the shell, a wide and gentle back side of the shell, and a circular platform on the shell surface with closely arranged scale-, brick- or mammilla-shaped granules (Feng et al. 2001). The shell described in this paper has two circular platforms on the surface. This species in shell features is generally closer to Maikhanella.

GenusMediata Feng et al., 2001 

Type species. Mediata kunyangensis Feng et al., 2001.

Mediata xixiangensis Shao et al., 2015 

Figure 9k and l 

Material. Nine specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are long and oval in outline in the lateral view. The apex is large, bare, roundly convex and protruding outside the aperture. The front side of the shell is narrow, but obviously broadening near the middle of the shell, and narrows towards the back margin. The dorsal surface is long and rectangular granules are regularly arranged along the growth line. On the middle of the dorsal surface is a parallel wide ridge (Fig. 9k and l).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Zhongyicun Member of eastern Yunnan, China.

Remarks. In terms of shell morphology, Mediata xixiangensis Shao and Mediata kunyangensis Feng are similar (Shao et al. 2015), but in these two species the dorsal ridge is different. In the middle of the dorsal surface of Mediata xixiangensis Shao, a parallel, wide ridge runs from the front to the back. The dorsal ridge of Mediata kunyangensis Feng disappears in the middle of the dorsum (Shao et al. 2015).

GenusRamenta Jiang, 1982

Type species. Ramenta cambrina, Jiang, 1982.

Ramenta cambrina Jiang, 1982

Figure 10a–c 

Material. About six specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are low cones, ovate from the apex view. The apex is on the front side of the shell. The front side of the shell is steep and oblique, whereas the dorsal side is gentle (Fig. 10a and c). The shell protrusion is made up of a specific pattern of scales that are rhomboidally packed in rows concentric with the shell border (Fig. 10b). The surface feature of the shell consists of scales around the apical area. The scales morphologically vary widely from round to square to elongate–rhomboid in different locations on the shell. Broken scale-like protrusions are internally hollow (Fig. 10c).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Zhongyicun and Dahai Members of eastern Yunnan, China.

Remarks. Jiang et al. (1982) established the genus Ramenta in 1982 based on Ramenta cambrina. Luo et al. (1982) regarded it as a brachiopod and called it Squamularia. Bengtson (1992) put this genus under the same name as the genus Maikhanella. Feng et al. (2001) suggested that Ramenta is closely related to Maikhanella, but there are obvious differences in many aspects. Ramenta is a valid generic name, and Feng et al. suggested changing it to Ramenta.

Ramenta superata Feng et al., 2001 

Figure 10d–h 

Material. About four specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are subcircular or rounded in top view (Fig. 10d–f) and low-shaped, bow–conical in side view (Fig. 10g and h). The shell apex is prominent, the exposed area small, situated anteriorly to the centre of the shell (Fig. 10e). The anterior side is narrow and short, the rest of the side broad and gentle, and the dorsal side slightly arched. The shell face is decorated with regular striking scale-like, long rhombic ornamentation (Fig. 10f and h).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, China.

Remarks. Compared with the type species Ramenta cambrina Jiang, 1982, Ramenta superata Feng, 2001 has a taller shell, with an ovoid mouth and an anteriorly positioned shell apex (Qin et al. 2019).

GenusPurella Missarzhevsky. 1974 

Type species. Purella cristata Missarzhevsky, 1974.

Purella sp.

Figure 10i–l 

Material. About 12 specimens from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The specimens are small, high or moderate cone shaped in side view. The front side of the shell is steep, the rear side is wide and gentle. On both sides of the shell surface, the shell decorations are extremely long and strip-shaped, arranged horizontally in parallel (Fig. 10i–l). The shell ornaments are well developed and the protrusions are triangular, with the largest protrusion located near the mouth margin in the centre of the back side of the shell (Fig. 10i and j).

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, China.

Remarks. The species is distinguished from other species of Purella Missarzhevsky by its large shell apex, central or near anterior, truncated anterior side of shell, rounded tail and triangular protrusion in the central shell at the rear side (Zhang 2019).

Phylum and class uncertain

Order Sachitida He, 1980 

Superfamily Siphogonuchitoidea Qian, 1977 

Family Siphogonuchitidae Qian, 1977 

Genus Siphogonuchites Qian, 1977 

Type species. Siphogonuchites triangularis Qian, 1977.

Siphogonuchites triangularis Qian, 1977 

Figure 10m–v 

Material. More than 70 phosphatic sclerite and interior mould particles from the Yangjiagou Member of the Dayingcun section of the Fucheng area, southern Shaanxi, China.

Description. The sclerites are 0.9–2.7 mm long and can be divided into three morphological types, straight, left or right sclerites (Fig. 10m–s). The sclerite is asymmetrical and irregularly curved, with irregular polygons, triangles or trapezoids in cross-section (Fig. 10t–v). Transversely arched upper or convex sclerite surfaces have one or three prominent longitudinal ridges and shallow longitudinal furrows between them (Fig. 10s). The concave lower surface is flattened transversely and is smooth.

Occurrence. Yangjiagou Member and Kuanchuanpu Formation of southern Shaanxi, Zhongyicun Member of eastern Yunnan, Yanjiahe Formation of eastern Hubei, Maidiping Formation of central Sichuan, Gezhongwu Formation of Guizhou Province, China; Siberia; western Mongolia.

Remarks. The Family Siphogonuchitidae was established by Qian (1977). The genera of the family are Siphogonuchites, Lopochites, Drepanochites and Lomasulcachites (Qian and Bengtson 1989). Siphogonuchites triangularis was first discovered in the phosphatic stratum of the Kuanchuanpu Formation of the Ningqiang area of Shaanxi (Qian et al. 1977). This type of fossil is characterized by a triangular cross-section, convex side depressed inward bilaterally, concave side smooth, individuals higher than wide, ornamented with a pleural ridge in the oblique direction (Qian 1989). So far, Siphogonuchites has been found in the early Cambrian of China, Mongolia, Iran, France, India and other countries (Qian 1999).

Abundant SSFs are described and illustrated from the Yangjiagou Member of the Dengying Formation at the Dayingcun section in the Fucheng area, southern Shaanxi. The key fossils recovered at this locality represent the Anabarites trisulcatus–Protohertzina anabarica assemblage zone and the succeeding Paragloborilus subglobosus–Purella squamulosa assemblage zone. However, the Watsonella crosbyi assemblage and younger assemblage zones are missing. Based on the fossil assemblage in the Yangjiagou Member of the Fucheng area, the member should belong to the lower Fortunian Stage of the Terreneuvian Series. This study provides new insights into the regional distribution patterns and stratigraphic correlation of SSFs during the earliest phase of the Cambrian Explosion.

We would like to thank Ruisen Fang, Xuan Yang, Zhao Wang, Rao Fu, Caibin Zhang, Jiayue Wang and Juanping Zhai for their help with fieldwork. We thank Qian Zhang and Shuangshuang Chen for their help with the SEM of the fossils and XRTC analysis of the thin sections. Thanks go to Xi'an Alberta Analysis and Testing Technology Co., Ltd. for processing rock sections, and to Ruihan Duan for help with the rock thin section figures.

ML: conceptualization (equal), data curation (lead), formal analysis (lead), investigation (lead), writing – original draft (lead), writing – review & editing (equal); GAB: conceptualization (equal), methodology (equal), writing – review & editing (equal); YL: methodology (supporting), software (supporting), writing – original draft (supporting); BS: formal analysis (supporting), methodology (supporting); YH: methodology (supporting), software (supporting), writing – original draft (supporting); FL: methodology (supporting), software (supporting), writing – original draft (supporting); ZZ: conceptualization (lead), investigation (lead), funding acquisition (lead), project administration (lead), resources (lead), supervision (lead), writing – review & editing (equal).

This research was supported by the National Key Research and Development Program of China (2023YFF0803601), the National Natural Science Foundation of China (NSFC 41890844 and 41720104002), the Department of Science and Technology of Shaanxi Province (2022TD-11) and the 111 Project (D17013).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

All data generated or analysed during this study are included in this published article.