First Report of Small Skeletal Fossils from the Upper Guojiaba Formation (Series 2, Cambrian), Southern Shaanxi, South China

Simple Summary Small skeletal fossils are reported for the first time from the early Cambrian Guojiaba Formation, southern Shaanxi, China. All specimens were recovered from bioclastic limestone interbeds and encompass a wide variety of skeletal clades, including brachiopods, sphenothallids, archaeocyaths, bradoriids, sponge spicules, echinoderm plates, and trilobite spines. The archaeocyaths described herein are considerably older than those described from the Xiannvdong Formation, which was previously assumed to contain the lowest archaeocyath-bearing horizons in South China. The brachiopod Lingulellotreta yuanshanensis is recorded for the first time from the Fucheng area, with previous records confined mainly to the Chengjiang Fauna-bearing Yu’anshan Formation in the lower Cambrian, the eastern Yunnan Province. Micro-morphological and elemental analyses of the small skeletal fossil assemblages were carried out using SEM, BSEM, and Micro X–ray fluorescence. The skeletal fauna in the Guojiaba Formation resembles the assemblages recovered from the upper Yu’anshan Formation (Chengjiang Fauna) in eastern Yunnan Province. Based on the recovered assemblage, the biostratigraphic age of the Guojiaba Formation correlates with the Chiungchussuan Stage (Stage 3 of Cambrian Series 2). Abstract A small skeletal fossil assemblage is described for the first time from the bioclastic limestone interbeds of the siltstone-dominated Guojiaba Formation, southern Shaanxi, China. The carbonate-hosted fossils include brachiopods (Eohadrotreta zhujiahensis, Eohadrotreta zhenbaensis, Spinobolus sp., Kuangshanotreta malungensis, Kyrshabaktella sp., Lingulellotreta yuanshanensis, Eoobolus incipiens, and Eoobolus sp.), sphenothallids (Sphenothallus sp.), archaeocyaths (Robustocyathus sp. and Yukonocyathus sp.), bradoriids (Kunmingella douvillei), chancelloriids sclerites (Onychia sp., Allonnia sp., Diminia sp., Archiasterella pentactina, and Chancelloria cf. eros), echinoderm plates, fragments of trilobites (Eoredlichia sp.), and hyolithelminths. The discovery of archaeocyaths in the Guojiaba Formation significantly extends their stratigraphic range in South China from the early Tsanglangpuian at least to the late Chiungchussuan. Thus, the Guojiaba Formation now represents the lowest known stratigraphic horizon where archaeocyath fossils have been found in the southern Shaanxi area. The overall assemblage is most comparable, in terms of composition, to Small skeletal fossil (SSF) assemblages from the early Cambrian Chengjiang fauna recovered from the Yu’anshan Formation in eastern Yunnan Province. The existing position that the Guojiaba Formation is correlated with Stage 3 in Cambrian Series 2 is strongly upheld based on the fossil assemblage recovered in this study.


Materials and Methods
All microfossils in this study came from the upper part of the Guojiaba Formation in the Dayingcun Section, Fucheng area, southern Shaanxi. The original samples of 160 kg were crushed into 7-15 cm diameter particles before being placed in 7-10% acetic acid solution. The insoluble residue was cleaned and dried after approximately three days. Fragments were manually picked using a binocular microscope. SEM images of goldcoated fossils were taken using a Phenom XL G2 SEM with a resolution of 8 kV and 0.10 Pa. Thin sections of all sampled layers were prepared at Northwest University. Micro Xray fluorescence was performed on thin sections to assess elemental composition. The similarity of the SSF fauna in the Guojiaba formation to other SSF faunas was undertaken using multivariate cluster analysis (based on Jaccard similarity), performed using PAST.

Materials and Methods
All microfossils in this study came from the upper part of the Guojiaba Formation in the Dayingcun Section, Fucheng area, southern Shaanxi. The original samples of 160 kg were crushed into 7-15 cm diameter particles before being placed in 7-10% acetic acid solution. The insoluble residue was cleaned and dried after approximately three days. Fragments were manually picked using a binocular microscope. SEM images of goldcoated fossils were taken using a Phenom XL G2 SEM with a resolution of 8 kV and 0.10 Pa. Thin sections of all sampled layers were prepared at Northwest University. Micro X-ray fluorescence was performed on thin sections to assess elemental composition. The similarity of the SSF fauna in the Guojiaba formation to other SSF faunas was undertaken using multivariate cluster analysis (based on Jaccard similarity), performed using PAST. All fossil specimens from this study are housed in the collections of the Northwest University Early Life Institute (ELI), Xi'an, China.

Small Skeletal Fossils and Elemental Analysis of the Guojiaba Formation
Abundant and diverse SSFs were recovered from the bioclastic limestone at the top of the Cambrian Guojiaba Formation. The skeletal fossils are mainly preserved by phosphatization. In terms of faunal composition, the SSF fauna of the Dayingcun Section includes brachiopods, sphenothallids, archaeocyaths, chancelloriids, hyolithelminths, oocysts, echinoderms, librigena (trilobites), and bradoriids.
All 451 SSF specimens were recovered from Guojiaba Formation residues. GJB+0 accounted for 43%, GJB+1 for 32%, GJB+2 for 19%, GJB+3 for 1%, and GJB+4 for 5% of the total abundance (Figure 2A). A total of 171 SSFs were recovered from sample site GJB+0. Brachiopods represented 52%, archaeocyaths 12%, oocysts 9%, sphenothallids 8%, hyolithelminths 6%, trilobites 5%, echinoderms 3%, chancelloriids 3%, and bradoriids 2% ( Figure 2B). A total of 127 SSFs were recovered from sample site GJB+1, of which 33% were brachiopods, 18% archaeocyaths, 14% oocysts, 13% sphenothallids, 5% hyolithelminths, 6% trilobites, 3% echinoderms, 2% chancelloriids, and 6% bradoriids ( Figure 2C). At sample site GJB+2, a total of 132 SSFs were recovered, of which 33% were brachiopods, 18% archaeocyaths, 13% sphenothallids, 5% hyolithelminths, 6% trilobites, and 2% chancelloriids ( Figure 2D). At sample site GJB+3, only five SSFs were found; three of hyolithelminths and two of oocysts ( Figure 2E). At sample site GJB+4, 21 SSFs were presented, of which 33% were brachiopods, 14% oocysts, 13% sphenothallids, and 5% hyolithelminths ( Figure  2F). Thin section results for the four sample sites are consistent across all four samples. We selected the thin section for GJB+0 as the best representative of all sample sites because this thin section is the most distinct and clear. Some skeletal fossils can be identified in the GJB+0 thin section, including hyolithelminths ( Figure 3A Thin section results for the four sample sites are consistent across all four samples. We selected the thin section for GJB+0 as the best representative of all sample sites because this thin section is the most distinct and clear. Some skeletal fossils can be identified in the GJB+0 thin section, including hyolithelminths ( Figure 3A Figure 3N). The main elemental components of the fossils are calcium phosphate ( Figure 3K,L) and iron, with the iron coating the shell surfaces ( Figure 3M).   The majority of hyolithelminthids are preserved as phosphatic internal molds without detailed shell ornamentation or microstructure ( Figure 4A-D). Chancelloriidae (Chancelloria sp., Onychia sp., Archiasterella pentactina, Chancelloria cf. eros) ( Figure 4E-H), oocysts ( Figure 4I,J), and echinoderm plates ( Figure 4K,L) are all relatively rare. In addition to trilobites ( Figure 4M-O), the arthropod assemblage includes a bradoriid carapace, identified as Kunmingella douvillei ( Figure 4P,Q). There is also a cup-like mollusc of indeterminate affinity ( Figure 4R). The remaining material consists of a large number of archaeocyath fragments that have undergone secondary phosphatization ( Figure 4S). Brachiopods represent the most abundant and diverse component of the overall assemblage ( Figure 2B-D,F). Seven brachiopod taxa are present, belonging to the orders Acrotretida and Lingulida (see Part 6 for further details) [40,41]. The most common taxa are the two species of acrotretoid brachiopods, Eohadrotreta zhujiahensis and E. zhenbaensis [13,24], which collectively account for 37.6% of the total number of brachiopods. The five remaining species, Kyrshabaktella sp., Spinobolus sp., Lingulellotreta yuanshanensis, Eoobolus aff. viridis, and Eoobolus sp. belong to the Lingulida. Most brachiopod valves are fragmented and incomplete; however, some juveniles have conjoined shells. Sphenothallids, including Sphenothallus sp. are the most abundant non-brachiopod element of the fauna and have a variety of skeletal forms. Some specimens have possible drill holes.
Material. Seventeen specimens with conjoined valves, eighteen specimens with ventral valves, and twelve specimens with dorsal valves from the Guojiaba Formation.
Diagnosis. See Zhang et al. [7]. Description. The commissural contour is usually transversely oval, and the shell is biconvex or slightly ventribiconvex ( Figure 5A-C). Growth lines are present in the later growth stages ( Figure 5C). The lateral and posterior margins of the commissural contour are strongly arched, whereas the anterior margin is gently rounded ( Figure 5C). Slender, flat-bottomed, hemispherical pits (approximately 1 µm in diameter) ornament the larval valve ( Figure 5D). The ventral pseudointerarea is rudimentary or weakly developed. The external pedicle foramen is circular and not enclosed by the larval shell. The dorsal valve is slightly convex ( Figure 5B). In the dorsal view, the dorsal larval valve has a circular or subcircular outline ( Figure 5B), and the dorsal pseudointerarea is orthocline. The average length of the ventral valve is 632 µm but varies from 478 µm to 907 µm ( Table 1). The ventral valve is cap-like ( Figure 5H), on average 95% as long as wide. The metamorphic shell is elevated above the shell surface. The comparatively large pedicle foramen is approximately 76 µm long and 99 µm wide, with an average length/width ratio of 106% ( Figure 5E-G). The tongue-like apical process encircles the pedicle foramen and occupies an average of 30% of the valve length. The ventral cardinal muscle scars are weakly developed and occupy approximately 22% of the valve length (Table 1).
Remarks. Eohadrotreta zhenbaensis is generally similar in shell outline to E. zhujiahensis, but there are substantial ontogenetic differences between the two species [8]. The pedicle foramen of E. zhenbaensis is never enclosed within the metamorphic shell, whereas, after the pedicle foramen enclosing stage, the pedicle foramen of E. zhujiahensis is enclosed within the metamorphic shell [8]. The ventral pseudointerarea of E. zhenbaensis is almost catacline to procline, divided by a narrow elongate intertrough, but the ventral pseudointerarea of E. zhujiahensis is almost catacline, divided by a slightly wider and short intertrough [8]. The ventral cardinal muscle scars are different for E. zhenbaensis versus E. zhujiahensis, with the former being more pronounced than the latter [8]. The dorsal pseudointerarea type of both are consistent between the two taxa and the dorsal cardinal muscle scars are well developed. Eohadrotreta zhenbaensis has a better-developed median septum than E. zhujiahensis. It has been previously proposed that E. zhujiahensis could represent a juvenile stage of E. zhenbaensis [13].
Occurrence. The upper part of the Guojiaba Formation (lower Cambrian, Chiungchussuan Stage) of southern Shaanxi Province; Shuijingtuo Formation of western Hubei Province, South China. Material. Five specimens with conjoined valves, eight specimens with ventral valves, and twelve specimens with dorsal valves, all from the Guojiaba Formation.
Diagnosis. See Holmer and Popov in [41]. Description. The shell is ventribiconvex, subcircular to transversely oval in commissural outline ( Figure 5I), on average 97% as wide as long (Table 2), with concentric growth lines on the shell surface. The ventral valve is low, conical to slightly convex, with a somewhat straight posterior margin bisected by a slightly recessed groove ( Figure 5J). The ventral apex is moderately convex, situated on a rounded ventral larval valve slightly posterior to the posterior 1/3 of the shell length (Table 2); the ventral larval valve is nearly circular in outline, but perforated by the pedicle foramen, which is not enclosed by the larval shell ( Figure 5K,L). The larval valve is ornamented by uniform flat-bottomed hemispherical pits approximately 1.03 µm in diameter. Concentric fila are present on the postlarval shell. The ventral pseudointerarea extends from apsacline to proline, and the interior is always poorly preserved. The dorsal valve is transversely oval in outline, on average 101% as long as wide ( Table 2). The median septum is poorly developed. The dorsal cardinal muscle scars are prominent, occupying approximately 32% of the valve length and 38% of the valve width.
Remarks. See remarks for E. zhujiahensis. Occurrence. The upper part of the Guojiaba Formation (lower Cambrian, Chiungchussuan Stage) of southern Shaanxi Province; the Shuijingtuo Formation of western Hubei Province; the middle Wulongqing Formation of eastern Yunnan, South China.  [43]. Description. Shell subcircular in outline, about 93% as wide as long ( Figure 5M, Table 3); outer surface of valves with pronounced growth lines ( Figure 5N-P); ventral pseudointerarea apsacline; foramen posterior to umbo and moderately developed; dorsal pseudointerarea narrow, occupying about half of valve width; ventral visceral area circular, not extending to mid-valve; dorsal mid-septum extending anteriorly, slightly exceeding mid-valve; pedicle without visible central cavity, thread-like, protruding from post-umbonal foramen.
Remarks. Kuangshanotreta malungensis has been reported from the lower Cambrian Chengjiang fauna of South China [43]; shell range in diameter from 0.8 mm to 2.2 mm. Specimens are relatively small, about 0.5 mm-0.9 mm. Obvious concentric growth lines are preserved on the shell surface of both valves ( Figure 5N-P). The shell outline and shape of Kuangshanotreta are somewhat similar to Eohadrotreta [24], but Kuangshanotreta has a less developed buttress, only has a weak dorsal median septum, and is generally larger than Eohadrotreta [43]. Nevertheless, it can be presumed that Kuangshanotreta malungensis specimens previously found manually from eastern Yunnan are equivalent to acid-etched specimens of Eohadrotreta zhujiahenis from the carbonates in western Hubei and southern Shaanxi provinces of South China [43]. Comparative studies and geometric analysis of specimens from these two genera have yet to be undertaken.
Occurrence   Figure 6A−B)  ( Figure 6A,B) Material. Two specimens with dorsal valves from the Guojiaba Formation. Diagnosis. See Skovsted et al. in [44]. Description. Valves oval to circular in outline ( Figure 6A); weakly biconvex, slightly longer than wide, and have a maximum width anterior of mid-length. Dorsal valves are 1.6-1.9 mm long (mean 1.75, N = 2) and 1.6-1.8 mm wide (mean 1.7, N = 2). The ventral valve is gently convex, acuminate posterior, and rounded anteriorly. The dorsal valve is gently convex and oval in outline. The metamorphic shell is almost circular and smooth. The dorsal pseudointerarea is moderately raised above the valve floor occupying about 63% of the valve width ( Figure 6B). The median groove is shallow broad, and weakly defined.
Remarks. On the basis of the outline of the ventral pseudointerarea with narrow propareas, a pedicle groove developing into an emarginature, and the low dorsal posterior pseudointerarea with broad margins, the species described above is referred to Kyrshabaktella [44,45]. Kyrshabaktella is assigned to the Family Zhanatellidae [45] as it lacks pitted larval and post-larval shells and possesses a smooth inner surface without special ornamentation. Kyrshabaktella mudedirri Kruse 1990, however, from the Middle Cambrian of Australia, has been previously shown to possess a badly preserved columnar shell structure [46]. Evidence of a well-preserved columnar shell structure is also evident in specimens from the lower Cambrian Harkless Formation of Nevada [44].
Occurrence  Figure 6C). The larval shell is smooth, with no ornamentation, pits, or pustules. The dorsal interior, with impressed posterolateral muscle scars, is bisected by an elongate median ridge that widens slightly and extends anteriorly along the median valve floor. The dorsal orthocline pseudointerarea is low and slightly elevated above the valve floor, separated by a broad, shallow median groove ( Figure 6D). Posterolateral muscle scars of the dorsal valve interior are not well preserved. There are no mantle canal impressions in specimens from the upper Guojiaba Formation.
Remarks. Spinobolus popovi was described from the early Cambrian Shuijingtuo Formation in the Three Gorges region of South China with distinctive spinose ornamentation [7]. The same species of Spinobolus was also reported in the early Cambrian Xinji Formation in Ruicheng County of North China [35], which extended the geographical distribution of the genus. The genus was first discovered in the Shuijingtuo Formation without pitting of the metamorphic shell structure, but Zhang (2018) [8] later showed that it is present in the metamorphic shell of Spinobolus. In regards to the taxonomic position of this genus, we follow Zhang (2018) [8] in retaining the assignment of Spinobolus to the Obolidae King, 1846, as originally proposed [7].
Occurrence  Figure 34, 1a-d in [40].  Table 4). The ventral pseudointerarea is elongated, triangular, about 56% longer in length, and occupying 88% of the total valve width in adults. Orthogonal pseudointerarea extends anteriorly to approximately 56% of the total valve length in adults ( Figure 6J,L) with well-developed bend lines ( Figure 6M). Elongated oval pedicle foramen located at the posterior tip of pseudointerarea with an average apical angle of 55.1 • . Pedicle foramen up to 0. 48 mm long and 0.1 mm wide, up to about 26% of the shell length in adults.
Remark. Lingulellotreta malongensis is one of the most distinctive early Cambrian linguloids from Kazakhstan and South China. The shell size of specimens from Kazakhstan is about half that of those recorded from South China [56,64]. The species is the earliest known taxon of the family Lingulellotretidae, and according to a detailed description by Holmer et al. (1997) [64], it is mainly recognized by its elongated foramen, internal pedicle tube, and smooth larval shell. These species were also found in the Chengjiang fauna with the digestive canal in a U-shaped arrangement [58]. Lingulellotreta ergalievi Koneva, 1983, is considered a junior synonym of Lingulellotreta malongensis [50]. The specimens presented here are only about half the length of the Kazakhstan specimens and a quarter of the length of the Chengjiang specimens [24]. This means that, from the Chiungchussuan Stage to the Tsanglangpuian Stage, the length of the pseudointerarea of Lingulellotreta gradually decreases, with a corresponding extension anterior to the posterior wall [64]. The elongated pseudointerarea may be a factor leading to the extinction of this species in the middle Cambrian [64]. The Chengjiang specimens of Lingulellotreta yuanshanensis were erected as a new species based on comparison to the type species Lingulellotreta ergalievi. Lingulellotreta yuanshanensis, when compared to L. ergalievi, has a more elongated ventral pseudointerarea, accommodating an extended body cavity, deepening posteriorly, with a recurved digestive tract [11].
Occurrence. The upper part of the Guojiaba Formation (lower Cambrian, Chiungchussuan Stage) in the Dayingcun Section, Shuijingtuo Formation in the Xiaoyangba Section of southern Shaanxi Province; Shuijingtuo Formation in the Aijiahe Section of Hubei Province, South China; Kazakhstan.    Material. Approximately 187 specimens from the Guojiaba Formation, the proximal part of the holdfast was not preserved.
Description. Each specimen is solitary and incomplete without a holdfast. Some specimens have circular holes ( Figure 7A). All specimens are composed mainly of calcium phosphate and organic material. The length of the specimens is between 1 mm and 3 mm ( Figure 7A-J), gradually tapered, open at the apical end, usually with the fracture edge at the apical end of each face forming a V-shaped ( Figure 7E) or U-shaped ( Figure 7D,J) notch. The inner surface is filled, and the outer surface is smooth or with numerous ( Figure 7C) transverse striae, straight or gently curved towards the opening.
Remarks. Sphenothallus Hall, 1847 occurs in Palaeozoic marine strata [25,[65][66][67][68][69]. Phosphatic examples of Sphenothallus fossils have received less attention over the last century than Sphenothallus from shale facies. Lower Cambrian examples of Sphenothallus from the Guojiaba and Xiannvdong Formations in the Shaanxi region were previously reported by Li et al. (2004b) [25]. Sphenothallus songlinensis from the Niutitang Formation and a potential Sphenothallus taijiangensis species from the Kaili Formation have both been described from Guizhou Province [65,70,71]. These previous specimens are all preserved in two dimensions, whereas the fossils from the Guojiaba Formation are preserved in three dimensions. Other unidentified Sphenothallus species have been found in the lower Middle Cambrian Burgess Shale Formation of British Columbia [72]. Some Sphenothallus species are from the Late Ordovician and are found in the United States and Baltica. These specimens have revealed the mineral composition and microstructure of phosphatic Sphenothallus species [72,73]. The characteristics and stratigraphic range of phosphatic Sphenothallus require further study.
Occurrence Sphenothallus sp. (Figure 7) Material. Approximately 187 specimens from the Guojiaba Formation, the proximal part of the holdfast was not preserved.
Description. Each specimen is solitary and incomplete without a holdfast. Some specimens have circular holes ( Figure 7A). All specimens are composed mainly of calcium phosphate and organic material. The length of the specimens is between 1 mm and 3 mm ( Figure 7A-J), gradually tapered, open at the apical end, usually with the fracture edge at the apical end of each face forming a V-shaped ( Figure 7E) or U-shaped ( Figure 7D,J) notch. The inner surface is filled, and the outer surface is smooth or with numerous (Figure 7C) transverse striae, straight or gently curved towards the opening.
Remarks. Sphenothallus Hall, 1847 occurs in Palaeozoic marine strata [25,[65][66][67][68][69]. Phosphatic examples of Sphenothallus fossils have received less attention over the last century than Sphenothallus from shale facies. Lower Cambrian examples of Sphenothallus from the Guojiaba and Xiannvdong Formations in the Shaanxi region were previously reported by Li et al. (2004b) [25]. Sphenothallus songlinensis from the Niutitang Formation and a potential Sphenothallus taijiangensis species from the Kaili Formation have both been described from Guizhou Province [65,70,71]. These previous specimens are all preserved in two dimensions, whereas the fossils from the Guojiaba Formation are preserved in three dimensions. Other unidentified Sphenothallus species have been found in the lower Middle Cambrian Burgess Shale Formation of British Columbia [72]. Some Sphenothallus species are from the Late Ordovician and are found in the United States and Baltica. These specimens have revealed the mineral composition and microstructure of phosphatic Sphenothallus species [72][73]. The characteristics and stratigraphic range of phosphatic Sphenothallus require further study.
Occurrence  Description. Cups are cylindrical or conical ( Figure 8A,C,E), with a minimum length of 1.83 mm ( Figure 8C) and a maximum of 5.91 mm ( Figure 8E). Diameter up to 1 mm or more ( Figure 8B,D). The outer wall with simple pores is arranged longitudinally ( Figure 8G,H), with additional bracts on the outside of the outer wall. The diameter of the pore is between 45.5 and 86.5 µm. The inner wall is thicker, 0.09-1.1 mm, with one row of large and round pores per intercept; the diameter of the pores is between 55.7 and 66.3 µm. No pores or only one row of pores in the septa ( Figure 8F). The cup cone angle is 22-29 degrees, but due to the variability of preservation, there is some margin of error in the measurements.
Remarks. Robustocyathus also occurs in South China, Siberia, eastern and western Gondwana, and western Laurentia [74]. Based on the identified fossil assemblages of archaeocyaths in Siberia, three zones are established [75]. Robustocyathus legitimus was found in the lower part of the Sekten Formation of the Siberian Platform [76]. Robustocyathus has also been found in the lower Cambrian reef facies of the middle Lena River [77]. In Nevada, USA, the archaeocyath species of Gold Point Reef are low in diversity in the lower Poleta Formation [78]. Robustocyathus infundibulus was discovered in the Qiongzhusian Stage of South China [74]. The identification of Robustocyathus [79], Usloncyathus [79], Stillicidocyathus [80], Metacyathellus [80], Dictyocyathus [80], and Agastrocyathus [81] is significant. Due to the restricted stratigraphic range of these taxa, direct correlations with Siberia, Eastern and Western Gondwana, and Laurentia are possible at a higher resolution level [28,74].
Occurrence  Figure 8L). Inner wall with one row of simple pores per intercept ( Figure 8G), formed by fluting of the inner edges of the septa; septa aporose to sparsely porous.
Remarks. The literature on Yukonocyathus is sparse. In the United States, the majority of irregular archaeocyath taxa belonging to the order Syringocnemidida and regular archaeocyaths are assigned to the family Ethmophyllidae [82]. The most abundant genera in the USA are Sekwicyathus, Yukonocyathus, Cordilleracyathus, Ethmophyllum, and Pesudosytingocnema [82]. In Sardinia, the genus Yukonocyathus is characterized by the presence of an outer wall with S-shaped canals and a simple inner wall [77], which are consistent with the specimens described here.
Occurrence. Upper part of the Guojiaba Formations (lower Cambrian, Chiungchussuan, and early Tanglangpuan stages) of southern Shaanxi Province, South China; Canada, United States.

Regional Fossil Occurrences and Stratigraphic Correlations of the Guojiaba Formation
The presence of three-dimensional preserved archaeocyathan fossils among the SSFs mentioned above is undoubtedly evident. There are relatively few reports of phosphatized archaeocyathan fossils [83,84]. Archaeocyath species were the earliest metazoan reef builders. They had calcareous skeletons and lived on the sea floor. The archaeocyathans flourished briefly in the early Cambrian. The fossils were almost ubiquitous in tropical regions during most of the early Cambrian [85]. For example, they were common in East Gondwana (Australia, East Antarctica), West Gondwana (Spain, France), and both coasts of Laurentia (eastern Canada, Nevada) [86][87][88][89][90]. Archaeocyaths first came to prominence in the Tommotian Stage of Siberia before the trilobites. The archaeocyaths were associated with SSFs. They are found together with chancelloriids, calcareous sponges, stromatoporoids, hyoliths, trilobites, lingulid brachiopods, and assorted echinoderms [85,87,91,92]. Archaeocyaths were first reported from the Three Gorges area of China [93]. The archaeocyathan fossils were successively collected in the Shuijingtuo Formation, Xiannvdong Formation, Tianheban Formation, Mingxinsi Formation, etc., of South China [28]. The oldest archaeocyaths in China are found in the Xiannvdong Formation of southern Shaanxi [74]. However, the discovery of phosphatized archaeocyathans from the Guojiaba Formation shows that the age of appearance of archaeocyaths extends to the Late Chiungchussuan Stage, which is the first report in southern Shaanxi of China. Yuan et al. (2001) proposed the evolutionary sequence of three Cambrian archaeocyathans assemblages, including the Chiungchussuan archaeocyathan assemblage, the Tsanglangpuian archaeocyathan assemblage, and the Tienhopan archaeocyathan assemblage [28]. These three archaeocyathans assemblages may be useful for the international correlation of lower Cambrian strata [28,94]. The Chiungchussuan archaeocyathan assemblage is similar to the archaeocyathan assemblage of the Tommotian to Botoman in Siberia, the Tsanglangpuian archaeocyathan assemblage is similar to the lower Toyonian Stage, and the Tienhopan archaeocyathan assemblage is similar to the upper Toyonian Stage (Figure 9) [28,94].
gy 2023, 12, x FOR PEER REVIEW 20 of Such stratigraphic correlations across the regions of South China have been well su ported by the discovery of SSFs in these regions. The Xihaoping Member of Dengying Formation in the Zhenba area contains the lobite Parabadiella huoi, but no Eoredlichia has been found [9]. The fossil assemblages of t Xihaoping Member of Dengying Formation consist mainly of Eoobolus incipiens witho the Eohadrotreta zhenbaensis, E. zhujiahensis, and Lingulellotreta yuanshanensis. It shou therefore, likely be assigned to early Stage 3 of Cambrian Series 2 ( Figure 9) [9]. The fos assemblage of the Shuijingtuo Formation is dominated by Eohadrotreta zhenbaensis a contains other taxa similar to those of the Hongjingshao Formation and Guanshan Fau in eastern Yunnan. This means the Shuijingtuo Formation is relatively younger than t Yu'anshan Formation ( Figure 9) [9,99,100]. The fossil assemblage of the Dailyatia obys Zone from Australia contains Parabadiella and other skeletal fossils [60]. The age of t Dailyatia obyssei Zone can be considered comparable to that of the Guojiaba Formati Based on the assemblage found in the Guojiaba Formation, it could be concluded that t Guojiaba Formation should belong to Stage 3 of Cambrian Series 2.  [7,9].
Cluster analysis shows that the lower Cambrian SSF assemblage from the Dayingc Section together with the eastern Yunnan Province and Three Gorges of China, althou the similarity between the three faunas is still marginal (Figure 10). What similarity exi is due to the co-occurrence of the genera Eohadrotreta, Kuangshanotreta, Kyrshabakte Spinobolus, Lingulellotreta, Eoobolus, Sphenothallus, Kunmingella, and Robustocyathellus. T presence of acrotretoids indicates that the Guojiaba Formation can be correlated with t upper silty shales of the Yu'anshan Formation in Yunnan Province (Figure 9) [7,11,43,10 The acrotretid Eohadrotreta zhenbaensis was obtained from the Shuijingtuo Formation  [7,9].
Hyolithelminths, chancelloriids, echinoderm plates, bradoriids, oocysts, etc., have not been described from the Guojiaba Formation in previous studies. The SSFs described in this study markedly increase the overall diversity known from Stage 3 Cambrian Series 2 in the Fucheng area. Trilobites Wutingaspis-Eoredlichia Zone has been discovered in the Guojiaba Formation, the lower Shuijingtuo Formation of the Hubei area, and the Yu'anshan Formation of the Yunnan area [95]. Most of these trilobites belong to the genus Eoredlichia but are poorly preserved [96]. The Wutingaspis-Eoredlichia Zone is widely found in various regions of the world, including South China, Siberia, Australia, Italy, and Morocco, suggesting a potential global correlation [95]. Two trilobite biozones have been identified in the Guojiaba Formation [97]. The lower part of the Guojiaba Formation is dominated by Parabadiella and Tsunyidiscus, the upper part by Eoredlichia and Wutingaspis, and other assemblages contain brachiopods and other fossils, suggesting that it could be correlated to the Yu'anshan Formation of Yunnan in terms of fossil assemblages [98].
Traditionally, the Yu'anshan Formation in eastern Yunnan is correlated with the Shuijingtuo Formation in southeastern Shaanxi and western Hubei provinces, both because the two formations represent the regional lowermost trilobite-bearing horizons respectively, and partly because they were deposited in the same sedimentary environment characterized by a sequence of black shales in the base of the two formations. As discussed by Zhang et al. (2016) [7], the multi-taxa trilobites recovered in the Yu'anshan and Shuijingtuo formations are most likely diachronous, and thus other fossils, notably SSFs dissolved from the carbonate interbeds, are of stratigraphical importance in the intra-and interregional stratigraphic correlation. More importantly, the Xihaoping Member of Dengying Formation, disconformably underlying the Shuijingtuo Formation in the Zhenba area, was recently revealed to yield the trilobite Parabadiella huoi, but no Eoredlichia found [9]. In terms of yielding the same trilobite species of Parabadiella, it is therefore assumed that the Xihaoping Member of Dengying Formation is well-correlated with the lower Guojiaba Formation, which contains rich exoskeletons of Parabadiella ( Figure 9). Such stratigraphic correlations across the regions of South China have been well supported by the discovery of SSFs in these regions.
The Xihaoping Member of Dengying Formation in the Zhenba area contains the trilobite Parabadiella huoi, but no Eoredlichia has been found [9]. The fossil assemblages of the Xihaoping Member of Dengying Formation consist mainly of Eoobolus incipiens without the Eohadrotreta zhenbaensis, E. zhujiahensis, and Lingulellotreta yuanshanensis. It should, therefore, likely be assigned to early Stage 3 of Cambrian Series 2 ( Figure 9) [9]. The fossil assemblage of the Shuijingtuo Formation is dominated by Eohadrotreta zhenbaensis and contains other taxa similar to those of the Hongjingshao Formation and Guanshan Fauna in eastern Yunnan. This means the Shuijingtuo Formation is relatively younger than the Yu'anshan Formation (Figure 9) [9,99,100]. The fossil assemblage of the Dailyatia obyssei Zone from Australia contains Parabadiella and other skeletal fossils [60]. The age of the Dailyatia obyssei Zone can be considered comparable to that of the Guojiaba Formation. Based on the assemblage found in the Guojiaba Formation, it could be concluded that the Guojiaba Formation should belong to Stage 3 of Cambrian Series 2.
Cluster analysis shows that the lower Cambrian SSF assemblage from the Dayingcun Section together with the eastern Yunnan Province and Three Gorges of China, although the similarity between the three faunas is still marginal (Figure 10). What similarity exists is due to the co-occurrence of the genera Eohadrotreta, Kuangshanotreta, Kyrshabaktella, Spinobolus, Lingulellotreta, Eoobolus, Sphenothallus, Kunmingella, and Robustocyathellus. The presence of acrotretoids indicates that the Guojiaba Formation can be correlated with the upper silty shales of the Yu'anshan Formation in Yunnan Province (Figure 9) [7,11,43,101]. The acrotretid Eohadrotreta zhenbaensis was obtained from the Shuijingtuo Formation of the Yangtze Platform [9,22,24], the Xinji Formation of the North China Platform [35], the Tethyan Himalaya [102], the Dailyatia dyssey Zone of South Australia [60], and the uppermost Shackleton Limestone [103] (Figure 10). The acrotretid Eohadrotreta zhujiahensis occurs in the Shuijingtuo, Xiannvdong, and Guojiaba Formations of South China [24,104]. The acrotretid Kuangshanotreta malungensis is recovered from the Yu'anshan Formation of South China. The lingulid Spinobolus extends the range from South China to North China [7,35]. Lingulellotreta yuanshanensis is reported in this study for the first time from the upper Guojiaba Formation, and specimens appear to be similar to the Lingulellotreta yuanshanensis from the silty shales of the Yu'anshan Formation in the Chengjiang area, with both possessing an elongated ventral pseudointerarea [11]. The lingulid Lingulellotreta yuanshanensis is restricted to South China [11], with limitations and territoriality. The biostratigraphy correlates closely with the brachiopods recently reported from the Yu'anshan Formation in the Yunnan area, as well as brachiopods found from the Dailyatia obyssei Zone of Australia ( Figure 10) [103]. This suggests the possibility of an association between these contemporary brachiopod faunas. Sphenothallids from the Guojiaba Formation are more abundant than those from the Chengjiang Formation, likely because the Guojiaba Formation represents a relatively shallower environment [105]. The current affinity for Sphenothallus is thought to be related to cnidarians [106]. Sphenothallus was widespread in Baltica [72], Laurentia [66,107], South China [25,65], and Siberia [104] and was considered a marine invertebrate [108]. Kunmingella douvillei has been previously found in the Shaanxi and Sichuan regions of the South China platform [39,97], with large quantities in the Yu'anshan Formation ( Figure 10) [109]. Chancelloriids of Chancelloria sp., Allonnia sp., and Archiasterella sp. have been discovered from the early Cambrian and widely distributed in South China [15], the Dokidocyathus regularis Zone of the Tommotian Stage in Siberia [84], the Laurentia [31,104], and North China [15]. The archaeocyathans of Robustocyathellus sp. are found in the Guojiaba Formation and Xiannvdong Formation of the Fucheng area, southern Shaanxi. Many of fossils discovered in the Guojiaba Formation have regional distribution characteristics, bearing little similarity with South Australia, the Baltic, Siberia, Avalon, North China, and other regions ( Figure 10). Given the links between the Guojiaba Formation taxa described here and those from other regions, the hyolithelminths, brachiopods, sphenothallids, archaeocyathans, chancelloriids, echinoderms, trilobites, and bradoriids reported here have the potential to expand the ability to correlate the lower Cambrian strata of South China with other strata both regionally and internationally. from the upper Guojiaba Formation, and specimens appear to be similar to the Lingulellotreta yuanshanensis from the silty shales of the Yu'anshan Formation in the Chengjiang area, with both possessing an elongated ventral pseudointerarea [11]. The lingulid Lingulellotreta yuanshanensis is restricted to South China [11], with limitations and territoriality. The biostratigraphy correlates closely with the brachiopods recently reported from the Yu'anshan Formation in the Yunnan area, as well as brachiopods found from the Dailyatia obyssei Zone of Australia ( Figure 10) [103]. This suggests the possibility of an association between these contemporary brachiopod faunas. Sphenothallids from the Guojiaba Formation are more abundant than those from the Chengjiang Formation, likely because the Guojiaba Formation represents a relatively shallower environment [105]. The current affinity for Sphenothallus is thought to be related to cnidarians [106]. Sphenothallus was widespread in Baltica [72], Laurentia [66,107], South China [25,65], and Siberia [104] and was considered a marine invertebrate [108]. Kunmingella douvillei has been previously found in the Shaanxi and Sichuan regions of the South China platform [39,97], with large quantities in the Yu'anshan Formation ( Figure 10) [109]. Chancelloriids of Chancelloria sp., Allonnia sp., and Archiasterella sp. have been discovered from the early Cambrian and widely distributed in South China [15], the Dokidocyathus regularis Zone of the Tommotian Stage in Siberia [84], the Laurentia [31,104], and North China [15]. The archaeocyathans of Robustocyathellus sp. are found in the Guojiaba Formation and Xiannvdong Formation of the Fucheng area, southern Shaanxi. Many of fossils discovered in the Guojiaba Formation have regional distribution characteristics, bearing little similarity with South Australia, the Baltic, Siberia, Avalon, North China, and other regions ( Figure 10). Given the links between the Guojiaba Formation taxa described here and those from other regions, the hyolithelminths, brachiopods, sphenothallids, archaeocyathans, chancelloriids, echinoderms, trilobites, and bradoriids reported here have the potential to expand the ability to correlate the lower Cambrian strata of South China with other strata both regionally and internationally.

Conclusions
The SSF assemblage of the upper Guojiaba Formation is reported for the first time. The earliest archaeocyathan, brachiopods of Acrotretida Eohadrotreta zhujiahensis and E. zhenbaen-sis and Lingulida Lingulellotreta yuanshanensis are discovered in this area. It also revealing the benthic diversity of the early Cambrian fauna dominated by Sphenothallus sp. This provides new fossil evidence for the early Cambrian stratigraphic correlation in the Micang Mountain area of southern Shaanxi.