2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
Apart from exquisite skeletons and astonishing biodiversity, radiolarians are one of the key components in the marine system with silica-biomineralized skeletons. Their origin and early history has long been studied extensively, including molecular data and the fossil record (Decelle et al., 2012; Ishitani et al., 2011; Pouille et al., 2011; Danelian and Moreira, 2004; Nazarov, 1975, 1973). Identifying the earliest record of radiolarians is undoubtedly important in regard to the rise of siliceous biomineralization in eukaryotic lineages and its impact on the evolution of the silica cycle (Zhang et al., 2013 and references therein). Moreover, they provide information on the origin and initial establishment of "modern"-type marine ecosystems during the transitional interval between the Ediacaran and Cambrian, since they constitute the main representatives of the heterotrophic plankton (De Wever et al., 2001; Anderson, 1983). However, little is known concerning the morphology of the earliest radiolarians, since fossil records are scarce, especially in old sequences. Despite their origin can be dated back to the Neoproterozoic by molecular data (Decelle et al., 2012), the earliest convincing fossil record of radiolarians is from the Middle Cambrian (Won and Below, 1999). The ancestor of radiolarians has been considered to belong to the order Archaeospicularia, and the spicular or needle-like morphology has been postulated to represent an ancient characteristic (Maletz, 2011).
However, history of radiolarians in the Early Cambrian or the Neoproterozoic is still poorly known. So far, reports concerning Early Cambrian radiolarians are still highly incomplete, among them, few specimens have been described (e.g., Braun et al., 2007a; White, 1986), some of the specimens are misinterpreted as nonbiologic objects or skeletal plates of the lobopod animal Microdictyon (Zhang and Aldridge, 2007; Lipps, 1992; Shu and Chen, 1989; Peng, 1984), some specimens are poorly preserved (Cao et al., 2014, Fig. 4l therein; Braun et al., 2007a). Besides, ambiguous age of some sequences in the Lower Cambrian poses another challenge since there are neither proper fossil zones nor geochemical data.
Here we report new siliceous microfossils recovered from the Yanjiahe Formation in the Luojiacun Section, Zigui County, Hubei Province, South China. According to SSFs biostratigraphy, the Yanjiahe Formation belongs to Early to Middle Meishucunian in age, corresponding to the Terreneuvian internationally (Guo et al., 2014). The fossils, recovered from the limestone nodules in the middle of Yanjiahe Formation, include?Blastulospongia sp. and unnamed spherical radiolarians, they both represent the possible earliest radiolarian fossil record. These fossils are less likely to be contamination because Blastulospongia is a genus that only reported from the Cambrian; the unnamed radiolarians are very similar to those reported from the Kuanchuanpu Formation (Terreneuvian) and the Shuijingtuo Formation (Series 2) (Cao et al., 2014; Braun et al. 2007a). Our findings support the idea by Braun et al. (2007a) that spherical radiolarian is an original representative and has an older history than previous thought. Along with sponges (e.g., Chang et al., 2017; Braun et al., 2007b), it is likely that the Cambrian invasion of siliceous skeleton-bearing radiolaria in the oceanic realm altered profoundly oceanic silica cycle during the Ediacaran–Cambrian transition. Their early biomineralization and flourish might be related to the high concentration of silica in the seawater caused by the breakup of the Rodinia supercontinent during the Neoproterozoic– Cambrian transition.1 GEOLOGICAL SETTING AND STRATIGRAPHY
The studied section crops out in a quarry (30°47'40.95"N, 110°54'49.81"E) closed to the Luojiacun Village, western Yichang, Hubei Province, China (Fig. 1). The Ediacaran to Lower Cambrian succession is well exposed. It includes, in ascending order, the Dengying, the Yanjiahe, the Shuijingtuo, and the Shipai formations. The basal Cambrian Yanjiahe Formation, disconformably overlies the Baimatuo Member of the Ediacaran Dengying Formation. There is a hiatus between the Yanjiahe and the Shuijingtuo formations (Fig. 2).
In biostratigraphy, Chen (1984) identified two small shelly fossils (SSFs) assemblage zones within the Yanjiahe Formation: Circotheca-Anabarites-Protohertzina assemblage zone and Lophotheca-Aldanella-Maidipingoconus assemblage zone. Guo et al. (2014) revised the SSFs into three assemblage zones, namely: the Anabaritestrisulcatus-Protohertzinaanabarica assemblage zone (Zone Ⅰ), the Purellaantiqua assemblage zone (Zone Ⅱ) and the Aldanellayanjiaheensis assemblage zone (Zone Ⅲ) in ascending order (Fig. 2). Based on SSFs, the Yanjiahe Formation was deposited in the Early to Middle Meishucunian and corresponds to the Cambrian Terreneuvian internationally.
In the studied area, the Yanjiahe Formation can be subdivided into 5 parts. Bed 1, characterized by basal dolomitic conglomerates, mostly consists of dolostone to sandy dolostone and banded black cherts. Bed 2 is dominated by siliceous phosphatic dolostone with flat pebble conglomerates. It is followed by a thick succession of alternation of shale, in which carbonate and phosphatic nodules are common, and limestone (Bed 3). Bed 4 comprises carbonaceous limestones. The uppermost Bed 5 is characterized by cherts and siliceous phosphatic dolostone with flat pebble conglomerates. The radiolarian fossils were recovered from the limestone nodules in Bed 3, which belongs to the Purellaantiqua Zone (Zone Ⅱ) in the Yanjiahe Section. This bed can be further correlated with the Upper Zhongyicun Member in eastern Yunnan, the Upper Maidiping Formation in western Sichuan, the Upper Gezhongwu Member in northern Guizhou (Guo et al., 2014).2 MATERIAL AND METHODS
Recrystallization in cherts during diagenesis process can strongly influence the microstructures of radiolarian fossils. This study focused on limestone nodules to detach radiolarians. A large amount of limestone nodules were collected from the middle part of the Yanjiahe Formation (Bed 3) in the field work. Sample was then crushed into small pieces of about 1 cm3. The fragments were placed in plastic barrels with 10% acetic acid solution (nine parts water to one part acid) for a period of about one week, the upper acid is discarded and the sample is washed until enough residues were detached. Subsequently, the residue was sieved (diameter=0.038 and 0.083 mm) carefully, dried at room temperature, and then hand-picked under a binocular microscope. Isolated specimens were mounted on stubs with latex then examined on stereoscan electron microscope (SEM) and electronic differential system (EDS) analysis in the State Key Laboratory of Geological Process and Mineral Resources (Wuhan).
In our study, abundant microfossils have been recovered, including radiolarians, SSFs, sponge spicules, etc. Among them, six radiolarian fossils were described. According to small shelly fossil biostratigraphy, the radiolarian fossils, including?Blastulospongia sp. and unnamed spherical radiolarians, belong to Purellaantiqua assemblage zone of the Yanjiahe Formation (Guo et al., 2014), which can be correlated with the second SSF biozone, Siphogonuchitestriangularis–Purellasquamulosa assemblage zone in eastern Yunnan (Steiner et al., 2007), corresponding to the Early Meishucunian Stage of Chinese usage, Upper Nemakit–Daldynian in Siberia and Terreneuvian, Fortunian Stage of international scheme (Peng et al., 2012; Khomentovsky and Karlova, 1993).3 SYSTEMATIC PALEONTOLOGY
All of the specimens described here are deposited in the Geological Museum of China University of Geosciences (Wuhan), China.
Class uncertain Family uncertain
Genus Blastulospongia Pickett and Jell, 1983
Diagnosis: Spherical to oval shell, perforated by numerous pores which are usually evenly distributed. Wall thin, interior of tests empty, the primary composition is uncertain.
Figure 3 (1–3)
Material: Two specimens: YJH03-11, YJH03-12.
Occurrence: Early Cambrian–South China, Siberia; Middle Cambrian, West Utah; Late Cambrian, Queensland.
Description: Globular tests, single and small shell, about 70 μm in diameter, with rounded or angular perforations evenly distributed all over the specimen. Spineless, wall thin. Composed of microcrystalline silica. Most of the perforations are nearly circular in outline, fairly and evenly spaced, about 5–8 μm in diameter, some angular ones might partially be attributed to recrystallization.
Remarks: Blastulospongia is an enigmatic genus that only receives specimens from Cambrian strata, with siliceous wall and varied diameter. It was originally interpreted as sponge by Pickett and Jell (1983). The type species, B. monothalamos, was described to be extremely large (1–1.9 mm in diameter). Bengtson (1986) reported B. mindyallica from the Late Cambrian of Queensland and proposed that this genus would be a radiolarian, if the original composition could have been siliceous. White (1986) reported an unnamed spumellarian from the Middle Cambrian, which is recognised as Blastulospongia for its strong similarities with this genus. In South China, Conway Morris and Chen (1990) reported B. polytreca from the Shuijingtuo Formation (the Cambrian Series 2). The B. polytreca was characterized by thin wall, bearing dimples and folds of varying openness. However, possibility of post-mortem damage cannot be ruled out for these dimples and folds, since the shells of B. polytreca are rather thin and would be easily influenced by sedimentary compaction. Conway Morris and Chen (1990) further provided evidence that the Blastulospongia was unlikely to be sponge for its siliceous composition that probably being primary. Kouchinsky et al. (2017) reported specimens recovered from Cambrian Terreneuvian in Siberia, by experiments of energy-dispersive X-ray analyses and their co-occurrence with calcitic shelly fossils, the siliceous composition of Blastulospongia was further confirmed to be original, rather than reflecting selective replacement by silica during diagenesis.
New materials described here are siliceous (Fig. 4a), co-occurred with calcitic shelly fossils. However, our specimens have some differences with those mentioned above. The specimens reported from Medvezhya Formation (Terreneuvian, Stage 2) in Siberia includes 2 specimens, the smaller specimen is ca. 350 μm and the larger one ca. 800 μm in diameter (Kouchinsky et al., 2017). Our specimens are rather small in diameter, the perforations are evenly spaced, and shows no deformation in the wall. Compared to B. polytreca, the perforations of our specimens are more sparsely distributed; compared to B. mindyallica, the shell of our specimens are thicker. The diameter of specimens from the Shuijingtuo Formation reported by Conway Morris and Chen (1990) is between 350 to 370 μm; the specimen described by White (1986) from the Middle Cambrian was about 380 μm, but the size range of his materials was not provided; the diameters of specimens reported from the Late Cambrian were between 280–520 μm (Bengtson, 1986). The diameter of this genus varies from 280 to 1 900 μm, some of the specimens have rather thin wall and tend to be deformed with dimples and folds (e.g., B. polytreca and one specimen from Siberia) but others do not show deformation. Our specimens share the morphology of single chambered, asiphonate, porate, without internal skeletal structures with Blastulospongia, but is rather small in size, it's possible that these differences represent two different lineages, so they are tentatively assigned to Blastulospongia.
As to this controversial question of whether Blastulospongia belongs to a sponge, radiolarian, or something else, observations of this study are in accordance with Conway Morris and Chen (1990) and Kouchinsky et al. (2017), which shows that Blastulospongia are less similar to sphinctozoan- grade. In morphology our specimens are most similar to that reported by White (1986) and they might belong to one species. In addition, if it is a radiolarian, to the familiar latticed architecture, radiolarians can also produce spherical tests of denser material with perforations, it is possible that radiolarian could have been diversified as a part of the 'Cambrian explosion'.
Blastulospongia is similar to a small shelly fossil, assigned as Aetholicopallaadnata (Conway Morris and Chen, 1990), in the round shape with perforations. Aetholicopallaadnata have been widely reported from the Lower Cambrian strata (Yang et al., 2014; Kouchinsky et al., 2013; Wrona, 2004; Elicki, 1998). But Aetholicopallaadnata is characteristic of double-walled shells connected by hollow pillars and preserved as phosphate (Wrona, 2004), which can be distinguished from Blastulospongia.
Suborder, family and genus unknown
Figure 3. 4–7
Material: Six specimens: YJH03-21, YJH03-22, YJH03-31, YJH03-32, YJH03-33, YJH03-34.
Occurrence: Early Cambrian, South China.
Description: Spherical, latticed shell, small in diameter, about 125 μm. The perforations are circular with pentagonal or hexagonal frames. Inner structure unclear. Siliceous in composition (Fig. 4b).
Remarks: These spherical radiolarians are very similar to those reported from the Kuanchuanpu Formation in Ningqiang, Shanxi Province, China (Braun et al., 2007a), they possessed latticed shell and spherical shape with small sizes, while the latter specimens possess short spines. In the Shuijingtuo Formation, Cao et al. (2014) reported similar spherical radiolarian, but its age is younger (Series 2, Stage 3) and its diameter is bigger (240 μm). The specimens described here are from the same region with those reported by Cao et al. (2014), not only validating the presence of radiolarians in the Early Cambrian sequences, but also providing opportunities to reveal characteristics of early radiolarian. The spherical morphology might represent an original characteristic of radiolarian, which was evolved much earlier than that has been generally accepted. The frame structure displayed similarities to some genera occurred in Ordovician, but due to recrystallization its inner structure was unclear. However, its systematic position and nomenclature requires additional material and more conspicuous characters showing the inner structure.4 DISCUSSION ON SIZE AND MORPHOLOGY OF RADIOLARIANS IN THE EARLY CAMBRIAN
In the Yanjiahe Formation, all these siliceous fossils yielded are small in size, with the average diameter being approximately 100 μm. The specimens reported from the Kuanchuanpu Formation are also relatively small, about 160 μm in diameter (Braun et al., 2007a). The specimens reported by Obut and Iwata (2000) from the Botomian (Lower Cambrian) in the Altai Mountains (Siberia) are even smaller. Indeed, the shell diameter of species Archaeocenosphaeramuricata Obut and Iwata ranges between 80 and 100 μm (Obut and Iwata, 2000). The specimens of Spongomassanannosphaera Won, described from the Middle Cambrian of Australia, varies between 93 and 130 μm in diameter (Obut and Iwata, 2000). The relationship of body size with oxygen availability has been discussed widely, with lots of studies showing that the increase of oxygen level is a critical trigger for evolution of life (Li et al., 2017; Zhang and Cui, 2016; Zhang et al., 2014; Payne et al., 2009; Canfield et al., 2007; Cloud, 1968). Specifically increased oxygen availability leads to increased sizes, as demonstrated by fossil records of protists (primarily in aerobic heterotrophs) and animals (Payne et al., 2009) and experiments in laboratory settings (Klok et al., 2009). Studies from geochemistry show that oxygen availability was still very low in South China (e.g., Liu et al., 2017; Jin et al., 2016), lithologically the wide spread black shall and cherts composition also indicate a relatively poor redox condition in the lower sequences of South China (e.g., Zhou and Jiang, 2009; Li et al., 1999). The small size of the?Blastulospongia and the spherical radiolarian fossils in the Yanjiahe Formation might be an original character and the low oxygen availability might be a factor that limited their size, but considerable complexity remains.
The ancestral representatives of radiolarians have been considered to belong to the order Archaeospicularia, and the simple spicular or needle-like morphology has been postulated to represent an ancient characteristic (Maletz, 2011). However, no elements of Archaeospicularia has been found in the Lower Cambrian till now, moreover, the simple, 'original' morphology described by Maletz (2011) could be easily confusing with Hexactinellida sponge spicules or some abiogenetic minerals for their similar outline, size and siliceous composition.
The record of radiolarians from the Lower Cambrian or even the Late Neoproterozoic would certainly provide a great breakthrough as regards to the radiolarian research as well as the Cambrian research. Recently there have been multi-disciplinary works on the Edicaran–Cambrian transition from South China (e.g., Ding et al., 2017; Mason et al., 2017; Hu et al., 2016; Yin et al., 2016; Zhang et al., 2016), as the Proterozoic to Early Cambrian strata are well exposed there, many publications provided evidence of radiolarian fossils shown in thin sections (He et al., 2013; Zheng et al., 2012; Zhao, 1999; Yin et al., 1994), and all of them possess spherical shell. But due to recrystallization the inner structures are difficult to observe. Reports regarding old radiolarian fossils from South China have a potential for our knowledge on the early history of radiolarians. Morphologically they possess latticed shell and spherical shape that are quite different from the postulated ancestral radiolarians (Archaeospicularia). The original character of radiolarian is still open to question, but evidence from both thin sections and acid etching analysis shows that the morphology of radiolarian in South China is different from that of Australia specimens during Cambrian. The hypothesis that oldest radiolarians belong to the order Archaeospicularia needs to be re-examined, and a phylogenetic analysis concerning old radiolarians must await more investigations.ACKNOWLEDGMENTS
This research was supported by the NSFC (Nos. 41430101, 41502014). We express our sincere thanks to Yan Zhang, Fenggang Lan, Wei Guo and Ke Zhang for help in the fieldwork. Professor Jonathon Aichison and anonymous reviewers are greatly appreciated for improving the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0960-0.
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