Vase-shaped microfossils from the Ediacaran Weng'an biota,Guizhou, South China
Yong Li a,⁎, Junfeng Guo a,b, Xingliang Zhang b, Wanqian Zhang a, Yunhuan Liu a, Weixian Yang a,Yuanyuan Li a, Lianqun Liu a, Degan Shu b
a College of Earth Science and Land Resources, Educational Ministry Key Laboratory of Geological Resources and Geological Engineering of Western China,Chang'an University, Xi'an 710054, China
b Early Life Institute, State Key Laboratory of Continental Dynamics, Department of Geology,Northwest University, Xi'an, 710069, China
Received 9 May 2007; received in revised form 24 September 2007; accepted 9 October 2007Available online 25 October 2007
The Ediacaran–Cambrian sequences of South China haverecently been intensively investigated for their vital importancein understanding synergic evolution of life and environmentafter the extreme climate of the Snow Ball Earth (Wang and Li,2001; Zhu et al., 2007). The Neoproterozoic assembly ofGondwana and its relationship to the Ediacaran–Cambrianbiological events are topics of global interest (McCall, 2006;Meert and Liberman, 2008-this issue). The Weng'an biota,which derives from the Ediacaran Doushantuo phosphorites ofGuizhou Province, South China, has long been a key subject of
interest since some earlier reports on Precambrian microfossilswere published in the 1980s (Chen and Liu, 1986; Zhang, 1989;Zhang and Yuan, 1992). Thus far, the fossil assemblage reportedfrom the Weng'an biota encompasses large spiny acantho-morphic acritarchs (Yin et al., 2001a; Grey, 2005), multicellularalgae (Zhang, 1989; Zhang and Yuan, 1992; Yuan et al., 1993;Yin et al., 2001b), Chlorophyta (Xue et al., 1995, 2001),Cyanophyta (Gao et al., 2002), Rhodolith-like fossils (Zhou andXue, 1999), Echinodermata (Xue et al., 1992; Li, 2002; Li et al.,2003), small shelly fossils (Li et al., 1997; Li, 2002), sponges(Li et al., 1998; Yin L.M. et al., 2001; Zhou et al., 1998b; Caoand Zhu, 2001), animal embryos (Xiao et al., 1998; Xiao andKnoll, 2000; Yin et al., 2001c; Chi et al., 2003), microtubularcnidarians (Xiao et al., 2000; Chen et al., 2002), protozoans(Wu and Wang, 2004), and problematic bilaterians (Chen et al.,2004a,b; Bengtson and Budd, 2004). Recent continuous
Fig. 1. SEM images of VSMs (internal mold) from the ore bed B of the Doushantuo Formation at Beidoushan, Weng'an, Guizhou Province, showing overall tear-shaped morphology and broken aperture of Protolagena papillata (Zhang and Li, 1991). A, WA/19-1-1, aperture view; B, WA/19-1-2, aperture view; C, WA/19-1-3,lateral view.
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investigation on the minute skeletonized fossils from theDoushantuo phosphorites led to the discovery of vase-shapedmicrofossils (VSMs), which are reported here for the first time.The new material is abundant and well preserved, which allowsfor taxonomic and chemical analysis.
VSMs were firstly recovered and documented from theVisingsö Formation of Sweden by Ewetz (1933). Later, Knolland Vidal (1980) documented VSMs from the same area andwere able to make a morphological comparison of the vase-shaped microfossils of Sweden with those from the Precambrianstrata in U.S.A., Brazil, Saudi Arabia and other areas, whichwere initially regarded as strikingly particular groups of micro-scopic fossils with uncertain affinities. This suggests that thevase-shaped microfossils occurred in distinct regions duringdifferent geological times could be homologous in origin (Knolland Vidal, 1980). More recently, Porter and Knoll (2000), and
Fig. 2. SEM images of VSMs, showing overall vase-shaped morphology, neck and apGaojiashan Member of Denying Formation at Ningqiang, Shanxi Province. A, WA/12view of Protolagena gaojiashanensis (Zhang and Li, 1991), showing the smoothmorphology and circular aperture.
Porter et al. (2003) discussed affinities between VSMs andtestate amoebae.
The study of vase-shaped fossils in China can be traced backto 1980s. At the beginning, all this type of microfossils, occurredtogether with the acritarchs and small shelly fossils, wereindividually described as chitinozoans, chitinozoan-like fossilsor an enigmatic taxa with unknown affinities (Yang et al., 1983).Until 1986, the term of “vase-shaped fossils” was introduced byDuan Cheng-hua into China when he investigated the vase-shaped microfossils from the Xihaoping Formation, FangxianCounty (Duan, 1986). A wide variety of vase-shaped micro-fossils have been recorded from the Doushantuo Formation inXiadong (Duan and Cao, 1989), the Gaojiashan Member of theDengying Formation in Ningqiang, Shaanxi Province (Zhangand Li, 1991; Ding et al., 1992; Zhang, 1994), Ediacaran strata ofWenxian County in Gansu Province (Zhao et al., 1990); the
erture. A and C from the same locality and horizon of fossils in Fig. 1, B from the, lateral view of Protolagena sp., note the multi-layered wall; B, GHF8-3, lateralsurface of the wall; C, after Yin et al. (2002), showing overall vase-shaped
Fig. 3. SEM images of Luxapertus wenganensis (Wu andWang, 2004) from the same locality and horizon of fossils in Fig. 1, showing overall tear-shaped morphology,lateral view. A, WA/12; B, WA/19-1-1; C, WA/19-1-2.
Fig. 4. Thin section (WA/3-1) of Protolagena papillata (Zhang and Li, 1991)from the same locality and horizon of fossils in Fig. 1, showing multi-layeredtexture of the test wall that is preserved as a mineralized cast coated with organicmaterial and inside filled with dark brown-colored organic remains.
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Yangjiagou Member of the Dengying Formation at XixiangCounty, Shaanxi Province (Duan et al., 1993); the KuanchuanpuFormation at Ningqiang County, Shaanxi Province (Cao et al.,1995; Ding et al., 1992, pl. III, fig.15); Taozichong Formation ofQing town, Guizhou Province (Xue et al., 2002) and otherlocalities in China. In sum, VSMs have been documented from20 localities distributed in nine provinces in China and fourstratigraphic units. In ascending order the units are: Pre-Ediacaran, the Ediacaran Doushantuo Formation and DengyingFormation, as well as the Meishucunian stage of the LowerCambrian. Among them, the VSMs from the DoushantuoFormation of the Yangtze Gorges area, western Hubei Province,and the Yangjiagou Member of the Ediacaran Dengying Form-ation prepared by Duan (1986) and Duan et al. (1993) throughthin-sectioning and maceration still remain controversial. TheWeng'an biota is known for its well preserved and abundantfossil embryos (Xiao et al., 2000; Chen et al., 2006), but no vase-shaped microfossils have been reported from these deposits todate, except for a supposed protozoan fossil based on the thin-section study (Wu and Wang, 2004). Therefore, the fossilspresented here not only enrich the contents of the Weng'an biotabut also record a significant extension of their distribution inChina. By comparing with the vase-shaped microfossilsoccurred from the other areas in the size, contour, oral openingand the microstructure of the wall, these minute fossils here beara general resemblance to the vase-shaped microfossils fromGaojiashan Member (the middle part of Dengying Formation),Hujiaba area, and the Kuanchanpu Formation (the Meishucu-nian stage of the lower Cambrian), Kuanchuanpu area,Ningqiang county, southern Shaanxi Province. However, thechemical composition of the wall of these minute fossils are ofphosphate, which is the same as the fossils from theKuanchuanpu Formation but different from the calcareousvase-shaped microfossils from the Gaojiashan Member.
2. Materials and methods
The fossils are recovered from the upper part of theDoushantuo phosphorites at Hill Beidoushan, Weng'an county,Guizhou Province, South China. The Doushantuo phosphorite
at Weng'an area is directly overlying the basal Nantuo dia-mictite, 29 m thick in total, and is subdivided into three units:the basal unit is siliceous phosphorite, about 12 m thick, and isoften referred as the ore bed A; the middle unit is dolomitic–phosphorite, 8–10 m thick, and referred as the ore bed B; the topunit is wavy to horizontally laminated dolostone, about 7 mthick. The overlying Denying Formation is more than 100 mthick and is composed of white thick-bedded dolomite. Detailedstratigraphical and locality information can be found in Li et al.(2003, Fig. 1). The fossils described here are from the ore bed B(dolomitic phosphorite), which are associated with acritarchs,multicellular algae, chlorophytes, echinoderms, small shellyfossils, and tubular cnidarian-like fossils. The acritarchs, multi-cellular algae and chlorophytes have also been found in the orebed A. The geologic age of the Doushantuo phosphorites inWeng'an area has been dated in a range from 599 to 584 Ma(Barfod et al., 2002). A marine environment has been suggestedfor the deposition of the Doushantuo phosphorites (Zhou et al.,1998a; Xiao et al., 2000; Chen et al., 2006).
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Three-dimensionally preserved microfossils were etchedwith 8–10% solution of acetic acid for several days, and thenhand-picked from the insoluble residues under a binocular mi-croscope. Thin-sections were obtained through cutting of therocks collected from the ore bed B at random.
3. Description of fossils
The fossils are vase-shaped to tear-shaped in outline, anarrow tubular neck gradually expanding into a spheroid that ishollow inside (Figs. 1, 2A and C). An oral opening is present onthe top of the neck in some specimens (Figs. 1, 2B and C, and 4)but absent in some others (Fig. 3). The height (the neck plus thespheroidal body) of the known specimens ranges from 800 μmto 1400 μm. The size of the neck region varies greatly inspecimens, 100–300 μm in height and 200–400 in diameter.The spheroidal body is about 400–700 μm across. SEM and thethin-section observations indicate that the test wall is about 10–12 μm in thickness and has a multi-layered texture, withinwhich at least two to three layers can be recognized (Figs. 2Aand 4). The Electron Microprobe analysis on the surface ofspecimens Wa/19-1-10 (Fig. 3C) and Wa/19-1-11 (Fig. 1A)shows the preserved tests is mainly composed of calciumphosphate with minor silica. The thin-section examination, de-monstrates the test wall is preserved as a mineralized castcovered by an organic film, and the dark brown-colored internalmold may results from its organic-rich composition (Fig. 4). Noornaments are observed on the external surface of themicrofossils.
4. Discussion
By comparison with previously described VSMs from SouthChina, the microfossils with oral opening are remarkably simi-lar to those from both the Lower Cambrian KuanchuanpuFormation and the Ediacaran Gaojiashan Member of theDengying Formation in southern Shaanxi Province in the size,overall shape and the composition of the test wall, and thosewithout oral opening (Fig. 3) resemble Luxapertus wenganen-sis (Wu and Wang, 2004) from the same biota, which waserected based on thin-section studies. Here, we first illustratesuch fossils in three dimensions as supplement to this species.Therefore, our material can be attributed to two genera andthree species: Protolagena papillata (Zhang and Li, 1991)(Figs. 1, 4), Protolagena gaojiashanensis (Zhang and Li,1991) (Fig. 2), L. wenganensis (Wu and Wang, 2004) (Fig. 3).When compared to the VSMs from the UpperWalcott DolomiteNodules (Porter and Knoll, 2000; Porter et al., 2003), the VSMsfrom the Weng'an biota are much bigger in size (0.6–1.4 mm),thicker in test wall (10–12 μm), and simpler in aperturemorphology.
The test wall of the VSMs from the Weng'an biota is calciumphosphate in composition. However, it may not represent theprimary composition but is a result of the post-mortemdiagenesis (Bloeser et al., 1977; Zhang and Li, 1991; Zhang,1994; Yin et al., 2001c; Xue et al., 2002; Hua et al., 2003; Liu etal., 2005). In some specimens the wall is broken (Fig. 2A) but
the test retains its shape, indicating its rigid nature duringdeformation, which is supported by the absence of crushed orflattened specimens.
It is generally argued that the VSMs are affiliated withprotozoans (Knoll and Vidal, 1980; Duan, 1986; Duan and Cao,1989; Ding et al., 1992; Zhou et al., 1993, 2002; Yin and Gao,1999). The abundance of the VSMs in the Weng'an biotasuggests that protozoans advent marine ecosystem as early assome 40–55 million years before the Cambrian explosion.
5. Conclusions
The VSMs are first documented from the Weng'an biota.They are morphologically grouped into three taxa, with twogreatly similar to the VSMs previously described from theyounger deposits in South China and thus assigned to P.papillata and P. gaojiashanensis, respectively, and another oneidentical to L. wenganensis (Wu and Wang, 2004) from thesame biota. The fossils presented in this paper not only providenew knowledge on the Weng'an biota, but also add new in-formation to the Late Neoproterozoic VSMs.
Acknowledgements
This work is supported by the National Natural ScienceFoundation of China (40332016), the Major Project ofMinistry of Education of China (Grant 104176), Ministry ofScience and Technology of China (2006CB806401) and theChina–Japan cooperative project on the study of Cambrian/Precambrian events. We would like to express our sincerethanks to Prof. Yin C.-Y. (Beijing) for courtesy of a photo inFig. 2C. Many thanks are due to Dr. Zhang Zhifei (NorthwestUniversity, Xian) for the linguistic help. We also thank GaoFengliang, Wang Yaopei andWu Chunying for the help in fieldwork.
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