Megascopic Symmetrical Metazoans from the Ediacaran Doushantuo Formation in the Northeastern Guizhou, South China

INTRODUCTION

The emergence of metazoans, especially of the bilateral symmetrical metazoan, is an important even in the evolutionary history of the life.Since the"Snowball Earth" (the Nantuo Glacial Epoch in China), the climate warming and the temperature rising became the necessary foundation for the evolvements and developments of organisms.The multicellular biotas from the Neoproterozoic Ediacaran Doushantuo Formation in South China including the Weng'an biota from southern Guizhou (Li et al., 1998; Xiao et al., 1998; Yuan et al., 1993Zhang and Yuan, 1992; Zhang, 1989), the Miaohe biota from western Hubei (Xiao et al., 2002; Ding e al., 1996; Chen et al., 1994a; Chen and Xiao, 1992, 1991; Zhu and Chen, 1984), the Lantian flora from southern Anhui (Tang et al., 1997; Chen et al., 1994bBi et al., 1987), and the Wenghui biota from northeastern Guizhou (Wang et al., 2007a, 2005; Zhao et al., 2004) are all implying that the macroscopic organisms, since the"Snowball Earth", might have distributed widely in the Yangtze Ocean (Wang et al.2007a, 2005; Tang et al., 2006; Wang and Wang2006).Although some megascopic metazoans found in the Miaohe biota (Ding et al., 1996; Chen et al.1994a; Chen and Xiao, 1992, 1991) have not been accepted because of the lack of some obvious structures and exoskeleton of metazoan (Xiao et al.2002; Yuan et al., 2002; Chen et al., 1994a), the researching on the Doushantuo Formation in northeastern Guizhou is characterized by metazoans because of the discovery of some macrofossils (carbonaceous compression) : the worm-like animal, characterized by annelid in morphology, with homologous metameres, parapodia, a possible tentacle, a line-like alimentary canal through the centre of body, and a fine dorsal vessel; the triradiate discoidal fossils with tree lobes attributed to Trilobozoa Fedonkin; and an octaradiate discoidal fossil with a central area on the centre of the disc and feather-like cilia on eight dextral-spinning radiate plates, which might be Ctenophora.

GEOLOGICAL AND ENVIRONMENTAL SETTINGS

In the section near Wenghui Village (26°50′07″N, 109°01′20″E), Jiangkou County, Guizhou Province, China, the Doushantuo Formation is about 70 m thick, which rests conformably above the tillites of the Nantuo Formation as well as below the siliceous stone of the Liuchapo Formation (Fig.1).The lower Doushantuo Formation consists mainly of ctenoid dolomite and medium-to thick-bedded dolomite (cap carbonate).The middle part is mainly composed of dolomite, muddy dolomite, and carbonaceous shale.The upper part is characterized by black carbonaceous shale, which yields abundant and diverse compression well-preserved fossils, including Anomalophyton, Baculiphyca, Beltanelliformis, Chuaria, Cucullus, Cyclomedusa, Doushantuophyton, Eoandromeda, Enteromorphites, Flabellophyton, Gesinella, Globusphyton, Jiangkouphyton, Linbotulichnus, Liulingjitaenia, Longifuniculum, Protoconites, Sectoralga, Wenghuiphyton, and Zhongbaodaophyton (Wang et al., 2007a, b, 2005; Wang and Wang, 2006; Zhao et al., 2004).These biota (the Wenghui biota), which were buried in situ, lived in a water-quiet shallow sea with light or even poor-oxygen conditions (Wang and Wang, 2006; Wang et al., 2005).On the basis of bio-stratigraphic and chemo-stratigraphic correlations with well-dated Neoproterozic sections located throughout the world, the depositional age of the Doushantuo Formation in Yangtze area is estimated to lie between 550 and 600 Ma (Knoll and Xiao, 1999).More recently, the U-Pb zircon ages of an ash bed above the Doushantuo Formation cap carbonate and near the boundary of the Doushantuo Formation as well as the Dengying Formation (the topmost Miaohe biota) in western Hubei yielded ages of (621±7) Ma and (555.2±6.1) Ma, respectively (Zhang et al., 2005).In addition, the overlying Liuchapo Formation in northeastern Guizhou is correlated with the Dengying Formation in western Hubei (Wang et al., 1987; Qin et al., 1984).

Figure  1.  Sketch maps showing location and horizons (c) of megascopic metazoan fossil occurrence, Wenghui Village (b), Jiangkou County, Guizhou Province (a), China.1.Siliceous stone; 2.carbonaceous mudstone; 3.muddy dolomite; 4.dolomite; 5.ctenoid dolomite; 6.tillite; 7.location and horizon of metazoan fossils.

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SYMMETRICAL METAZOANS

Bilateral Symmetry

The worm-like fossil with elongated body, from the Doushantuo Formation of northeastern Guizhou, is characterized by annelid in modern taxology in morphology.It is composed of many homologous metameres (Figs.2a–2g).A circular oral cavity is located on the first segment (Fig.2b), and a small anus is at the end of the body (Fig.2f).A curved carbonaceous film, elongated and stick-like, shoots from the dorsa of the third segment, and diminishes gradually from the base distally (Fig.2b).But it cannot be distinguishable between a tentacle of the body and a macroalgal fragment piled up.Its eleventh or twelfth segment being longer than other metameres, which looks like a ring, might be a clitellum, a function of procreation (Figs.2b, 2g).On the front segments of the body, from the rear of the second segment to the front of the clitellum, the inter-segment furrows are so deep and wide that the skin protuberates to form parapodia (Figs.2b, 2c, and 2g).On the surface of the body, there are three different setae: the metameric seta, which is straight or substraight needle-shaped, on all segments irregularly (Figs.2b, 2g); the twin parapodium seta, which is straight spine-like on every parapodium (Fig.2c); and the anal seta, which is of a triangle-form, around the anus (Fig.2f).A line-like alimentary canal links the mouth to the anus and passes through the centre of body evidently (Figs.2a, 2b, and 2d–2g).Above the alimentary canal, the dorsal vessel is fine, elongated and curved (Fig.2b).The worm-like fossil, in morphology, shows the phenomenon of heteronomous metamerism.The front segments on the body are more evident than the rear part.The clitellum, which migh be a function of procreation, is located in the fore-middle part of the body.The parapodia with the parapodium seta might be a function of movement and the possible tentacle might be a sense organ.So the worm-like animal would be able to raise its forepart to sense the surroundings and could sinuously move or crawl on the depositional surface.In addition the digestive and vascular system, one of the primary characteristics of annelid in modern taxology, could enhance the assimilation and transportation of nutritious matters, and could promote the metabolism of the animal body.

Figure  2.  Photographs showing the worm-like fossils from the Doushantuo Formation of the Neoproterozoic Ediacaran in northeastern Guizhou.Ac.Alimentary canal; An.anus; As.anal seta; Cl.clitellum; Dv.dorsal vessel; Ms.metameric seta; Oc.oral cavity; Pa.parapodium; Ph.Pharynx; Ps.parapodium seta; Te possible tentacle.Entire specimen (a), (e), (g); structures of the body forepart (b); details of front abdomen on clitellum (c); the rear part of the body (d); detail of body's end (f).

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Radiate Symmetry

The triradiate discoidal macrofossils from the Doushantuo Formation consist of a circular plane and three radial lobes although they are of different shapes and sizes (Figs.3a–3c).A large discoidal fossil with a star-triradiate symmetry (Fig.3a) is similar to Albumares brunsae Fedonkin, and Anfesta stankovskii Fedonkin from Vendian in White Sea, Russia (Fedonkin, 1984, 1976), but it is a carbonaceous compression and is larger than the latter in the diameter of the plane.Its lobes, elongated subtriangle-shaped, preserved in the form of carbonaceous compresses, radiate from the center of the plane, whereas the point of the lobe is towards the edge of the plane.Differing from the former, a smal discoidal fossil bears three fan-shaped lobes on the plane (Fig.3b).Similar to Tribrachidium heraldicum Glaessner from Ediacara, South Australia (Glaessner1959), the fan-shaped lobes radiate from the centre of the plane.Other small discoidal fossils have also three radial lobes, but the pattern of the triradiate symmetry as a triangle, is constituted by three oval lobes around the central of the plane but with no radiation from the central of the plane (Fig.3c).These discoida macrofossils with a triradiate symmetry could attribute to metazoan Triloboza Fedonkin (Fedonkin, 1985).

Figure  3.  Photographs showing the discoidal fossils from the Doushantuo Formation of the Neoproterozoic Ediacaran in northeastern Guizhou.The triradiate discoidal fossils (a–c); the octaradiate discoidal fossils, Eoandromeda octobrachiata Tang et al. (d–i).

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Another macrofossil, an octaradiate discoidal fossil with dextral-spinning radiate plates, from the Neoproterozoic Doushantuo Formation, is characterized by metazoan but different from Triloboza (Figs.3d–3i).On the disc, the eight radiate plates constitute a circular or subcircular central area in the center of the disc and then extend dextrally to the margin of the disc and disappear in the margin of the disc (Figs.3d, 3h, and 3i).The spinning radiate plates cause the wave-shaped margin of the disc (Figs.3d, 3f, 3h, and 3i).Between both radiate plates, a smooth and shallow groove connects the central area to the margin of the disc and is commonly changeless in width (Figs.3d–3i).In addition, a central ridge preserved in the form of carbonized compression covers the centre of every radiate plate (Figs.3d–3g).Many feather-like cilia, narrow band-shaped, vertical to or subvertical to the central ridge, arrange regularly on the radiate plates and pass over the central ridge (Figs.3e–3g).The octaradiate discoidal fossil was reported to be found in only two specimens (published one specimen) in the Miaohe biota from western Hubei, South China, and named Eilscaptichnus ichnofossil-kind, but no generic name and specific name, no type species and holotype (Li and Ding, 1996).Tang et al. (2008)collected three specimens of the octoradiate organism (published two specimens) from the upper Doushantuo Formation of northeastern Guizhou, and named as Eoandromeda octobrachiata.Tang et al. (2008)considered that the extant group of Eoandromeda is unknown in taxonomy.Eoandromeda is abundant from the Doushantuo Formation in northeastern Guizhou and most of them are well preserved.The end-parts of the feather-like cilia on the radiate plates and central ridges, in various specimens, incline to various directions (Figs.3e–3g), therefore, the band-shaped feather-like cilium could be interpreted as lamella that could be formed by soft tissues and could quiver ceaselessly toward the long axes of radiate plate to produce a current.The eight currents produced from the dextral radiate plate gather to cause a rotational flow looking for foods or moving its body up and down in seawater.In addition, the circular or subcircular central area in the centre of the disc may be the statocyst of ctenophores in morphology.Therefore, Eoandromeda with an octaradiate symmetry could be Ctenophora.

Discussion

The emergence of the symmetrical metazoans could relate with the abundance and diversity of macroscopic metaphytes.In the Chinese Ediacaran biotas including the Wenghui biota (Wang et al., 2007a, b, 2005; Zhao et al., 2004), the Miaohe biota (Xiao et al., 2002; Ding et al., 1996; Chen et al., 1994a; Chen and Xiao, 1992, 1991; Zhu and Chen, 1984), and the Lantian biota (Tang et al., 1997; Chen et al., 1994b; Bi et al., 1987), there are numerous macroscopic algae that lived erect on the sea-floor (Wang et al., 2007a, 2005; Wang and Wang, 2006; Chen et al., 1994a), and their appearance broke the reign of micro-organisms lying on the depositional surface.Therefore, the erecting macroalgae multiplied advances the photosynthetic capacity and oxygen release to increase the oxygen contents in the seawater (Wang et al., 2007a, 2005; Wang and Wang, 2006; Chen et al., 2000).Accordingly, the environment with oxygen provided the essential conditions for the life and propagation of macroscopic metazoans that depended on oxygen.

The symmetrical metazoans from the Doushantuo Formation in northeastern Guizhou could not be the most original type because of their complex structures.In addition, the macroscopic algae, a primary producer in the ecosystem, began feeding animals and changing their taste.The increased oxygen and these new food resources established a new environment and ecological chain to accelerate the evolution and development of metazoan.

CONCLUSION

The emergence of the symmetrical metazoans might relate with the abundance and diversity of macroalgae and the increase of oxygen contents in the seawater.The worm-like fossils and discoidal fossils from northeastern Guizhou indicate that the symmetrical metazoans have appeared in the Neoproterozoic Ediacaran, but they are not the most original type.Especially, the worm-like animal with complex structures, characterized by modern taxological annelid, implies that the triploblastic and bilaterally symmetrical animal originated at least before 550 Ma or earlier.