Ediacara-Type Fossils from Lower Cambrian Strata in Qingzhen County, Guizhou Province, South China

INTRODUCTION

The Ediacara fauna, generally believed to contain the oldest metazoan fossils, has been described from about 25 localities around the world (Hofmann et al., 1990). It was commonly considered that this fauna was restricted to Late Neoproterozoic (Glaessner, 1984). These were suggested to be mostly nonlocomotory quasi-autotrophs that were wiped out during the mass extinction at the Neoproterozoic– Cambrian boundary (Seilacher, 1984). The oldest Ediacara-type body fossils were discovered below Proterozoic glacial deposits (Hofmann et al., 1990), but mainly restricted to Late Neoproterozoic (Glaessner, 1984). However, some elements of this fauna were continually discovered in Cambrian strata (Hagadom and Waggoner, 2000; Hagadom et al., 2000; Jensen et al., 1998; Crimes et al., 1995; Narbonne et al., 1991).

In Qingzhen County, Guizhou Province, South China, there is a well exposed stratigraphic succession spanning the Neoproterozoic–Cambrian boundary. Here, an Ediacara fauna, including Tirasiana qingzhensis and Cyclomedusa sp., is described from a horizon of Lower Cambrian Taozichong Formation, about 15 m above Sinian Dengying Formation (Neoproterozoic). These Ediacara-type fossils described below from Qingzhen County can be considered as the first one discovered in Cambrian strata in South China.

The Ediacara-bearing horizon is featured by fine-grained sandstones and siltstones. This horizon is Early Cambrian, constrained by an underlying occurrence of Anabarites-Protohertzina Zone, and by an overlying occurrence of lowest trilobite Zunyidiscus sp..

DESCRIPTION OF FOSSILS

The fossil specimens are preserved as the very simple convex discoid and annulation relieved on the lower surfaces of thin sandstone and siltstones, and siliceous rock beds. Two types of Ediacara-type fossils can be recognized.

The first type (Figs. 1A–1E) was commonly present at the base of the upper member of Taozichong Formation. Most fossil specimens were present in a cape-shaped form (Figs. 1B, 1C, 1D). A nipple-like protuberance was found in the center with 1.0–2.0 mm in diameter and 1.0 mm in height. Six ridges surround the nipple-like protuberance, and the ridges become slightly wide outwards (Figs. 1A, 1B). Fossils were present in a relatively flat depression, and cross sections show the structures extend only a fraction of a millimeter into the sediment. Fossil specimens are closely overlapped in some sites (Figs. 1C, 1E).

Figure  1.  Ediacara-type fossils in Taozichong Formation, Guizhou Province, South China. (A)–(E). Tirasiana qingzhenensis; (F) Cyclomedusa sp.. Scale bars represent 10 mm.

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The second type of Ediacara-type fossil (Fig. 1E) is represented by a single fossil specimen found in the siltstone bed, a horizon 4 m above the bed bearing the first type of Ediacara-type fossils. The fossil comprises a bipartite circular impression, which looks like a disc with 35 mm in diameter and 0.5 mm in relief. It exhibits a central tubercle 4 mm in diameter, and a complete but irregular annular low raised ridge 1–2 mm wide, and an inner disc 10–13 mm in diameter surrounded by a raised rim about 1 mm wide. The distance of the outer rim to the inner rim is 3–7 mm. No radial structures were found in association with the rims.

DISCUSSION

The biogenicity of the disc fossils found in Neoproterozoic–Cambrian in the world needs to be addressed. The disc fossils from Neoproterozoic– Cambrian were present in an impression, and show some similarity with the gas-evasion marks in Devonian rocks in New York (Cloud, 1960). Hofmann et al. (1990) considered that the disc fossils are not pseudofossils formed by gas evasion. The gas cave construction is featured by a central hole, differing from the disc fossils from Neoproterozoic–Cambrian. The disc fossils show a remarkable variety not only in size but also in morphology. The tremendous variations in both size and morphology are found in Cyclomedusa, Ediacaria, Tirasiana and Nimbia. The large variations between species and even between genera led to the suggestions that these discs are the different facets preserved for the same fauna, and many taxa proposed are actually synonymous (Jenkins, 1992; Sun, 1986). Most of these discs seem to have been preserved by being impressed on the underlying bed. The morphological variability of the disc fossils is very obvious; this phenomenon was described by Sun (1986), Jenkins (1992), and Crimes and Mcilroy (1999).

The fossil specimens found in Qingzhen Cambrian rocks were present in a more clearly biomorphological character than those in Canada, Namibia, Australia, Russian platform, strongly supporting a biological origin.

CONCLUSIONS

The Cambrian Taozichong Formation contains the fossil specimens attributed to Tirasiana qingzhenensis and Cyclomedusa sp.. Although such fossils were reported from the Vendian of Eastern Europe (Martin et al., 2000), and the Cambrian of North Norway (Crimes and Mcilroy, 1999), northern Sweden (Jensen et al., 1998; Føyn and Glaessner, 1979), Newfoundland (Narbonne and Aithen, 1995), Namibia (Glaessner and Wade, 1966) and South Australia (Jensen et al., 1998; Fedonkin and Waggoner, 1997; Glaessner, 1984), the exact biostratigraphic correlation is not implied and their stratigraphic range may be longer than previously thought. In Guizhou Province, South China, these Ediacara-type fossils occur in Lower Cambrian, indicating that at least some Ediacaran taxa may have had direct Phanerozoic descendants with some going extinct in the terminal Neoproterozoic mass extinction event. Additionally, this finding indicates that organisms diversify prior to the well-known "Cambrian explosion", and some animals from Neoproterozoic to Cambrian may be different only in the skeletal form.