In the course of studies on Devonian radiolarian faunas from Nanning, Guangxi, SW China (Luo et al., 1997), numerous smooth siliceous microspheres (Fig. 1) are encountered together with abundant spherical entactiniid radiolarians. The present investigation reveals that these microspheres are siliceous casts formed through precipitation of silica between the medullary and cortical shells of entactiniid radiolarians. As these microspheres are commonly the most abundant particles encountered in some Devonian samples, an understanding of their origins is desirable.
Examined material is from two bedded chert samples collected from the upper part of the Tanhe Formation at Wuxiangling, 5 km SE of Nanning, Guangxi Province, SW China. Tanhe Formation consists of thin dark grey, brown grey cherty rock and cherty mudstone containing abundant radiolarians (entactiniids, ceratoikiscids and palaeoscenidiids) and tentaculitids. Based on zonation of tentaculitid faunas this formation is Lower to Middle Devonian (Upper Emsian – Lower Givetian) (Zhong et al., 1992; Kuang et al., 1996). A standard method (Pessagno & Newport, 1972) for extraction of radiolarians from cherts using hydroflouric acid was applied.
Free microspheres are the most common individuals (Pl. 1, figs 1, 8). All have smooth surfaces and possess some relatively larger holes or pores. Average microsphere diameter (20 specimens) is 92 μm. Hole shapes vary from three-bladed to round with spines rarely seen penetrating the microspheres (Pl. 1, figs 7, 10). A full range of material from microsphere-bearing entactiniids in which there are remnants of the original spongy radiolarian shells (Pl. 1, figs 5–7, 9) to solitary microspheres . . .
Funding towards this research was provided by the Hong Kong Research Grants Council. We thank the two journal reviewers for their constructive comments.
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