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Chongyu Yin, Institute of Geology, Chinese Academy of Geological Sciences (China)
Pengju Liu, Institute of Geology, Chinese Academy of Geological Sciences (China)
Feng Tang, Institute of Geology, Chinese Academy of Geological Sciences (China)
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The acanthomorph Tianzhushania is abundant and geographically widespread in South China, and its type species of Tianzhushania spinosa, an abundant acanthomorph species both in Yangtze Gorges and Weng'an area is characterized by a large vesicle covered with cylindrical processes (sometimes bundled) and multilamellar structures. In a careful thin sections study, Yin et al. (2004) discovered some Weng'an fossils have tubercular sculpture and on top of that cylindrical processes. They further argued that the dimples in Megasphaera ornata (Xiao et al., 2000) are the attach sites of the cylindrical processes. Because of the presence of cylindrical processes in both Tianzhushania spinosa and Megasphaera ornata, Yin et al. synonymized these two genera. Tianzhushania takes priority over Megasphaera, and thus Megasphaera ornata became Tianzhushania ornata. However, Tianzhushania ornata have not been found in Yangtze Gorges for a long time.
Here we report the new discovery of Tianzhushania ornata from the lower Doushantuo Formation in the Jiulongwan section at Sandouping town, Zigui County, Hubei Province. Unlike Weng'an area, both Tianzhushania spinosa and T. ornata were all found from cherty thin sections cut parallel with beddings. The latter first appears at the bed 16 (12 m above the base of the Doushantuo Formation.) simultaneous with the former, but individuals of T. ornata are obviously less than T. spinosa. In the bed 18 (24 m above the base of the Doushantuo Formation.), they become more abundant and more than 54 individuals can be found from a single thin section. According to a biostratigraphical investigation in Yangtze Gorges, two different silicified microfossil assemblages respectively appear in Member 2 and Member 3 of the Doushantuo Formation. Cherty nodules and bands in the Member 2 contain abundant and diverse acanthomorphic acritarchs, as well as multicellular algal thalli and macroscopic carbonaceous compressions like Chuaria have been found from black shale intercalations. The other silicified assemblage found in the Member 3 includes abundant Leiosphaeridia, as well as acanthomorphic acritarchs, but absent from Tianzhushania. The Member 2 assemblage in Yangtze Gorges can be taxonomically compared with the Weng'an biota found from the upper ore-beds of the Doushantuo Formation in Weng'an area (Yin et al., 2007).
The new discovery of Tianzhushania ornata in Yangtze Gorges provides new evidence for the correlation of the upper ore-beds in Weng'an area with the Member 2 of the Doushantuo Formation in Yangtze Gorges. The fossiliferous upper ore-beds in Weng'an area yields Pb-Pb dates of 599±4 Ma in the lower part and 574±14 Ma in the upper part (Barfod et al., 2002; Chen et al., 2004). It indicates that the age of the Member 2 assemblage in Yangtze Gorges should be older than 580 Ma, when the typical Ediacara biota begins to diversify elsewhere.
Supported by NNSFC (40672022), CGS (1212010511607) and MST (2006FY120300).
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