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1 Introduction The Cambrian through Early Ordovician was a lengthy interval when microbialites once again dominated after their decline from a peak in the Middle Proterozoic (e.g., Riding 2006a). These microbialites declined again in the Middle–Late Ordovician, although they remained locally common (e.g., Webby 2002; Shapiro 2004; Riding 2006a). The Cambrian to Early Ordovician microbialites exhibit the following characteristics: (1) abundant stromatolites, although their diversity in growth form is limited compared with typical Proterozoic stromatolites; in fact, the distinctive coniform and microdigitate forms are absent, and branched and columnar stromatolites are scarce (Awramik 1971; Riding 2000); (2) in contrast, thrombolites and dendrolites became established and diversified (Riding 2000; Shapiro 2004); and (3) calcimicrobes (e.g., Girvanella, Renalcis, and Epiphyton) were also distinct and became diversified during this interval (e.g., Riding 2000, 2006b; Shapiro 2004). The Proterozoic stromatolites, with the exception of silicified stromatolites, rarely preserve tubular or vesicular microfossils (e.g., Riding and Sharma 1998; Riding 2000), whereas the Cambrian to Early Ordovician stromatolites include calcimicrobial traces such as Girvanella (e.g., Toomey and Nitecki 1979; Can˜as and Carrera 1993; Dang et al. 2009). Previous studies of Cambrian to Early Ordovician stromatolites have examined depositional environments, environmental controls on stromatolite growth form, and morphostratigraphy (e.g., Pratt and James 1982; de Freitas and Mayr 1995;

N. Adachi (*), J. Liu, and J. Cao School of Earth and Space Sciences, Peking University, Haidian, Beijing 100871, PR China e-mail: [email protected] Y. Ezaki Department of Geosciences, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan

J. Reitner et al., Advances in Stromatolite Geobiology, Lecture Notes in Earth Sciences 131, DOI 10.1007/978-3-642-10415-2_28, # Springer-Verlag Berlin Heidelberg 2011

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Fig. 1 Index map showing the Jianxin and Majialing sections at Anhui Province, South China. The inset map shows an Early to Middle Ordovician palaeogeographic reconstruction [modified from Zhang and Jin (2007)]. The star symbol shows the location of the larger map

Shapiro and Awramik 2000); however, little is known of their microstructures and the responsible calcimicrobes (e.g., Girvanella). Stromatolite-bearing reefs associated with well-preserved calcimicrobes and metazoan (e.g., lithistid sponges, receptaculitids) can be found in the Lower Ordovician Hunghuayuan Formation (late Tremadocian–early Floian) in Anhui Province, South China (Fig. 1). Although these stromatolites show restricted morphological diversity and are small in size compared with typical Proterozoic stromatolites, the presence of abundant calcimicrobial traces provides clues to their construction. The Early Ordovician is also a particularly intriguing time, when a transition in reef type occurred between microbial- and metazoan-dominated re