The Weng’an Biota (Doushantuo Formation): an Ediacaran window on soft-bodied and multicellular microorganisms

The Weng’an Biota is a fossil Konservat-Lagerstätte in South China that is c. 570 – 609 myr old and provides an unparalleled snapshot of marine life during the interval in whichmolecular clocks estimate that animal clades were diversifying. It yields fossils that are three-dimensionally preserved in calcium phosphate with cellular and sometimes subcellular fidelity. The biota includes candidates for the oldest animals in the fossil record, including embryonic, larval and adult forms. We argue that, although the Weng’an Biota includes forms that could be animals, none can currently be assigned to this group with confidence. Nonetheless, the biota offers a rare and valuable window on the evolution of multicellular and soft-bodied organisms in the prelude to the Cambrian radiation. Received 21 November 2016; revised 28 February 2017; accepted 1 March 2017 The origin and evolutionary assembly of animal body plans comprises one of the most formative episodes in the history of life. Animals are ecosystem engineers and their appearance fundamentally changed our planet’s ecology (Butterfield 2011a). Despite the importance of this evolutionary episode, many aspects of the timing and nature of the event remain poorly constrained. Molecular-clock analyses estimate that animals originated by the Cryogenian and diversified through the Ediacaran (Peterson & Butterfield 2005; Erwin et al. 2011; dos Reis et al. 2015), but fossil evidence of animals from before the Cambrian is controversial (Erwin et al. 2011; dos Reis et al. 2015; Cunningham et al. 2017). The Weng’an Biota is one of the few Lagerstätten from the critical interval in which early animals are expected according to molecularclock studies. In this Ediacaran fossil assemblage, organisms are phosphatized in cellular and even subcellular detail, providing a rare glimpse of soft-bodied and multicellular life at this time. Early research appeared to fulfil expectations of the presence ofmetazoans with reports of embryonic (Xiao et al. 1998), larval (Chen et al. 2000, 2002) and adult (Xiao et al. 2000; Chen et al. 2002, 2004; Yin et al. 2015) animals from the Weng’an deposit. However, subsequent analyses have cast doubt on this view, and there is currently much disagreement over these interpretations (Bailey et al. 2007a; Huldtgren et al. 2011; Bengtson et al. 2012; L. Chen et al. 2014, Xiao et al. 2014a). Here, we review the stratigraphic position, geological age and environmental setting of the deposit, and present an overview of the biota and an assessment of the phylogenetic affinities of the various taxa. Stratigraphy and age The Weng’an Biota occurs within the Ediacaran Doushantuo Formation (551 – 635 Ma, Condon et al. 2005) of south China (Fig. 1). In addition to the phosphatized microfossils fromWeng’an, this formation has yielded silicified microfossils (Yin et al. 2004) and macrofossils that are preserved as 2D carbonaceous compressions including macro-algae (Xiao et al. 2002) and the putative ctenophore Eoandromeda (Tang et al. 2008, 2011). The Weng’an Biota itself is known from localities in Weng’an County, Guizhou Province. The Doushantuo Formation overlies the Marinoan glacial tillites of the Cryogenian Nantuo Formation that can be dated to 635 Ma (Condon et al. 2005). It is overlain by the Ediacaran Dengying Formation, which contains fossils of the classical Ediacara macrobiota (Sun 1986; Xiao et al. 2005; Z. Chen et al. 2014). The base of the Dengying Formation can be dated to 551 Ma (Condon et al. 2005). In Weng’an, the Doushantuo Formation is composed of five units that have been described in detail by Xiao et al. (2014b) and Yin et al. (2015). The Weng’an Biota occurs mainly in Unit 4, the Upper Phosphorite Member, but also in Unit 5. Unit 4 is divided into 4A, a black phosphorite, and 4B, a grey dolomitic phosphorite (Dornbos et al. 2006; Xiao et al. 2014a,b; Yin et al. 2015). The age of the biota has been debated (Budd 2008; Erwin & Valentine 2013; Xiao et al. 2014a; Yin et al. 2015), with arguments focusing on the correlation of two karstic surfaces, one at the top of Unit 3 and the other within Unit 5 (for detailed discussion of Doushantuo correlation see Zhu et al. (2007), Zhu et al. (2013) and Yang et al. (2015)). If the lower surface is correlated to the c. 582 Ma Gaskiers glaciation (Condon et al. 2005) then the biota would be younger than 582 Ma. However, the lower surface may be older (Yin et al. 2015) and, if the upper karstic surface correlates to the Gaskiers glaciation (Xiao et al. 2014a), then the biota would be older than 582 Ma. Direct radiometric dates for Unit 4 at Weng’an have been inconclusive, giving Pb–Pb isochron ages of 572 ± 36 Ma for Unit 4A (Y. Chen et al. 2009) and 599 ± 4 Ma for Unit 4B (Barfod et al. 2002). However, a recent U–Pb date of 609 ± 5 Ma from a tuff immediately above Unit 4 at Zhancunping, in Hubei Province (Zhou et al. 2017), suggests that the Weng’an biota is probably older than 609 ± 5 Ma and probably older than the © 2017 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/3.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Review Focus Journal of the Geological Society Published Online First https://doi.org/10.1144/jgs2016-142 by guest on January 30, 2018 http://jgs.lyellcollection.org/ Downloaded from