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The Ediacaran Aspidella-Type Impressions in the Jinxian Successions of Liaoning Province, Northeastern China

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The Ediacaran Aspidella-Type Impressions in the Jinxian Successions of Liaoning Province, Northeastern China The Ediacaran Aspidella-type impressions in the Jinxian successions of Liaoning Province, northeastern China MAŁGORZATA MOCZYDŁOWSKA AND FANWEI MENG Moczydłowska, M. & Meng, F. 2016: The Ediacaran Aspidella-type impressions in the Jinxian successions of Liaoning Province, northeastern China. Lethaia, DOI: 10.1111/let.12173. Macroscopic impression fossils from the Xingmincun Formation of the Jinxian Group, Liaoning Province of northeastern China, are identified as members of the Aspidella plexus of Ediacaran age. This is the first recognition of the taxon in the Liaoning Province, although such fossils have been previously recorded in the succes- sion, but were referred to as new species and relegated to an earlier Neoproterozoic age. A revision of the taxonomic interpretation and relative age estimation of the pre- vious record is provided, as well as an evaluation of abiotic vs. biotic processes that could produce similar structures to studied impressions. The mode of preservation of the fossils is considered from a biochemical point of view and along with the proper- ties of organic matter in the integument of soft-bodied metazoans. The selective preservation of the Ediacaran organisms, including metazoans, as impressions (moulds and casts) against the organically preserved contemporaneous cyanobacterial and algal microfossils, and an exceptionally small number of terminal Ediacaran meta- zoan fossils (Sabellidites, Conotubus and Shaanxilithes), demonstrates the non-resistant characteristics and the very different biochemical constitution of the Ediacaran meta- zoans compared with those that evolved in the Cambrian and after. The refractory biomacromolecules in cell walls of photosynthesizing microbiota (bacterans, cutans, algaenan and sporopollenin groups) and in the chitinous body walls of Sabellidites contrast sharply with the labile biopolymers in Ediacaran metazoans known only from impressions. The newly emerging biosynthesis of resistant biopolymers in metazoans (chitin and collagen groups) initiated by the annelids at the end of Ediacaran and fully evolved in Cambrian metazoans, considered with the ability to biomineralize, made their body preservation possible. The Chengjiang and Burgess Shale metazoans show evidence of this new biochemistry in body walls and cuticles, and not only because of the specific taphonomic window that enhanced their preservation. □ Aspidella plexus, early metazoans, organic biochemistry, selective preservation, taphonomy. Małgorzata Moczydłowska [[email protected]], Department of Earth Sciences, Palaeobiology, Uppsala University, Villav€agen 16, SE 752 36 Uppsala, Sweden; Fanwei Meng [[email protected]], State Key Laboratory of Palaeontology and Stratigra- phy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China; manuscript received on 25/04/2015; manuscript accepted on 3/12/2015. Ediacaran macroscopic fossils are renowned for their the rationale used to describe their organic proper- preservation as impressions (moulds and casts), less ties. The exceptions are the organically preserved ter- frequently as carbonaceous compressions (contain- minal Ediacaran annelidan Sabellidites, whose robust ing organic biofilms) or traces, and exceptionally as wall is chitinous (Moczydłowska et al. 2014; M. organically preserved and biomineralized body fos- Moczydłowska, unpubl. data), and the carbonaceous sils (Gehling et al. 2000; Narbonne 2005; Fedonkin compressions of Conotubus, Shaanxilithes, Maw- et al. 2007; Hua et al. 2007; Cai et al. 2011; sonites, Eoandromeda and Khatyspytia of as-yet- Laflamme et al. 2012; Moczydłowska et al. 2014; unresolved biochemistry and affinities (Hua et al. Tarhan et al. 2015). Recognized in various preserva- 2007; Zhu et al. 2008; Grazhdankin 2014; Tarhan tion modes and ecological niches on shallow shelves et al. 2014). Among them, Eoandromeda was inter- vs. deep marine slopes, the fossils represent soft- preted as a ctenophore (Tang et al. 2011). This is in bodied organisms including metazoans (Zhu et al. sharp contrast to contemporaneous uni- and multi- 2008; Ivantsov 2009; Xiao & Laflamme 2009; Sper- cellular fossils of cyanobacterial and algal origin that ling & Vinther 2010; Narbonne 2011; Narbonne synthesized cell walls and trichome sheaths that are et al. 2014; Liu et al. 2015). They comprised body extremely resistant to biodegradation, diagenesis, walls made from non-resistant organic matter, a fact and laboratory acid treatment (Moczydłowska et al. verified by observations in all known sites and 1993; Xiao et al. 2002; Grey 2005; Moczydłowska modes of preservation with a few exceptions, and 2008). Macroscopic carbonaceous compressions of DOI 10.1111/let.12173 © 2016 The Authors. Lethaia published by John Wiley & Sons Ltd on behalf of Lethaia Foundation. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. 2 M. Moczydłowska & F. Meng LETHAIA 10.1111/let.12173 the Lantian and Miaohe biotas (early and late Edi- 2013), or even non-metazoan, microbial organisms acaran respectively) showing complex morphologies (Grazhdankin 2014; Liu et al. 2015). of fan-shaped and branching algal thalli and colonial Aspidella fossils have been recently noted in South cyanobacteria (Yuan et al. 1999, 2011; Xiao et al. China, in the Shibantan Member of the Dengying 2002) are also within the group of photosynthesizing Formation in the Yangtze Gorges area, Hubei Pro- organisms. Unnamed carbonaceous compressions vince (Meyer et al. 2013; Chen et al. 2014), although from the Ediacaran Khatyspyt Formation in Siberia not yet systematically described or compared with may be microbial colonies or seaweeds (Grazh- other modes of preservation. Specimens of a Sprig- dankin et al. 2008; Grazhdankin 2014). gia-type holdfast reported by Chen et al. (2014, fig. Biological affinities and phylogenetic relationships 3D) from the same area and stratigraphical interval of most Ediacaran taxa are not yet resolved, nor is of the Shibantan Member are considered here to be there consensus on whether certain taxa are meta- synonymous with Aspidella, as are those identified as zoans or of enigmatic affinities unknown among a possible Ediacaran-type fossil by Duda et al. (2014, modern phyla (Erwin et al. 2011; Narbonne 2011; fig. 2A). These, together with the present specimens Yuan et al. 2011). It is generally accepted that the from the Liaoning Province, are the current records Ediacaran biota comprised representatives of pori- of Aspidella in China. fera, ctenophorans, cnidarians, aplacophorans, and We report a new record of discoidal impressions stem- and crown-group bilaterian metazoans, identified as an Aspidella form-genus preserved in including molluscs (Narbonne 2005; Fedonkin et al. several taphonomic morphs consistently bearing the 2007; Ivantsov 2009; Xiao & Laflamme 2009; Vinther characteristic of the Aspidella plexus, which is pri- et al. 2012; Laflamme et al. 2012; Menon et al. 2013; marily the disc-shaped body with concentric rings Gehling et al. 2014; Narbonne et al. 2014; Yin et al. and a central boss (Gehling et al. 2000; Fedonkin 2015). Recognition of siboglinid affinity of Sabel- et al. 2007; Laflamme et al. 2011). This is the first lidites suggests the presence of the crown-group recognition of the Aspidella plexus in the Liaoning Annelida (Moczydłowska et al. 2014; but see also Province of northeastern China and includes revised Parry et al. 2014). Alternative interpretations that taxa previously assigned to junior synonyms by certain taxa are fungi (Peterson et al. 2003), terres- Zhang et al. (2006). The age of strata containing the trial lichens and slime moulds (Retallack 2012, assemblage is estimated to be Ediacaran at c. 580– 2013), or microbial colonies (Grazhdankin et al. 550 Ma based on the global chronostratigraphical 2008; Grazhdankin 2014) have not received wide- range of such fossils (Grazhdankin 2014). spread support (Menon et al. 2013; Xiao et al. 2013; The assemblage studied here should not be con- Chen et al. 2014). fused with structures recorded approximately The disc-like, flat- to high-relief impressions pre- 1600 m below in the underlying Changlingzi Forma- served on bedding planes are the predominant fos- tion (Figs 1 and 2) that originally were attributed to sils within the Ediacaran assemblages in siliciclastic the Cyclomedusa fossils (Xing & Liu 1979) and sub- successions worldwide (Fedonkin et al. 2007; sequently interpreted as pseudo-fossils and repre- Laflamme et al. 2013), but they also occur in car- senting sedimentary, gas escape structures (Sun bonate facies (Grazhdankin et al. 2008; Chen et al. 1986; see below). 2014). Originally described as morphologically dis- parate genera and species, they have been substan- tially reduced as a result of depositional and taphonomic analyses that have eliminated superflu- Geological setting and age of ous synonyms (Gehling et al. 2000; Narbonne 2005; succession Fedonkin et al. 2007). The most common discoidal impression is the Aspidella plexus, which comprises The collection of fossils derives from clayey shale in a great variety of morphs, and its type species is A. the middle member of the Xingmincun Formation, terranovica Billings, 1872. It has been recognized as Jinxian Group, at the Yangjue and Yangtun localities a buried holdfast
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