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Journal of Paleontology 92(3):323–335


Canada), the Yangtze Gorges area of South China, and the Ediacara Member of the Flinders Ranges of South Australia. All known Arboreomorpha are benthic epifaunal fronds that effectively partitioned the water column on a macroscopic level, resulting in complex tiered Ediacaran ecosystems (Clapham and Narbonne, 2002; Ghisalberti et al., 2014). The limited taphonomic windows offered by most


Ediacaran localities preserve impressions of two-dimensional casts and molds in medium to coarse sandstones (Narbonne, 2005; Kenchington and Wilby, 2014), limiting morphological information to external structures. Taphonomic studies distin- guishing between genuine structures and morphological “mistakes” resulting from bending, folding, or overlapping relationships have been essential in forwarding taxonomic and phylogenetic research (Laflamme et al., 2007; Liu et al., 2011; Brasier et al., 2013; Matthews et al., 2017). Rare glimpses into three-dimensional morphology are offered by exceptional sites in Namibia (Vickers-Rich et al., 2013; Ivantsov et al., 2016), Newfoundland (Narbonne, 2004; Narbonne et al., 2009), and the White Sea (Grazhdankin, 2014; Ivantsov, 2016). The discovery of several exquisitely preserved, three-


dimensional examples of the classic Ediacaran frond Arborea (previously Charniodiscus, see systematic taxonomy) from the Ediacara Member, Rawnsley Quartzite of South Australia, allows for the reevaluation of this genus and adds insight into the internal construction and likely external morphology of this frond. Preservation of inferred internal anatomy, combined with detailed preservation of the branching architecture, also helps refine the classification of Arboreomorpha.


Geological setting and stratigraphy


The Ediacaran assemblages found within the Flinders Ranges of South Australia contain some of the highest diversity of Ediacara biota (Droser et al., 2006; Droser and Gehling, 2015), the broadest range of occupied ecological niches (Bambach


offer the greatest abundance of specimens (Gehling and Droser, 2013); however, the mass-flow sands facies has been responsible for exquisite, three-dimensional preservation of large Arborea fronds allowing for unprecedented levels of morphological detail. For our purposes, 3D preservation is dis- tinguished from typical Ediacaran two-dimensional preserva- tion as representing moldic sand infills that can be isolated and removed from external molds in the surrounding matrix.


Taphonomy


The influence of taphonomy on Ediacaran taxonomy is profound. Although emerging taphonomic studies demonstrate a common reliance on clays and bacterially mediated pre- cipitation of iron sulfides associated with Ediacaran preserva- tion (Gehling, 1999; Laflamme et al., 2011, Darroch et al., 2012; Schiffbauer et al., 2014; Liu, 2016), Ediacaran-type preserva- tion spans a remarkably large lithological spectrum, resulting in an equally expansive range of taphomorphs (Grazhdankin et al., 2008; Xiao et al., 2013; Kenchington and Wilby, 2014). Distinct preservational styles (of Narbonne, 2005) are typically limited to single localities, representing an intimate association between sedimentary facies, depositional context, and ambient water energy irrespective of the qualities of biological tissue constructing these organisms. Preservation of the soft-bodied Ediacara biota from South


et al., 2007; Bush et al., 2011; Laflamme et al., 2013), and the first probable examples of stem-group Bilateria anywhere in the world (Fedonkin and Waggoner, 1997). Ediacaran fossils in South Australia are typically restricted to the Ediacara Member of the Rawnsley Quartzite (Gehling, 2000; Gehling and Droser, 2013) in the upper portion of the Pound Subgroup, Wilpena Group, which overlies the global Marinoan glacial tillite that marks the base of the Ediacaran Period (Knoll et al., 2006). Volcanic ash layers are unknown from the Flinders Ranges; however, paleobiological studies have linked the Flinders Ranges with the White Sea assemblage of Russia (Waggoner, 2003; Boag et al., 2016), which has been radiometrically dated at 555 Ma (Martin et al., 2000). For over a decade, diligent excavations of complete Edia-


caran surfaces from the National Heritage Listed Ediacara Fossil Site at Nilpena (herein referred to as Nilpena) in the Flinders Ranges of South Australia (see reviews in Droser et al., 2006; Gehling and Droser, 2013; Droser and Gehling, 2015) have uncovered a diverse assemblage of Ediacara biota occupying specific ecological niches and representative sedimentary facies including shoreface sands, wave-base sands, delta-front sands, sheet-flow sands, and mass-flow sands. Although Arborea is known from all these facies, wave-base and sheet-flow sands


Australia results from the intimate relationship between the organisms and expanses of seafloor microbial mats (Gehling, 1999; Gehling et al., 2005; Gehling and Droser, 2009; Darroch et al., 2012). The ‘death mask’ model (Gehling, 1999) predicts that after burial by storm deposits, new colonies of microbial mats restricted the transport of oxygenated pore waters into the underlying sands. In the presence of decaying organic material (i.e., buried microbial mats and Ediacara biota), sulfate trapped in the pore waters was reduced through the metabolic activities of sulfur-reducing bacteria, resulting in pyritic coatings that form the basis of Flinders-type preservation (Narbonne, 2005). Evidence such as pyritized bacterial mats (Gehling et al.,


2005), ‘old elephant skin’ (Gehling, 1999), preserved microbial filaments (Gehling et al., 2005; Callow and Brasier, 2009), and microbially induced sedimentary structures (Gehling and Droser, 2009) on the beds containing fossils all suggest a direct link between bacterial mats and Ediacaran preservation. Recently, it has been suggested that the relatively higher silica saturation state of Neoproterozoic oceans prior to the advent of abundant silica biomineralizers aided in rapid sediment lithification via early-stage precipitation of silica cements (Tarhan et al., 2016). New discoveries of exquisitely preserved Ediacaran fronds


from Nilpena Farm in South Australia exhibit three-dimensional casting of soft-bodied Ediacaran organisms within fine-grained event beds resulting from storm or mass-flow events. The three-dimensional casting has been essential in differentiating multiple petaloids in multifoliate fronds such as Rangea and Swartpuntia (Narbonne et al., 1997; Vickers-Rich et al., 2013) and typically results in finer-detailed preservation (Narbonne, 2004; Narbonne et al., 2009). It was proposed by Dzik (1999) and Narbonne (2005) that Nama-style preservation may allow for the casting of internal rather than external features, therefore


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