EDIACARAN DISTRIBUTIONS IN SPACE AND TIME
(579Ma; Van Kranendonk et al. 2008) and remain globally distributed until their abrupt disappearance at the Proterozoic–Cambrian boundary (~541Ma; Narbonne et al. 1997; but see Jensen et al. 1998; Hagadorn et al. 2000). Although the phylogenetic affinities of many of these taxa are contentious (e.g., Glaessner 1984; Gehling 1991; Seilacher, 1992; Seilacher et al. 2003; Budd and Jensen 2015), emerging paleon- tological studies suggest they represent an amalgamof stem- and crown-group metazoans in addition to disparate, higher-order eukaryo- tic clades with no modern representatives (Xiao and Laflamme 2009; Brasier et al. 2012; Laflamme et al. 2013; Rahman et al. 2015). Recent molecular clock analyses of early animal divergence estimate the origins ofdemosponge, cnidarian, and bilaterian crown groups deep within the Cryogenian (Peterson et al. 2008; Erwin et al. 2011), suggesting that the Ediacara biota, irrespective of their uncertain metazoan affinities, were contemporaneous with early metazoans (Fedonkin et al. 2007b) and their associated innovations of skeletonization, predation, and bioturbation (Grotzinger et al. 2000; Hua et al. 2003; Liu et al. 2010; Carbone and Narbonne 2014; Darroch et al. 2015). As such, their inclusion within a holistic macroevolutionary and ecological framework is critical to study the development of early animal life. The challenges of conducting evolutionary
studies and identifying associated selective pressures (e.g., ecological, evolutionary, environmental, and subsequent physiological changes) on Ediacaran organisms that lack systematic agreement have long been recognized. In light of this problem, Waggoner (1999, 2003) resolved to test hypotheses that sought to explain the distribution of Ediacaran macrofossils in stratigraphic space and time as a function of biogeographic, ecological, and paleoenvironmental factors. Using a modified cladogram to perform parsimony analysis of endemism (wherein localities replaced taxa and presence/absence of taxa replaced characters), three statistically distinct biotic “assemblages,” termed the Avalon, White Sea, and Nama, occurring in loosely ascending stratigraphic order, were identified. The Avalonian assemblage (579–559Ma) includes
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the oldest Ediacaran communities, consisting of Rangeomorph and Arboreomorph taxa in deep-water marginal slope and basinal facies (Darroch et al. 2013; Liu et al. 2015). These assemblages are often preserved under ash beds in “Conception style” preservation (Narbonne 2005; Kenchington and Wilby 2014) or on the soles of or intrastratally within contourites and turbidites in deep-water sandstone packages (Narbonne et al. 2009, 2014). The White Sea biota (558–550Ma) represents a diverse grouping dominated by Bilateralomorph, Dickinsoniomorph, and Kim- berellomorph taxa typically preserved in shal- lower prodeltaic shelf settings with pervasive microbial mats, creating “Flinders style” cast- and-mold preservation (Martin et al. 2000; Narbonne 2005; Gehling and Droser 2013; Zakrevskaya 2014). The youngest assemblage, the Nama (549–541 Ma), largely consists of depauperate communities of Erniettomorph and Rangeomorph taxa in shallow shelf– shoreface settings, often preserved as three- dimensional molds preserved within beds of storm-deposited sand and channel-fill deposits (Narbonne et al. 1997; Narbonne 2005; Vickers-Rich et al. 2013; Ivantsov et al. 2015; Darroch et al. 2015). The Nama assemblage is also unique in hosting the earliest skeletonizing macrofauna, including the globally distributed Cloudina (Warren et al. 2011), in addition to unresolved tubular taxa (e.g., Carbone et al. 2015). Tectonic (paleogeography), paleoenviron-
ment (lithology and bathymetry), and temporal (evolutionary succession) factorswere identified by Waggoner (2003) as underlying controls on the known Ediacaran macrofossil record. However, due to a sparse data set (21 localities, 70 genera) and limited geochronological data, the relative impacts of these factors could not be quantitatively defined. In the subsequent decades, dedicated paleontological work has aimed to identify the controlling factors of these assemblages at an outcrop scale. Recent studies suggest these core assemblages may represent discrete temporal intervals (i.e., evolutionary stages; Xiao and Laflamme 2009; Grazhdankin 2014), environmental partitioning (i.e., an eco- logical response to bathymetry; Grazhdankin 2004; Gehling and Droser 2013), taphonomic
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