Decoupling Preserving Lithology with Moldic Preservation

Taphonomic biases can influence any quantitative paleontological study; this concern is heightened when dealing with exceptional soft-tissue preservation (Briggs Reference Briggs2003). Several taphonomic windows were open during the Ediacaran Period (Kenchington and Wilby Reference Kenchington and Wilby2014), preserving macrofossils in a range of sedimentological facies and depositional environments, albeit in highly disproportionate regularity. We aimed to resolve whether any large-scale lithological barriers are present in order to (1) assess the impact on the overall fidelity of the taxonomic record and (2) constrain any potential lithological dependencies guiding the known Ediacaran taphonomic windows.

Due to a combination of unique paleoenvironmental conditions, microbially induced moldic “death masks” of soft tissues in siliciclastic sandstone and siltstone dominate the Ediacaran fossil record (Gehling Reference Gehling1999; Gehling et al. Reference Gehling, Droser, Jensen and Runnegar2005). Moldic preservation occurs through a complex interplay between two processes: (1) mold formation initiates as labile tissues decay rapidly and are infilled with sediment; and (2) sulfate-reducing bacteria then exploit this decaying organic material, converting sulfate (SO₄ ^2-) to hydrogen sulfide (H₂S), which combines with iron in the sediment to precipitate early diagenetic minerals such as pyrite (FeS₂). Ultimately, these molds are believed to be the product of multiple depositional factors at the time of burial, including presence of clay minerals, microbial mat prevalence, and ensuing chemistry of pore waters (Narbonne Reference Narbonne2005; Callow and Brasier Reference Callow and Brasier2009; Darroch et al. Reference Darroch, Laflamme, Schiffbauer and Briggs2012, Kenchington and Wilby Reference Kenchington and Wilby2014). In light of this complexity, we instead tested the degree to which overall preserving lithology affects this taphonomic character in order to inform potential outcrop-scale biases in collection methods.

Database results reinforce a clear ubiquity of moldic preservation across Ediacaran genera; convex hulls for each lithology show limited separation toward any of the traditional assemblages, perhaps with the exception of the Avalonian biota, which are tightly correlated with Conception-style preservation in ash (Fig. 3, Supplementary Fig. S.4.C). This inference is strengthened by the results of beta-diversity analyses, which show that lithology accounts for the second-lowest dissimilarity values among tested variables after temporal binning (Table 2). Although moldic preservation may appear to create a taphonomic filter for epifaunal taxa (Kenchington and Wilby Reference Kenchington and Wilby2014), with the exception of the unique Conception-type, moldic preservational styles are observed in a wide range of paleoenvironments (shallow shoreface to deep-water slope; see Bouougri et al. Reference Bouougri, Porada, Weber and Reitner2011; Narbonne et al. Reference Narbonne, Laflamme, Trusler, Dalrymple and Greentree2014). Furthermore, moldic preservation appears to be largely decoupled from any strictly lithological controls, such as carbonate facies (Grazhdankin et al. Reference Grazhdankin, Balthasar, Nagovitsin and Kochnev2008; Chen et al. Reference Chen, Zhou, Xiao, Wang, Guan, Hua and Yuan2014; Fig. 3, [Kh_khat] and [Muz2Sh]). This can make the identification of any major taxonomic–taphonomic moldic biases beyond in vivo position relative to the sediment–water interface difficult.

Figure 3 Distribution of macrofossil-preserving lithologies at each locality, overlain as polygons across our updated taxonomic ordination space. Siliciclastic coarse-grained sandstone (Coarse sandstn.), fine-grained siltstone (Mixed siltstn.), and limestones (Limestone) appear broadly distributed across taxa, while siliciclastic and carbonate shale/mudstones (Shale/mudstn.) and ash (Ash) show greater taxonomic restriction.

There are several implications of these results. First, Ediacaran taxonomy does not appear to suffer from any litho-preservational barriers among taxa preserved via moldic impressions in siltstone–sandstone grained siliciclastic settings. As discussed, this trend extends into the heavily undersampled carbonate facies (e.g., Chen et al. Reference Chen, Zhou, Xiao, Wang, Guan, Hua and Yuan2014). Second, from a worker effort or “search image” perspective, further explorative work for new Ediacaran localities should remain focused on identifying heterolithic sedimentary packages with sedimentological and structural properties suitable for moldic preservation: abundant MISS, cleavage planes that align with original bedding surfaces, and well-defined bedding soles, which are often formed via contourite and/or turbidity currents (Sperling et al. Reference Sperling, Carbone, Strauss, Johnston, Narbonne and Macdonald2015), and not limited to just siliciclastic sandstone and siltstone lithofacies.

Finally, these data may point to the importance of microbial mats, and not necessarily lithology, in the overall mold-forming process. It is well documented that Ediacaran seafloors were marked by global evidence for extensive microbial mats and absence of deep bioturbation (Mángano and Buatois Reference Mángano and Buatois2014; Davies et al. Reference Davies, Liu, Gibling and Miller2016). These mats would therefore have supported large populations of decay bacteria and created an effective seal over newly buried organisms. This seal would have largely restricted the flux of an oxygenated water column into dysoxic–anoxic pore waters beneath the sediment–water interface, producing geochemical conditions that would heavily favor early diagenetic precipitation of preserving minerals such as pyrite (see above; Darroch et al. Reference Darroch, Laflamme, Schiffbauer and Briggs2012). In addition, scavenging and macrophagy are conspicuously absent until the latest Ediacaran, creating ecosystems with minimal biological biostratinomic destruction (Kenchington and Wilby Reference Kenchington and Wilby2014). These conditions have led several authors to suggest the overall fidelity of Ediacaran soft-tissue preservation was vastly superior to that of the Phanerozoic (Brasier et al. Reference Brasier, Antcliffe and Callow2010). As such, exceptional moldic preservation in the Ediacaran should permit a robust representation of true biological patterns with relatively few taphonomic biases in comparison with Phanerozoic settings. Furthermore, as microbial mats appear ubiquitous in their temporal, geographic, and bathymetric distribution in the Ediacaran prior to large-scale metazoan bioturbation, the spatial/temporal partitioning of mats as a potential taphonomic bias in the Ediacaran appears largely decoupled from the other factors tested in this study (Laflamme et al. Reference Laflamme, Schiffbauer and Narbonne2011a, Reference Laflamme, Darroch, Tweedt, Peterson and Erwin2013).

Despite the pervasiveness of the moldic taphonomic window in the Ediacaran, it is important to note that moldic preservation may exert a bias against epifaunal taxa that extend into the water column (Kenchington and Wilby Reference Kenchington and Wilby2014). Consequently, much of the Ediacaran fossil record is dominated by taxa that are proximal to the sediment–water interface, such as the discoidal taxon Aspidella (Laflamme et al. Reference Laflamme, Schiffbauer, Narbonne and Briggs2011b), regarded to be the anchoring holdfasts for fronds (Gehling et al. Reference Gehling, Narbonne and Anderson2000). Rarely are both holdfast and frond preserved on the same bedding surface (e.g., Laflamme et al. Reference Laflamme, Narbonne and Anderson2004; Narbonne et al. Reference Narbonne, Laflamme, Trusler, Dalrymple and Greentree2014: Fig. 4.4). Rather, fronds appear either without associated holdfast in instances where the disk remained buried within the sediment both in vivo and during preservation (Laflamme et al. Reference Laflamme, Narbonne, Greentree and Anderson2007, Reference Laflamme, Schiffbauer, Narbonne and Briggs2011b), or more commonly, the frond itself will be absent, leaving only bedding surfaces covered in holdfasts in varying styles of preservation (Tarhan et al. Reference Tarhan, Droser and Gehling2010; Kenchington and Wilby Reference Kenchington and Wilby2014). The ubiquity of holdfasts and comparative rarity of preserved fronds is a critical unresolved taphonomic bias present in the current Ediacaran record (Laflamme et al. Reference Laflamme, Schiffbauer, Narbonne and Briggs2011b). As holdfasts cannot be rigorously correlated with specific frondose taxa (Burzynski and Narbonne Reference Burzynski and Narbonne2015), this reinforces that both the diversity and stratigraphic range of frondose taxa are likely highly underrepresented in localities with dense holdfast assemblages (Darroch et al. Reference Darroch, Sperling, Boag, Racicot, Mason, Morgan, Tweedt, Myrow, Johnston, Erwin and Laflamme2015).

Figure 4 Localities that have been geochronologically constrained are binned into Avalon, White Sea, and Nama temporal intervals. These have then been expressed as polygons on top of our taxonomic ordination space to assess the degree to which these suggested biostratigraphic stages represent discrete taxonomic intervals.

Sampling Intensity and Lithology Limit Carbonaceous Compressions

Although the mid-late Ediacaran macrofossil record is dominated by moldic soft-tissue preservation, in several cases carbonaceous compressions have been recovered. The most well-known instances of this occur in several Chinese localities, including the Gaojiashan Lagerstatte, Shaanxi Province (Cai et al. Reference Cai, Schiffbauer, Hua and Xiao2012), and the Miaohe and Wenghui biotas in the Hubei and Guizhou provinces, respectively (Xiao et al. Reference Xiao, Yuan, Steiner and Knoll2002; Zhao et al. Reference Zhao, Chen, Peng, Yu, He, Wang, Yang, Wang and Zhang2004) (Fig. 3: [Gaoj], [Miah1], [Jiang]). Carbonaceous compressions, formed via kerogenization (polymerization of organic molecules), are best known from the iconic Cambrian Burgess Shale; hence they are often referred to as Burgess Shale–type (BST) preservation (Orr Reference Orr2014). Kerogenization is essential to BST preservation; however, it can be accompanied by two other mineralization processes: authigenic pyritization and aluminosilicification, which can co-occur as a range of taphonomic end members that together contribute to the BST taphonomic mode (Cai et al. Reference Cai, Schiffbauer, Hua and Xiao2012; Schiffbauer et al. Reference Schiffbauer, Xiao, Cai, Wallace, Hua, Hunter, Xu, Peng and Kaufman2014).

Database analysis has shown significant undersampling of fossiliferous shale lithofacies, identifying only 6 BST candidates from our 86 Ediacaran localities and >45 geological units: [Ong], [MatoGDS], [Jiang], [Gaoj], [Kh_khat],and [Miah1] (Fig. 3). These deposits all form in low-energy, deep-water distal offshore shelves or restricted basinal lagoons. Perhaps the most uncertain feature of Ediacaran BST deposits is the apparent exclusion of frondose taxa from this preservational window (Steiner and Reitner Reference Steiner and Reitner2001; Grazhdankin et al. Reference Grazhdankin, Balthasar, Nagovitsin and Kochnev2008). The Khatyspyt Formation in Siberia [Kh_khat] is interpreted to be a distal, low-energy carbonate ramp, below storm-weather wave base (SWWB). Although it contains exquisitely preserved Rangeomorphs such as Charnia Ford, Reference Ford1958, occurring as molds in authigenic carbonate cementation, these same frondose taxa are relegated to “phantoms” within interbedding carbonaceous compression windows (Grazhdankin et al. Reference Grazhdankin, Balthasar, Nagovitsin and Kochnev2008: Fig. 3a–h). New Ediacaran localities with BST-type preservation may therefore help to confirm the presence/absence of certain Ediacaran taxa from this mode of preservation, thus resolving issues associated with taphonomic effect versus ecological biofacies (Zhu et al. Reference Zhu, Gehling, Xiao, Zhao and Droser2008; Gehling and Droser Reference Gehling and Droser2013; Kenchington and Wilby Reference Kenchington and Wilby2014; Carbone et al. Reference Carbone, Narbonne, Macdonald and Boag2015).

Chronostratigraphic Overlap of the Avalonian and White Sea Assemblages

Evolutionary succession has often been suggested as an important factor in the distribution of taxa in the Ediacaran macrofossil record. However, the ability to test the biotic assemblages as discrete temporal intervals has been limited by a lack of high-quality geochronology. We use a collection of updated publication data to reanalyze Waggoner’s core assemblage concept as temporally distinct by binning Ediacaran localities into the traditionally accepted biostratigraphic intervals for the Avalon (579–559 Ma), White Sea (558–550 Ma), and Nama (549–541 Ma) (Narbonne et al. Reference Narbonne, Xiao and Shields2012; see Supplementary Table S.5). If the hypothesis that the three assemblages represent a response to a temporal control on the distribution is correct, then temporal binning should also display a clear polygonal separation similar to the initial taxonomic separation (Fig. 1). Instead, NMDS ordination plots of time-binned localities reveal that temporal overlap is present between both the Avalon–White Sea and White Sea–Nama assemblages. These results agree with the current, albeit limited geochronological record (Fig. 4, discussed below), and are supported by beta-diversity (dissimilarity) metrics showing that temporal binning produces the lowest diversity dissimilarity when compared with all other variables being tested (Table 2). However, upon further statistical analyses, the data produce contradictory results. When confidence ellipses were drawn around the class centroids for each time bin, they did not overlap, suggesting the assemblages are in fact temporally distinct at CI=95% (Supplementary Fig. S.4.D). These inconsistent results may reflect differences in the relative sample size of tested variables: abundant diversity data yield higher degrees of confidence and therefore a narrower overall confidence interval, while the limited number of geochronological dates requires significantly wider confidence intervals (see Supplementary Table S.5). At present, both NMDS and beta-diversity data support previous local-scale observations of contemporaneous but unrelated taxa partitioning into different depositional environments (Grazhdankin Reference Grazhdankin2004; Wilby et al. Reference Wilby, Carney and Howe2011; Gehling and Droser Reference Gehling and Droser2013; Noble et al. Reference Noble, Condon, Carney, Wilby, Pharaoh and Ford2014). Meanwhile, confidence testing reinforces the need for more critical age constraints of fossiliferous Ediacaran strata that may bridge traditional assemblage time intervals.

Recently updated geochronology of the Charnian Supergroup of Charnwood Forest, England, has placed much of the classic Avalonian assemblage into a clearer biostratigraphic context. For instance, a date of 561.85±0.34 Ma (Noble et al. Reference Noble, Condon, Carney, Wilby, Pharaoh and Ford2014) now provides a maximum age for the Bradgate Park Formation (Fig. 4: [NQ_D], [NQ_A], [NQ_B]). Additionally, although no dates were recovered from the stratigraphically highest fossiliferous surfaces, which preserve the greatest taxonomic diversity, they lie ~200 m below the Hanging Rocks Formation, which has been dated 556.6±6.4 Ma. This effectively constrains the age of the Avalonian frondose taxa in the Bradgate Park Formation to ca. 561–557 Ma (Noble et al. Reference Noble, Condon, Carney, Wilby, Pharaoh and Ford2014). Taking uncertainty into account, the localities at Bradgate Park appear likely to overlap temporally with the lowermost fossiliferous strata in the White Sea region in Russia. The prodeltaic and outer-shelf succession of the Lyamsta Formation is constrained by a youngest possible age of 558±1 Ma from the overlying Verkhovka Formation (Grazhdankin Reference Grazhdankin2004). It contains a highly disparate population in comparison with deep-water frondose communities at Bradgate Park, including Dickinsonia Wade, Reference Wade1972, Parvancorina Glaessner, Reference Glaessner1958, and sac-like Inaria Gehling, Reference Gehling1988 [LR_pd], [LR_ds] (Grazhdankin Reference Grazhdankin2004). Thus, with current, albeit limited geochronological data, it is apparent that highly diverse, yet taxonomically dissimilar assemblages were likely to have co-occurred at least during the Avalon–White Sea transition. These data lessen the case for evolutionary succession exerting a first-order control on Avalon–White Sea assemblages. However, a lack of precise geochronology highlights the effect that an uneven chronostratigraphic record can have on interpreting trends in both global diversity and evolution. Specifically, data illustrating the relative youth of the uppermost Bradgate Park Formation place heightened emphasis on obtaining similarly accurate age constraints for the Avalon Peninsula Conception Group. While the Conception Group records the oldest occurrence of Ediacaran body fossils (578.8±0.5 Ma; Van Kranendonk et al. Reference Van Kranendonk, Gehling and Shields2008; Schmitz Reference Schmitz2012) from the upper Drook Formation, the stratigraphic range of its Avalonian taxa are still poorly constrained by a maximum age of 565±3 Ma (Benus Reference Benus1988), taken from the upper Mistaken Point Formation. The ~1400 m of overlying fossiliferous strata of the Trepassey and Fermeuse formations on both the Avalon and Bonavista peninsulas of Newfoundland lack any geochronological constraint (Williams and King Reference Williams and King1979; Wood et al. Reference Wood, Dalrymple, Narbonne, Gehling and Clapham2003; Hofmann et al. Reference Hofmann, O’Brien and King2008), and may provide a much younger maximum age for the Avalonian assemblage.

It is also important to evaluate both taphonomic and stratigraphic gaps in the known record when assessing global trends in taxonomic diversity and evolution. Both first and last appearances of specific taxa can be highly concentrated along physical or preservational disconformities that truncate the stratigraphic range of taxa that would otherwise have first appeared or gone extinct in an unrecorded interval of time (Kidwell and Holland Reference Kidwell and Holland2002). A similar effect can be induced by rapid up-section changes in paleoenvironment. These factors are both present in the uppermost stratigraphy of Newfoundland and Charnwood Forest. While fine- to medium-grained deep-water turbiditic facies preserve the fossil communities from the Bradgate Park Formation, the overlying Hanging Rocks Formation is bounded by an erosional surface. It is composed of poorly sorted volcanic epiclastic conglomerates and pyroclastic components, overlain by medium-grained sandstones and tuffaceous siltstones interpreted as turbidity currents carrying reworked detritus from much shallower fluvial or nearshore conditions (McIlroy et al. Reference McIlroy, Brasier and Moseley1998). A similar shift in paleoenvironment occurs within the stratigraphy at Bonavista Peninsula. There, weak turbidity currents with thin interbedded fossil-preserving ash beds of the Fermeuse Formation grade into shallower prodelta and delta-front facies of the Renews Head Formation (Williams and King Reference Williams and King1979; O’Brien and King Reference O’Brien and King2005). These facies shifts may have removed both the environmental conditions for a deep-water Avalonian biotope to exist and the Conception-style taphonomic window that preserves the majority of epifaunal fronds (Hofmann et al. Reference Hofmann, O’Brien and King2008). Factors such as these reinforce the need for caution when interpreting the absence of certain taxa from the current Ediacaran record as a true biological signal (Sperling et al. Reference Sperling, Carbone, Strauss, Johnston, Narbonne and Macdonald2015). Global studies of macroevolutionary patterns must take place within a stratigraphic framework that can statistically account for varying likelihoods of preservation, be it taphonomic, environmental, or a superposition of the two (Holland Reference Holland1995, Reference Holland2003). Given the current uncertainty in correlating the global Ediacaran stratigraphic record, the most rigorous method of assessing global trends in biodiversity should remain dedicated to geochronologically constrained outcrop-scale studies, which can better account for these preservational impacts on observed diversity (Darroch et al. Reference Darroch, Sperling, Boag, Racicot, Mason, Morgan, Tweedt, Myrow, Johnston, Erwin and Laflamme2015).

Assessing Biotic Turnover in the Depauperate Nama

The temporal overlap observed between the White Sea and Nama assemblages is more complex. With the exception of the Khatyspyt Formation in the Olenek Uplift of Siberia (Grazhdankin et al. Reference Grazhdankin, Balthasar, Nagovitsin and Kochnev2008), there is significant taxonomic separation between terminal Ediacaran strata (549–541 Ma) and older White Sea localities (Fig. 4). The deep-water Khatyspyt Formation [Kh_khat] is a unique assemblage in itself, plotting closely with other White Sea localities (Waggoner Reference Waggoner2003; this study), but lacking the iconic Dickinsoniomorph, Bilateralomorph, Kimberellomorph taxa that define the White Sea Region, Russia, and poorly age-constrained 556±24 Ma, Australian assemblages (Preiss Reference Preiss2000). The Khatyspyt consists of the fronds Charnia and Khatyspytia Fedonkin, 1985 (in Fedonkin et al. Reference Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007a), numerous Aspidella-type morphs, the serial Palaeopascichnus Palij, Reference Palij1976, and sac-like Inaria preserved in nodular bituminous limestones (Knoll et al. Reference Knoll, Grotzinger, Kaufman and Kolosov1995; Grazhdankin et al Reference Grazhdankin, Balthasar, Nagovitsin and Kochnev2008). Interstratified within these limestones are calcareous mudstones, which preserve an assemblage of carbonaceous compressions of taxa similar to the >551.09±1.02 Ma Miaohe assemblage in the uppermost Doushantuo Formation of south China (Condon et al. Reference Condon, Zhu, Bowring, Wang, Yang and Jin2005; Xiao et al. Reference Xiao, Yuan, Steiner and Knoll2002).

Several authors have referred to the Khatyspyt Formation as “Avalon-type,” based on its relatively deep-water distal carbonate ramp depositional setting, presence of fronds, and absence of typical White Sea biota (Grazhdankin et al. Reference Grazhdankin, Balthasar, Nagovitsin and Kochnev2008; Rogov et al. 2012). Interestingly, our ordination plots do not place the Khatyspyt within the Avalon-type taxonomic designation, and instead agree with the results of Waggoner (Reference Waggoner2003) by placing it squarely within the White Sea assemblage ([Kh_khat] in Figs. 1, 4). From a stratigraphic perspective, a maximum age for the Khatyspyt assemblage is provided by a volcaniclastic breccia unit, which crosscuts the overlying Kessyusa Formation and is dated at 543.9±0.3 Ma (Bowring et al. Reference Bowring, Grotzinger, Isachsen, Knoll, Pelechaty and Kolosov1993; Knoll et al. Reference Knoll, Grotzinger, Kaufman and Kolosov1995). However, the presence of Cambrian shelly tubular taxa Cambrotubulus and Anabarites in the intermediary Turkut Formation (Brasier et al. Reference Brasier, Shields, Kuleshov and Zhegallo1996; Maloof et al. Reference Maloof, Porter, Moore, Dudás, Bowring, Higgins, Fike and Eddy2010) reinforces that the underlying Khatyspyt assemblage remains poorly constrained as an important deep-water end member for the terminal Ediacaran.

Traditional shallower-water Nama-type localities display unusually depauperate populations of taxa and are principally separated into two populations. Limestones and micritic mudstone horizons preserve mineralizing taxa such as Cloudina Pflug, Reference Pflug1972, Namacalathus Grotzinger et al., Reference Grotzinger, Watters and Knoll2000, and Namapoikia Wood et al., Reference Wood, Grotzinger and Dickson2002 [Nam_drie], [Sali_1], [Lijian], [Omn] (Grotzinger et al. Reference Grotzinger, Watters and Knoll2000; Hofmann and Mountjoy Reference Hofmann and Mountjoy2001; Wood et al. Reference Wood, Grotzinger and Dickson2002; Amthor et al. Reference Amthor, Grotzinger, Schröder, Bowring, Ramezani, Martin and Matter2003), often co-occurring with the tubular genus Corumbella Hahn et al., Reference Hahn, Hahn, Leonardos, Pflug and Walde1982 [MatoGDS], [Loc1], [Loc2] (Babcock et al. Reference Babcock, Grunow, Sadowski and Leslie2005; Warren et al. Reference Warren, Fairchild, Gaucher, Boggianai, Poiré, Anelli and Inchausti2011; see Figs. 2, 4). The global distribution of these taxa, coupled with their abundant and consistent preservation in terminal Ediacaran (549–541 Ma) carbonate successions favors this association as an excellent candidate for a globally correlative biozone (Grant Reference Grant1990; Warren et al. Reference Warren, Fairchild, Gaucher, Boggianai, Poiré, Anelli and Inchausti2011). The few soft-bodied Ediacara biota in the Nama assemblage such as Pteridinium Gürich, Reference Gürich1930, Rangea Gürich, Reference Gürich1933, Charniodiscus Ford, Reference Ford1958, Namalia Germs, Reference Germs1968, and Nasepia Germs, Reference Germs1973 (and perhaps Swartpuntia Narbonne et al., Reference Narbonne, Saylor and Grotzinger1997—see Ivantsov and Fedonkin Reference Ivantsov and Fedonkin2002) are in fact also found in assemblages taxonomically assigned to both the White Sea and Avalon (Gehling and Droser Reference Gehling and Droser2013; Narbonne et al. Reference Narbonne, Laflamme, Trusler, Dalrymple and Greentree2014). This is an important detail that suggests the disparate taxonomic ordination visible for the Nama assemblage (Fig. 4: [Nam_hf], [Nam_aa], [Nam_sw], [Car_SB], [DeaV3], [Muz2Sh]) is mostly due to the absence of diverse White Sea clades (Bilateralomorphs, Dickinsoniomorpha, and Triradialomorpha) from the terminal Ediacaran rather than the emergence of novel taxa (such as at [Gaoj] and [Lijian] localities; Fig. 2). The extent to which this represents a genuine biological signal has been explored in recent studies (Gehling and Droser Reference Gehling and Droser2013; Darroch et al. Reference Darroch, Sperling, Boag, Racicot, Mason, Morgan, Tweedt, Myrow, Johnston, Erwin and Laflamme2015), which recognize that the absence of White Sea fauna in less diverse Nama-aged strata of analogous paleoenvironmental and taphonomic conditions may represent a global-scale episode of protracted extinction. If the depauperate Nama assemblages do in fact represent surviving taxa with a shared ecological association with White Sea biota, this hypothesis would also posit that surviving taxa (such as Erniettomorphs and Rangeomorphs) should be composed of ecological generalists or opportunists with broad niche tolerances (see next section; Darroch et al. Reference Darroch, Sperling, Boag, Racicot, Mason, Morgan, Tweedt, Myrow, Johnston, Erwin and Laflamme2015). Such a scenario may also be reflected by the utilized morphospaces of Ediacara-type biota, which remained largely unchanged between the White Sea and Nama after initial expansion observed within the Avalonian assemblage. A lack of significant morphological diversification among Ediacara taxa thereafter may indicate that these organisms could have become developmentally or ecologically entrenched (Shen et al. Reference Shen, Dong, Xiao and Kowalewski2008) and would therefore be gradually outcompeted by skeletonizing, tubular, and trace-making animals that were increasing in both diversity and autecological complexity throughout the terminal Ediacaran.