586


BOAG ET AL.


preserve an assemblage of carbonaceous com- pressions of taxa similar to the >551.09±1.02Ma Miaohe assemblage in the uppermost Doushantuo Formation of south China (Condon et al. 2005; Xiao et al. 2002). 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 (Grazh- dankin et al. 2008; Rogov et al. 2012). Interest- ingly, our ordination plots do not place the Khatyspyt within the Avalon-type taxonomic designation, and instead agree with the results of Waggoner (2003) 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±0.3Ma (Bowring et al. 1993; Knoll et al. 1995). However, the presence of Cambrian shelly tubular taxa Cambrotubulus and Anabarites in the intermediary Turkut Formation (Brasier et al. 1996; Maloof et al. 2010) 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 local-


ities 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, 1972, Namacalathus Grotzinger et al., 2000, and Namapoikia Wood et al., 2002 [Nam_drie], [Sali_1], [Lijian], [Omn] (Grotzinger et al. 2000; Hofmann andMountjoy 2001; Wood et al. 2002;Amthor et al. 2003), often co-occurring with the tubular genus Corumbella Hahn et al., 1982 [MatoGDS], [Loc1], [Loc2] (Babcock et al. 2005; Warren et al. 2011; see Figs. 2, 4). The global distribution of these taxa, coupled with their abundant and consistent preservation in terminal Ediacaran (549–541Ma) carbonate successions favors this association as an excellent candidate for a globally correlative biozone (Grant 1990; Warren et al. 2011). The few soft-bodied Ediacara biota in theNamaassemblage such as Pteridinium Gürich, 1930, Rangea Gürich, 1933, Charniodiscus Ford, 1958, Namalia Germs, 1968, and Nasepia


Germs, 1973 (and perhaps Swartpuntia Narbonne et al., 1997—see Ivantsov and Fedonkin 2002) are in fact also found in assemblages taxonomically assigned to both the White Sea and Avalon (Gehling and Droser 2013; Narbonne et al. 2014). 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 Edia- caran 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 2013; Darroch et al. 2015),which recognize that the absence ofWhite 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 depauperateNama assemblages do in fact represent surviving taxa with a shared ecological association with White Seabiota,thishypothesiswould also positthat surviving taxa (such as Erniettomorphs and Rangeomorphs) should be composed of ecologi- cal generalists or opportunists with broad niche tolerances (see next section; Darroch et al. 2015). 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. Alackof significant morphological diversifica- tion among Ediacara taxa thereaftermay indicate that these organisms could have become developmentally or ecologically entrenched (Shen et al. 2008) and would therefore be gradually outcompetedby skeletonizing, tubular, and trace-making animals thatwere increasing in both diversity and autecological complexity throughout the terminal Ediacaran.


Paleoenvironmental Distribution of the Ediacara Biota


To link temporal changes in global diversity


with responses to ecological disparity in the Ediacaran record, it is critical to identify patterns in paleoenvironmental partitioning


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