588


BOAG ET AL.


FIGURE 6. Relative water-depth contour averaging of localities with ordinated Ediacaran taxa overlain. From left to right: (11) slope/basin; (10) outer shelf (below SWWB); (9) middle shelf (well below FWWB, above SWWB); (8) inner shelf (between FWWB and SWWB); (7) lower shoreface (above FWWB); (6) middle shoreface; (5) upper shoreface; (4) reef margin complex. Not significant: (3) lagoonal/restricted; (2) peritidal (intertidal); (1) fluvial/deltaic. Overlain dashed polygons represent the Waggoner (2003) Avalon, White Sea, and Nama assemblages.


as Rangea, Charnia, Pteridinium, and Charnio- discus are repeatedly found in eurytopic water depths ranging from offshore middle shelf (depth rank 10: [Nil_mf], [Nil_sf]) to middle shoreface (depth rank 6: [Nam_hf]), with isolated occurrences of Namalia in depths as great as slope/basinal (depth rank 11: [SJb_Nor2]) and Charniodiscus as shallow as upper shoreface (depth rank 5: [Nil_ss]) (Bouougri and Porada 2007; Gehling and Droser 2013; Narbonne et al. 2014). Addition- ally, many of these taxa also display extremely long stratigraphic ranges (Narbonne et al. 1997; Fedonkin et al. 2007a, and references therein; Grazhdankin et al. 2008; Liu et al. 2013; Narbonne et al. 2014). These results provide strong global evidence that depauperate Nama communities were composed of stratigraphi- cally long-ranging cosmopolitan survivors with broad environmental and likely ecologi- cal tolerances. In contrast, although White Sea localities with apex diversity such as Nilpena (Australia) share similar bathymetry [Nil_ss], Nil_wb], [Nil_df] to those in Namibia [Nam_hf], [Nam_sw], [Nam_aa], older envir- onmentally tolerant taxa such as Dickinsonia


and Tribrachidium (see Gehling and Droser 2013; Hall et al. 2015) are conspicuously absent from later Nama communities. Previous work- ers have suggested this absence of polyfacies taxa in the latest Ediacaran could indicate a true global extinction signal occurring abruptly at the end of the White Sea; however, it was presumed premature to conclude that the low-diversity terminal Nama assemblages represent evidence of either early extinction or changed preservational circumstances (Gehl- ing and Droser 2013). As we find globally a high correlation between remaining Ediacara taxa in the present Nama assemblage and large bathymetric-range tolerance, this provides support for a model in which surviving taxa were ecological generalists that were able to colonize a latest Ediacaran Period (<549Ma) punctuated by conditions that were now limiting to preceding Ediacarans until their ultimate disappearance at the Proterozoic/ Cambrian boundary (Laflamme et al. 2013; Darroch et al. 2015). Clearly, there were likely many other direct,


indirect, and resource gradients that may have controlled the distribution of Ediacaran taxa.


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