584


BOAG ET AL.


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.


Wilby et al. 2011; Gehling and Droser 2013; Noble et al. 2014). 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 biostrati- graphic context. For instance, a date of 561.85±0.34Ma (Noble et al. 2014) now pro- vides a maximum age for the Bradgate Park Formation (Fig. 4: [NQ_D], [NQ_A], [NQ_B]). Additionally, although no dates were recov- ered from the stratigraphically highest fossiliferous surfaces, which preserve the greatest taxonomic diversity, they lie ~200m below the Hanging Rocks Formation, which has been dated 556.6±6.4Ma. This effectively constrains the age of the Avalonian frondose taxa in the Bradgate Park Formation to ca. 561– 557Ma (Noble et al. 2014). Taking uncertainty into account, the localities at Bradgate Park appear likely to overlap temporally with the


lowermost fossiliferous strata in theWhite Sea region in Russia. The prodeltaic and outer-shelf succession of the Lyamsta Formation is constrained by a youngest possible age of 558±1Ma from the overlying Verkhovka Formation (Grazhdankin 2004). It contains a highly disparate population in comparison with deep-water frondose communities at Bradgate Park, including Dickinsonia Wade, 1972, Parvancorina Glaessner, 1958, and sac-like Inaria Gehling, 1988 [LR_pd], [LR_ds] (Grazh- dankin 2004). 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


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