EDIACARAN DISTRIBUTIONS IN SPACE AND TIME


heightened emphasis on obtaining similarly accurate age constraints for the Avalon Penin- sula Conception Group. While the Conception Group records the oldest occurrence of Ediacaran body fossils (578.8±0.5 Ma; Van Kranendonk et al. 2008; Schmitz 2012) from the upper Drook Formation, the stratigraphic range of its Avalonian taxa are still poorly constrained by a maximum age of 565±3Ma (Benus 1988), taken from the upper Mistaken Point Formation. The ~1400m 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 1979; Wood et al. 2003; Hofmann et al. 2008), 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 preser- vational 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 2002). A similar effect can be induced by rapid up-section changes in paleoenviron- ment. 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 Hang- ing 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. 1998). A similar shift in paleoenvironment occurs within the stratigraphy at Bonavista Peninsula. There, weak turbidity currents with thin inter- bedded fossil-preserving ash beds of the Fermeuse Formation grade into shallower prodelta and delta-front facies of the Renews Head Formation (Williams and King 1979;


585


O’Brien and King 2005). 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. 2008). 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. 2015). 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 1995, 2003). 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. 2015).


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. 2008), there is significant taxonomic separation between terminal Ediacaran strata (549–541Ma) and olderWhite Sea localities (Fig. 4). The deep-water Khatyspyt Formation [Kh_khat] is a unique assemblage in itself, plotting closelywith other White Sea localities (Waggoner 2003; this study), but lacking the iconic Dickinsonio- morph, Bilateralomorph, Kimberellomorph taxa that define the White Sea Region, Russia, and poorly age-constrained 556±24Ma, Australian assemblages (Preiss 2000). The Khatyspyt consists of the fronds Charnia and Khatyspytia Fedonkin, 1985 (in Fedonkin et al. 2007a), numerous Aspidella-type morphs, the serial Palaeopascichnus Palij, 1976, and sac-like Inaria preserved in nodular bituminous limestones (Knoll et al. 1995; Grazhdankin et al 2008). Interstratified within these limestones are calcareous mudstones, which


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