EDIACARANS ON THE EDGE: ENVIRONMENTAL CONTROLS ON DIVERSITY

Recent paleoecological studies of the Ediacara biota in the Late Ediacaran, Rawnsley Quartzite, on the western margins of the Flinders Ranges, demonstrate remarkable variations in the diversity of assemblages from suites of beds situated within relatively small stratigraphic and geographic range. While vertical heterogeneity has been demonstrated for successive beds, the overall assemblage is relatively uniform in particular sedimentary facies. However, facies changes along strike are accompanied by contrasting fossil assemblages and styles of preservation. Fossil assemblages preserved by event–sands, close to storm wave-base, represent samples of benthic communities.

In deeper, slope-facies isolated thicker sandy event beds preserve single-species dominated assemblages on the bed soles. Large, fragmented fronds, bag-shaped forms, and fractal plumes, occur within massive sandstones deposited as mass-flow or slumped sands.

These examples of rich Ediacara fossil assemblages in contrasting sandy-facies represent the juxtaposition of shallow-water communities and microbial mat grounds with submarine canyon edge and canyon-fill facies that preserve transported Ediacara organisms. Holdfasts and torn stalks in shallower facies suggest that erect fronds were once present, but were torn away and transported into deeper water. Where such fronds were partly filled with sand during transported, they were re-sedimented within mass-flow beds together with various other three-dimensional forms.

Ediacaran mega-fossil assemblages cluster into the Avalon association (575-560 Ma) of discoidal forms and rangeomorphs from Newfoundland and England, the White Sea association (~560-550 Ma) of rangeomorphs, stem-group bilaterians and trace fossils from Russia, NW Canada and Southern Australia, the Nama association (~550–542 Ma) of rangeomorphs, trace fossils and calcified tubes and cups from Namibia and western North America, and the Yangtze association (551-542 Ma) of metaphytes and trace fossils from China and India. Although environmental factors appear to have influenced the diversity of Ediacaran mega-fossil assemblages around the globe, it is the presence of trace fossils and bilaterian body fossil assemblages, younger than 560 Ma, which distinguishes them from the older Avalon association.