Name: Ediacara Assemblage
The geological setting of the Ediacara assemblages varies around the world. The supercontinent Rodinia was in existence at the time, although it rifted into Lauentia and East Gondwana at about this time. Most of the Ediacarian assemblages are believed to have been deposited in littoral (beach) (Glaessner & Wade, 1966) benthonic (sea-floor), well-lit shallow marine conditions. Seilacher (1993) interpreted the forms as immobile soft-bottom dwellers some of which reclined flat, some were "erect elevators" and others were "sediment stickers", which were smothered by storm sands.
At the Ediacara assemblage in South Australia the deposit has been interpreted as lying on sandy shoals which occasionally emerged, as shown by dessication (drying) cracks on some bedding planes. The arenaceous (sandy) sediments show cross-bedding and ripple marks; interbedded argillaceous (grains less than 4mm) laminae indicate that there were occasional tranquil conditions between shifting currents (Glaessner & Wade, 1966).
This calm environment was clearly occasionally disturbed by catastrophic sediment inputs; at the Mistaken Point Formation in Newfoundland this took the form of a volcanic ash bed (Grotzinger et al, 1995). However, this same formation has been interpreted eslewhere as a deep water turbidite (turbulent suspended sediment) containing heterotrophic fauna (shallow water faunas have been interpreted as autotrophic).
The Ediacara fauna may have evolved in response to the transgression of sea over continents following the end of the Varanger Glaciation 610 Ma ago. This resulted in extensive shallow water habitats, which were subsequently exploited by soft bodied creatures like the Ediacara fauna. At this time the atmospheric and ocean chemistry was in transition; carbon dioxide levels decreased while atmospheric oxygen increased, and carbonate platforms expanded toward the base of the Cambrian.
Age of the Ediacara Assemblage
The age of the Ediacara assemblage has been debated, and is still not resolved. It is generally agreed to be from the early Vendian, also known as the Ediacaran, a late stage in the Pre-Cambrian Neoproterozoic. Estimates include ~ 25 million years (my) ± 10-15 my before the Precambrian-Cambrian boundary between 600 and 530 my ago (Sepkoski, 1979).
A volcanic-ash bed which is interbedded with Ediacara fossils at Mistaken Point Formation in Newfoundland gave a U-Pb zircon age of 565 ± 4 my. The Slawatycze Formation in Poland yielded a U-Pb zircon age of 551 ± 4 my, but this is indirectly gained through assumed lithological correlation with a volcanogenic sequence in the Ukraine. The lower age limit for Ediacara fossils has been calculated at 548.8 ± 0.3 my using zircon dating (Grotzinger et al. 1995).
The age of the assemblage is significant because of its implications for the importance of the Ediacara in the early evolution of life; many workers have tried to tie-in the appearance and apparent later extinction of the Ediacara assemblages with changes in Earth's chemistry which may have lead to the innovation of biomineralisation.
Section author: Kate Yarrington
This section is part of a Fossil Lagerstätten web site which has been built up as a result of the efforts of the 2002-3 MSc Palaeobiology class in the Department of Earth Sciences at University of Bristol, as part of a course in Scientific Communication.
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