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Name: Ediacara Assemblage |
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Geological
Setting and Age
Geological
Setting
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.
Department of Earth Sciences
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