Name: Ediacara Assemblage
The mode and mechanisms of preservation of soft-bodied faunas in particular may be intrinsic to resolution of the debate over the affinity of the fauna. It is important to attempt to understand how the Ediacaran fossils were preserved because recognition of the diagnostic characteristics is hampered by the nature of the preservation.
Gehling (1986) suggested that the unusual preservation is the result of cyanobacterial films, which protected and hindered aeration. This seems feasible, as cyanobacterial films were already present in algal stromatolites at this time. Recent experiments by Jones (2002) on bacterial gelatinous layers have demonstrated their efficacy in preserving the delicate spines of modern Spirograptus (a graptolite).
Other hypotheses explaining the preservation include the idea of a high density of bottom population allied with rapid burial in sediment. The nature of the sediments indicates that 'flash' events such as turbidity flows and ash-fall were involved in many (but not all) of the Ediacarian deposits.
Preservation could be due to a low degree of biological processing by deposit feeders. Sepkoski (1979) suggested that the apparent extinction of many Ediacarian taxa at the base of the Cambrian was due to an increase in deposit feeders, thus removing the evidence of later forms. Others have suggested that an absence of active filtrators prevented circulation and thus a reducing environment formed.
Glaessner (1984) believes that the organisms are preserved because there were no predators at that time. However, Hutchinson (1961) proposed that the earliest predators could have been soft-bodied, as in the Burgess Shale and therefore might not have been preserved.
There appears to be no common environmental factor that could explain the unusual preservation of the Ediacarian fauna. It may be that many of the factors described above played a role or that factors such as 'flash' events lead to preservation in some cases while other factors were important in the quieter 'shifting current' conditions suggested by Glaessner & Wade (1966) to explain the quartzite sandstone. Seilacher's (1992) view that the unusual preservation was a consequence of the 'unique construction' depends on acceptance that the fauna was indeed uniquely constructed.
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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