Name: Ediacara Assemblage
Location: Worldwide - named after locality in Australia. Also found in Namibia, Sweden, Eastern Europe, Canada, England, Wales, New Foundland,
Age: Neoproterozoic

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Fauna and Flora


The worldwide Ediacara assemblage represents soft-bodied, trace and rare skeletonised fossils. The affinities of the fossils are the subject of much controversy, with much opinion polarising towards either Seilacher's "Doomed Experiment" whereby the organisms represent an evolutionary sideline that disappeared at the Pre-Cambrian - Cambrian boundary, or Glaessner's traditional 'Ancestral Metazoa' hypotheses. The macrofossils have been classified variously with four of the five accepted living kingdoms and with at least two extinct kingdoms (Waggoner, 1996). There have been three main approaches:

The debate: The nature of the Ediacara fauna

Most opinion on this matter has polarised towards Glaessner's (1984) view that the fauna were ancestors of modern extant metazoans and Seilacher's (1989) view that the whole assemblage represented a "doomed experiment" or "evolutionary sideline".

More recently most of the original protagonists have agreed on the middle ground. In 1994, Seilacher & Buss claimed that the assemblage represented only an extinct metazoan phylum and Glaessner (1966, 1984) admitted that although many of the fossils could be fitted into known classifications there were still some mystery fossils, such as Tribrachidium.

The resolution of the problem of the Ediacara rests on consensus on the classification of the organisms. There is no clear taxonomy of the Ediacara and thus the various hypotheses are based on completely different interpretations of the evidence (See Table.1).

Australians

Fedonkin

Seilacher

 

 

 

Coelenterata

Coelenterata

Coelenterata

Hydrozoa

Cyclozoa(*)

Psammocorallia(*)

Ovatoscutum

Ovatoscutum

Beltanelliformis

Scyphozoa

Beltanelliformis

 

Ediacaria

Ediacaria

Trace Fossil

Mawsonites

Mawsonites

Mawsonites

Anthozoa

Trilobozoa(*)

 

Charnia

Albumares

 

Charniodiscus

Tribrachidium

 

Rangea

 

 

Petalonamae(*)

Petalonamae(*)

Vendozoa(*)

 

Charnia

Charnia

 

Charniodiscus

Charniodiscus

Pteridinium

Pteridinium

Pteridinium

Phyllozoon

Phyllozoon

Phyllozoon

 

Rangea

Rangea

Annelida

Proarticulata(*)

Ovatoscutum

Dickinsonia

Dickinsonia

Dickinsonia

Spriggina

Praecambridium

Spriggina

Arthropoda

Arthropoda

Albumares

Praecambridium

Spriggina

Tribrachidium

Incertae Sedis

 

 

Albumares

 

 

Tribrachidium

 

 

Table 1 . Three Alternative Classifications of Some of the Core Members of the Ediacara Fauna.

The examples outlined below should illustrate the problems involved. Mawsonites spriggi was named by Glaessner & Wade (1966). They regarded it as a jellyfish of uncertain affinity. On the other hand, Seilacher (1989) interpreted Mawsonites as the complex burrow of a bilateral metazoan (i.e. not a body fossil at all).

The tracks, trails and burrows confirm that mobile animals lived in the Ediacarian assemblage. Faecal pellets indicate that some animals had one-way guts. Seilacher (1989) suggested that this trace fossil evidence indicates the presence of mobile organisms with digestive systems, and which probably ate microbial mats.

Nemiana (fig.3.1) is a round, convex shape and was a common occurrence in the Ediacara. It was placed in an extinct group of cnidarians (e.g. corals) by Seilacher (1989). Seilacher invoked a physiology whereby the sea anenome-like organism weighed itself with sand ingested via the mouth. However most other workers believe that Nemiana represents a burrow, or the filling of a community sea anenome after death.

Seilacher's "Weird Fauna" hypothesis rests on his interpretation of the quilted appearance of most of the assemblage. He classified nearly all (except e.g. Nemiana) body fossils into a new extinct, monophyletic clade. Many of these body fossils have apparently little in common, in terms of form.

Various workers (e.g. Gehling, 1986) have attempted to disprove Seilacher's hypothesis by showing that the quilted construction of the different fossils evolved independently, perhaps as convergent evolution in response to a common environmental pressure.

Dickinsona and the superficially similar Phyllozoon were compared by Gehling.  They were contemporaneous and both grew by adding successive segments.  However superficially similar, close study revealed that Dickinsonia is bilaterally symmetrical across the body and asymmetrical down the middle, while Phyllozoon is asymmetrical in both directions.

There are additional differences between the two genera.  Dickinsonia grew continuously by the addition of tiny new segments to what was believed to be its posterior, as the whole organism grew, the new segments similarly increased in size.  Furthermore a rare specimen of Dickinsonia appears not to have Seilacher's diagnostic mattress design (although as a rare occurrence the alternative morphology could be a relict of diagenesis).  Phyllozoon differs from Dickinsonia at least in terms of symmetry and possibly construction.

One of the principal arguments put forward by Seilacher (1989) and other workers for the assignment of the Ediacara biota to a new kingdom or phylum is the apparent lack of true symmetry of the fossils.  The segments in many of the specimens (e.g. Spriggina) appear to be slightly offset across the midline, whereas in more familiar forms (e.g. trilobites) the segments are completely symmetrical.  Symmetry is indeed an important diagnostic characteristic but it seems unwise to place so much emphasis on this apparently 'weird' feature.  Complete symmetry is not the norm, and more importantly, the processes by which the Ediacara biota were preserved are not understood.  The more parsimonious solution would be the production of slightly offset symmetry by a taphonomic process.                                                    

Seilacher (1989) also proposed that the animals in the Ediacara fauna obtained energy by chemosymbiotic processes, in the manner of deep-sea vent communities.  Seilacher (1989) compared the "gutless" vent fauna (e.g. Riftia, a worm) to the Ediacarian fauna.  "Gutless" fauna have no mouth, digestive system or anus - apparently similar to the Ediacara fauna, but only if the preservation of the fauna to be perfect.  If Seilacher's (1989) "weird fauna" represents a separate kingdom then a chemosymbiotic mode of life would be feasible, but the fact that some of his kingdom do not fit casts doubt on the whole hypothesis.

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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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