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Name: Ediacara Assemblage |
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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:
- They have been classified as metazoans of extant phyla
("metazoan ancestors") in or near extant classes or orders by,
for example, Glaessner (1984) and the original workers, including Sprigg.
- Traditionally this view has been strongly opposed by Seilacher
(1989, 1992) who classified all the organisms as a "weird fauna"
or "doomed experiment" in an extinct kingdom called Vendobionta,
later however, Seilacher and Buss (1994) modified this to an extinct
metazoan phylum.
- Fedonkin adopted a third approach in the 1980's, based on the
idea that the Ediacaran fauna should be considered "not as ancestors
of the Phanerozoic fauna, but as descendants of the older Precambrian
metazoans, still unknown to us". He used differences in symmetry to
place the fauna in major taxonomic groups.
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).
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Australians |
Fedonkin |
Seilacher |
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Coelenterata |
Coelenterata |
Coelenterata |
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Hydrozoa |
Cyclozoa(*) |
Psammocorallia(*) |
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Ovatoscutum |
Ovatoscutum |
Beltanelliformis |
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Scyphozoa |
Beltanelliformis |
|
|
Ediacaria |
Ediacaria |
Trace Fossil |
|
Mawsonites |
Mawsonites |
Mawsonites |
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Anthozoa |
Trilobozoa(*) |
|
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Charnia |
Albumares |
|
|
Charniodiscus |
Tribrachidium |
|
|
Rangea |
|
|
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Petalonamae(*) |
Petalonamae(*) |
Vendozoa(*) |
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Charnia |
Charnia |
|
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Charniodiscus |
Charniodiscus |
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Pteridinium |
Pteridinium |
Pteridinium |
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Phyllozoon |
Phyllozoon |
Phyllozoon |
|
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Rangea |
Rangea |
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Annelida |
Proarticulata(*) |
Ovatoscutum |
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Dickinsonia |
Dickinsonia |
Dickinsonia |
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Spriggina |
Praecambridium |
Spriggina |
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Arthropoda |
Arthropoda |
Albumares |
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Praecambridium |
Spriggina |
Tribrachidium |
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Incertae Sedis |
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Albumares |
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Tribrachidium |
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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.
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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