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made sense to prevent total loss of pressure
(and hence death) when only one or two
segments were punctured. They were
quilted organisms, similar to inflatable
mattresses. The high surface area to mass
ratio suggests that they respired through
the skin; they may have been
photosynthetic, as suggested by
McMenamin (1998), used photosynthetic
symbionts like modern corals, perhaps were
chemosymbiotic (with chemosynthetic
symbionts so they could survive in deep-
water, reducing environments), or may
have ingested materials through the body
wall. Seilacher suggested that such
organisms could survive without bony
skeletons or shells because there was little
predation, but that the kingdom became
extinct with the onset of predation at the
beginning of the Cambrian.
In the 1980s, Fedonkin devised a
classification scheme for the Ediacaran
fossils based on their symmetry and
thus independent of any biological
interpretation (1990). He divided the
organisms into two major groups: Radiata
(disc-shaped, no bilateral symmetry) and
Bilateria (bilateral symmetry apparent).
Radiata can be further subdivided
into: Cyclozoa, with a concentric pattern
(e.g. Aspidella ); Inordozoa, with a radial
pattern; and Trilobozoa with a three-rayed
symmetry (e.g. Triforillonia ). Other radiate
forms were considered to belong to the
cnidarian classes Conulata or Scyphozoa.
Bilateria may show no particular head or
tail (i.e. bidirectional, e.g. spindle-shaped
forms), or have a distinct head or rooting
structure (i.e. unidirectional, e.g. frondose
forms). Intermediates exist, for example,
between concentric and radial forms of
Aspidella . One problem with this scheme,
as pointed out by Jim Gehling, is that it only
works if the organisms are flattened, yet
many reconstructions show the creatures as
quite three-dimensional.
In a strange twist to an already bizarre
story, Greg Retallack of the University of
Oregon looked at the indeterminate
growth pattern of many of the Ediacaran
organisms, which suggested that they had
no upper limits nor definite bounds to
their size and shape (Retallack, 1994). He
also studied taphonomic aspects of the
Ediacaran biota, particularly their
compression. The conclusion? That
Ediacaran fossils were lichens! Lichens are
composite organisms formed by a
symbiotic relationship between green
algae (which provide photosynthesis) and
fungi (which provide bulk). Few
paleontologists have accepted the
arguments presented by Retallack. While
some Ediacaran organisms show a distinct
holdfast (or head), and their growth
patterns may be less determinate than
worms, for example, they are not random;
and the sheer size and bulk of many forms
is quite unlike that of modern lichens.
Moreover, modern lichens are terrestrial,
not marine.
So what is the consensus? We can
perhaps go someway towards a
reconciliation of these apparently
disparate theories by taking a slight retreat
from Seilacher's (1989) view that they
represent a separate kingdom, and
considering instead that they represent
a separate phylum (Buss and Seilacher,
1994), an animalian grade of organization
more developed than simple sponges,
in which few cell types are present, but
far less evolved than the higher Eumetazoa
in which tissues and organs have
developed (see also Dewel et al ., 2001).
Many of them seem to be colonial and
similar in many respects to cnidarians
(jellyfish, anemones, and corals),
although not possessing all of the defining
cnidarian features (such as a mouth
surrounded by a ring of tentacles).
Perhaps, therefore, they are on an
evolutionary pathway towards the
cnidarians (which are, after all, the
simplest forms of eumetazoans), but have
not yet attained the true cnidarian state.
In this case we can consider them (or at
least most of them) to belong to a single
clade, which is at an early phase of the
development of the metazoan body plans
that we know today. Classifying these as a
'cnidarian-like' phylum of animals is not
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