the Ediacaran animals with modern groups. Thus, we have Ediacaran jellyfish,
corals, and worms -a continuity of evolutionary relationship across the
greatest of all geological boundaries. Yet, as I argued just a few months
ago in my column on conodonts (July 1983), the traditional ploy of forcing
old and problematical animal fossils into modern taxonomic categories often
fails badly. We must recognize that the early history of life should be
studded with failed experiments--small groups that never achieved much
diversity and bear only distant relationship with any modern animal. We
might expect that our oldest fauna should contain a large number of such
curiosities--yet all Ediacaran animals have been shoehorned, often with
considerable effort, into modern groups.
Dolf Seilacher now argues, turning the old view completely on its head,
that the Ediacaran fauna contains not simply a few creatures with no modern
analogues--but that every animal in it shares a basic mode of organization
quite distinct from the architecture of living groups. The entire Ediacaran
fauna, in other words, represents a unique and extinct experiment in the
basic construction of living things. Our planet's first fauna was replaced
after a mass extinction, not simply improved and expanded.
Dolf began by showing that the similarities of Ediacaran and modern
animals are misleading and superficial, and that the Ediacaran forms could
not work as their supposed living counterparts. Nearly all Ediacaran fossils
have been falsely fit into three modern groups: jellyfish, corals, and
segmented worms. Living jellyfish move by contracting a prominent ring
of concentric muscles located at the outer edge of their bell, radial
grooves for feeding lie within the concentric muscles, toward the center.
But the so-called Ediacaran medusoids have a reversed arrangement that
could not work in the same way: concentric structures surround the center,
and radial grooves lie on the outside.
Modern alcyonarian corals ("soft" corals, or sea pens) invariably
bear distinct branches, often springing from a common stem. The branches
must be separated so that water, bearing oxygen and nutrients, can reach
the individual polyps (members of the colony) growing on them. At first
glance, the Ediacaran "sea pens" look superficially like their
modern counterparts in general shape, but they form a continuous, quilted
structure, not a set of separated branches--and could therefore not
operate like a modern soft coral colony. The Ediacaran "worms"
are segmented and bilaterally symmetrical like their supposed modern analogues,
but so are many other creatures--and such a basic and repeatable architecture
need not imply close relationship. In other respects, the Ediacaran creatures
are most unwormlike. They may be up to a meter in length and flat as a
pancake--more like films than the substantially thickened bodies of
most modern segmented worms.
After tracing the differences between Ediacaran animals and their supposed
modern counterparts, Seilacher examined the similarities that unite all
Ediacaran forms. They seem to share an architecture only rarely utilized
by modern animals--and not by any living creature ever linked to an
Ediacaran fossil. They look like ribbons, pancakes, and films, sometimes
slightly "blown up" as air mattresses with a foliate or quilted
structure.
The Ediacaran animals evolved before any creature had invented mineralized
skeletons or any external hard parts. Perhaps their unique Bauplan
(to use the convenient German term for a basic scheme of organic architecture),
or "building plan," records a pathway to large size that animals
without supporting hard parts might follow--light and thin structures,
woven together for added strength. In any case, and following a favorite
theme of these columns for more than a decade, the Ediacaran fossils seem
to represent one of two possible solutions--the one not followed
by modern animals--to the basic structural problem of large size:
the imposed decline of surfaces relative to volumes since surfaces (growing
as length squared) must increase more slowly than volumes (growing as
length cubed) as objects of similar shape get bigger. Since so many organic
functions depend upon surfaces (respiration and feeding, to name just
two) yet must serve the entire body's volume, this decline in relative
surface cannot be tolerated for long.
Of the two possible solutions, most modern animals have retained their
rounded or globular shapes but have evolved internal organs to increase
surface areas--lungs for respiration and the complexly folded surface
of the small intestine for absorption of food, for example. Another potential
solution, followed rarely today but exploited by some large parasites,
including tapeworms, permits large size without any internal complexity
by changing the body's basic shape into something very thin--a ribbon
or pancake--so that no internal space will be far from the external
surface, the only locus of respiration and absorption of food in the absence
of internal organs. The Ediacaran animals, as a group, have followed this
second pathway to large size and therefore
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