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The Ediacara
fauna
The Ediacarans were life’s first experiment in
large-scale living – or so it was believed for a long time. We now know that algae had beaten them to it,
in the form of seaweeds.
The Ediacarans are classed as animals, although it’s
not too clear why. They had none of the
internal structure that we associate with animals. The experiment ultimately failed. Few if any creatures from life’s next stage
appear to have descended from the Ediacarans.
So
these fossils are the oldest large-sized examples known. They stem from the late Precambrian, which
had never yielded up fossils of any kind before (more).
In fact the late Precambrian rocks were teeming with fossils. But they were all microscopic, and nobody was
using microscopes to look for fossils at the time (more).
The first Ediacarans were found in the 1940s, in the Ediacara Hills of
The Ediacara creatures had no hard
parts, so most of them rotted completely away as soon as they died. But occasionally an underwater mud slide will
despatch an entire zone of living organisms – and preserve them for
posterity. And we have a few of these
to thank for knowing that these strange creatures ever existed. (See also the Burgess
Shale). And once palaeontologists knew where to look,
they began to turn up in various places around the world.
The Ediacara creatures were quite unlike anything that
scientists had seen before. So it’s not
surprising that it took a while for scientists to make sense of them. The story changed quite a bit over the years.
But a recent article in New scientist (14 Apr. 07) reckons that they’ve now 
been
sorted. This drawing of how the
Ediacarans might have looked comes from the article. It seems that they came in two quite separate
groups, one after the other (so to speak).
The first group were the rangeomorphs. They
emerged about 575 million years ago.
This was just 5 million years after the end of the terrible Varangarian ice age. (The N.S.
calls it ‘the Gaskiers’, but I’m sticking with Varangarian for the time
being.) They inhabited exclusively the
deep sea bed. They lasted for 15 million
years, and then slowly died out leaving no issue.
The rangeomorphs came in various sizes. Most would fit
comfortably into a shoe box, but the largest were over 4 metres long. On the face of it, this is ridiculous. Everything, but everything, that had gone
before was microscopic. 
And
now all of a sudden, we have these huge creatures appearing out of
nowhere. How was this possible?
The theory goes like this. In the aftermath of the ice age, even the
deep oceans became heavily oxygenated.
In addition, the oceans will have been bursting with organic matter,
released by the melting glaciers. So all
a creature had to do was to stand there with its mouth open, and food would
pour in. In fact the Ediacarans didn’t
have mouths. They absorbed the nutrients
through their surfaces. This meant that
the bigger you were, the better.
I don’t pretend to understand the following. But apparently the rangeomorphs were able to
grow big quite so fast because they had a ‘fractal’ design.
However after 15 million years conditions
changed. Either the supply of free food
dried up or the competition for it became too fierce. And the rangeomorphs had no answer. The last rangeomorph disappears from the
fossil record about 545 million years ago.
However, about 560 million years ago, a new breed
appeared, this time occupying the shallow waters. The arrival of the shallow-water Ediacarans
led to a golden age, with bio diversity reaching a peak for 10 million years. Most of them have no clear link to the Cambrian animals that came immediately
afterwards. But it seems that a few went
on to spawn genuine Cambrian creatures, and thus continue their line.
There are tantalising hints that some of the shallow-water
Ediacarans were remarkably jellyfish like.
Now a creature has been found, from well within the Cambrian period (151
million years to be precise) which palaeontologists think was a true descendent
of one of these. They’ve called it Stromatoveris. It’s the fossil on the left, which comes from
New Scientist 13 May 06. The picture on the right is of a modern ‘comb
jelly’, and it’s thought that the entire phylum of comb jellies (ctenophores) are direct
descendents of this little lad, Stromatoveris.
The fossil record
This creature has filled in an important gap in the fossil record.
Until it was found, there seemed to be a complete break between the
mysterious Ediacara fauna and the trilobites and others of the Cambrian
period. It was if there had been an
almost total mass extinction between the two periods. Now we can see that some creatures at least
lived through any upheavals there may have been. Indeed there may well have been no extinction
at all – just this gap in the record.
© C B Pease, December 07