Big only in the sense that there is nothing else...they really are quite
small...but since that is all there is, they are big. Precisely why they
should not be used to discredit the Cambrian explosion. They, like the
Ediacara whatever-they-ares, are just below the usual Precambrian boundary.
They still are not deciphered, and it looks like most of them are probably
of plant origin...but even if they do represent animal morulae, where are
the adults, and why do we only find morulae? These are of course questions
everyone else, especially the discoverers are eagerly seeking answers
to...again suggesting we not be too quick to blow the whistle on the
Cambrian explosion.
The following account describes an event that took place at a GSA meeting I
attended a couple of years ago in New Orleans. I quote from one of my
colleagues:
"One of their [evolutionary scientists] biggest assumptions was that the
molecular clock is reliable.... When Levinton gave his paper [at the 1996
GSA meetings in New Orleans] he stated that the molecular clock can be
best compared to a sun dial in the shade, which isnāt very encouraging
for his method, but he and his colleagues still believed that it yielded
data sufficient to test the theory of the rapid evolution of life at the
base of the Cambrian....
From their molecular clock data they concluded that the initial
divergence of metazoan life forms occurred about 1.2 billion years ago
(+/- 50 to 250 million years) . The base of the Cambrian is currently
dated at about 543 million years ago , so their conclusions require a half
billion years of metazoan history before the Cambrian. They also
concluded that the beginning of Metazoan phyla was not an explosion,
but was somewhat spread out during that half billion years.
A couple of days later these papers were discussed in a 'Hot topics
discussion' during the noon hour. Four scientists gave brief
presentations on the new ideas about the Cambrian explosion, followed
by audience questions and comments. Many questions dealt with
technicalities of their research method, but two questions stand out. A
little background is necessary before dealing with these questions. The
proposal that complex metazoan animals, ancestral to such things as
molluscs, trilobites, vertebrates, sea urchins, corals, and many others,
existed for a half billion years before the Cambrian implies that they
lived all that time without leaving a fossil record. This pretty much
requires that before the Cambrian they existed as soft worm- or
larvae-like forms, with the general genetic make-up of the Cambrian
groups but without their skeletonized morphology.
Now the questions. The first of the two questions was - why are trace
fossils (fossil tracks, trails, and burrows) so rare before the base of
the
Cambrian, if these animals existed for that half billion years? An
internationally recognized expert on trace fossils stood up, presumably
to answer the question. However, he talked about other things and the
very important question never was answered. At the end of the
discussion another scientist stood up and commented on the implication
that all the skeletonized phyla developed skeletons at about the same
time in the Cambrian. He asked - why are all these types of animals living
for so long and then all making skeletons all at once? He then asked,
with some vigor - 'Why are you avoiding the real question?' After a
pause, one member of the original presenters answered 'because itās
really hard (a hard question)'. He went on to say that they hoped
answers would come from further study of developmental biology.
These two questions were apparently not asked by people who doubted
the evolution theory, but by evolutionary scientists willing to ask the
hard questions that need to be addressed as they try to test between
different hypotheses. The fact remains that the Cambrian explosion is
one of the big challenges to naturalistic theories that still remains
unanswered."