>I can think of one possibility[for teh sudden appearance]. If animals
lived off of microbes in the
>precambrian, and then began eating each other in the cambrian, it would
>give rise to a selective pressure for protection i.e. shells. That would
>make the animals of the Cambrian much more preservable.
Good try, tho not original, after all, what else can you say? but a bit of
a stretch to understand why all of these phyla should do so simultaneously,
and with completely different, mature mechanisms, that still function the
same way today (ie. not experimental). This is all assuming they even had
the capacity to develop hard parts (which clearly they did, since they all
developed them, didn't they???).
Art
http://biology.swau.edu