I'll tackle this, at the risk of saying something innacurate based upon
insufficient info-seeking.  //Regarding early mollusc diversification,
this is a good site to start with:
http://www.palaeos.com/Paleozoic/Cambrian/Cambrian.htm - of course an
internet search can't compare to what could be gleaned from a University
library, but if you dig long enough and hard enough, some good info can
usually be found.  Molluscs were not a major componant of the Cambrian
fauna - the fossil record suggests they radiated a lot more in the
Ordivician.  That said, most of the major molluscan groups were present
by the end of the Cambrian.  So, if there indeed was as pronounced an
"exposion" of higher order taxa in the Cambrian, molluscs would have
been a part of it, not before or after.

-Ross M.