To add some reference for people who are confused on what |existed" in what time-frame, here is a site I like to look at from time to time. Its not too in depth, makes it easy for lazy people who hate wading through too much text.
Geological Time Scale
Edit: Although this creature was thought to be a Permian Period creature... Maybe Mollusks where dominant back then too but this period ended with "extinction" so who knows...
edit on 11/10/2012 by TheSparrowSings because: (no reason given)