In addition to the scarcity of early fossils (addressed somewhat by increasingly indtensive searching), there is the difficulty of deciding what qualifies as a genuine mollusk versus almost there but not quite.

The oldest animal that has been identifed as a mollusk is the late Precambrian (ca. 550 million years ago) Kimberella.  It is thought to have had a radula, as scrape marks similar to radular feeding traces have been found in the same deposits.  Definite early mollusks occur in the earliest Cambrian, around 540 million.  There are also some mollusk-like things in the Cambrian that may be early mollusks, transitional forms between mollusks and other related phyla, or something else.

Molecular data suggest that mollusks are closely related to brachiopods, annelids, sipunculids, etc.  Based on common features of these organisms, and on what seem to be characteristics of primitive mollusks, we can speculate on what the ancestor of mollusks might be like, but there's no direct evidence beyond what we can tell about Kimberella and the like.

As to identifying the ancestor of a particular family, it depends on how picky you are.  It's almost impossible to be absolutely certain that a particular fossil is the direct ancestor as opposed to an uncle or cousin.  However, a close connection can be inferred in many cases.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa