>'What came first, the mollusk or the shell' or does nobody know?<

The answer is a little tricky because the shell is usually what gets fossilized.  Thus, a mollusk without a shell is very unlikely to leave much trace.  Also, there is the question of what counts as a shell and what counts as a true mollusk.

The oldest known mollusk-like animal is Kimberella.  It did not have a hard shell, but probably had a tough organic layer on its back.  Also in the late Precambrian, there are fossil scratches that look just like feeding traces from a mollusk scraping algae with its radula.  No shells are known from this time.

The modern aplacophorans (a groups of worm-like mollusks preserving some primitive features) do not have a shell, but they do have tiny spicules over much of the body.  The shells of more advanced mollusks may be derived from the fusion of such spicules into a more solid shell.

Chiton shells are very different from those of the other shelled mollusks, and it is suspected that they got their shells independently of the others.

There are also fossils with part of the body covered with spicule-like structures and part with larger plates like a chiton.  Halkieriids are one such group.

Thus, it looks like the first mollusk probably did not have a solid shell, but it might have had a coat of tiny spicules or a tough organic layer, the precursor to the shell.

    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