Ponder and Lindberg (eds), 2008, Phylogeny and Evolution of the
Mollusca, U Calif. Press, would be another good source.

There are two different types of objections to HAM (Hypothetical
Ancestral Mollusk).  One is the general aversion that cladistics has
to ancestors.  As many mollusks have good fossil records, we probably
do often deal with genuine ancestors  with some frequency and so
probably need not take that too seriously.   However, the more
important issue is whether the classic HAM actually matches up with
current ideas on what the acestral mollusk would be like.  Models for
mollusk (and arthropod, for that matter) evolution up to very recently
often suffer from the assumption that the ultimate ancestor would have
been a polychaete annelid or something very much like a polychaete.
Although annelids are relatively close kin to mollusks (along with
bryozoans, brachiopods, phoronids, nemerteans, entoprocts, etc.), the
common ancestor of mollusks and annelids would be relatively generic,
not a full polychaete.  In particular, the ancestral mollusk would
very likely not be segmented, and it's questionable as to whether
there would be any serial duplication of organs.  Kimberella, a
probable mollusk relative or early mollusk from the Precambrian, seems
to have a tough but non-mineralized dorsal "shell" and a creeping
foot; a radula is possible (no direct evidence in specimens, but
traces like those left by radular grazing co-occur).  Several early
Paleozoic mollusks or mollusk-like things had multiple small shells,
not to mention modern aplacophorans and chitons, so it's not clear how
many shells HAM ought to have and how much of the "shell" is
mineralized.


--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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