>Dear Conch-l,
>
>I was wondering... what the current thought is as to when molluscs first
>appeared in history?  What did molluscs descend from?  What order did
>molluscs appear?  What descended from molluscs?

The oldest known mollusks are late Precambrian, about 570 million years
ago.  Kimberella is generally accepted as a primitive mollusk, and there
are also some traces of radula scratches, showing that some primitive
mollusk had been feeding on encrusting algae.

Also, there are a variety of small shelly fossils of uncertain affinities.
Many of these may have been small pieces of armor from a larger animal with
many individual plates (a bit like a chiton, but often with many more
plates per animal).  Some are mollusk-like, but the exact affinities of
them are very uncertain.  This also gets into the problem of defining
mollusks.

Various other invertebrate phyla are thought to be close relatives of
mollusks.  Recent DNA work and studies of developmental patterns suggest
that within the protostomes there are two major groups.  The arthropods and
other phyla that regularly shed their outer layer form one group.  The
other, including mollusks, annelids, brachiopods, bryozoans, and various
other groups, tends to have a trochophore larva.  Within this group, the
exact relationships among phyla are uncertain.  Sipunculid worms and
endoprocts have been suggested as close relatives of the mollusks based on
anatomical data.  Brachiopods seem to be close relatives of mollusks in my
DNA work, but more data are needed on other groups of mollusks and other
phyla.

Kimberella does not fit into any modern group of mollusks.  Various
monoplacophorans, bivalves, and probable gastropods are present in the
Early Cambrian, about 540 million years ago.  Chitons and cephalopods are
known from the late Cambrian, although Ellis Yochelson questions whether
Matthevia is truly a chiton.  Scaphopods appear by the Ordovician.  Several
extinct classes also appeared in the Cambrian, including paragastropods,
rostroconchs, and hyolithids.  Some paleontologists consider hyolithids as
a separate phylum, closely related to mollusks.  Aplacophorans have almost
no fossil record; however, Amelie Scheltema suggsts that the Middle
Cambrian Wiwaxia may be related to the aplacophorans, or possibly a generic
primitive mollusk.

Apart from the question of earliest ancestry, mollusks are not thought to
have given rise to any other groups.  However, there are some strange
things that turn out to be mollusks.  Abbott claimed that the shellless
pteropods (Gymnosomata) were made from the scraps left over from making
normal mollusks.  Entoconchids are gastropods but look more like some
parasitic worm, being endoparasites of sea cucumbers.  Xenoturbella is a
small saclike worm with hardly any development of internal structure, yet
its development and DNA suggest that it is a modified nut clam (Nuculoida).


Dr. David Campbell

"Old Seashells"

Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA

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919-962-0685
FAX 919-966-4519

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug