>3) The transition from univalved helionellid mollusks
>(monoplacophorans) to bivalves through a group of rostroconchs in the
>early Cambrian seems now quite well established. There are now know
>intermediate specimens that occur in the chronological position.
>(Gubanov, Kouchinsky, and Peel, 1999, The first evolutionary-adaptive
>lineage within fossil molluscs: Lethaia, 32:155-157.)
There is some debate on the exact classification of the forms. The
monoplacophoran-bivalve transitional forms may be better classified as
monoplacophorans than as rostroconchs. (I.e., my advisor calls them
monoplacophorans and most people now seem to at least admit they are not
well-developed rostroconchs.) Watsonella is the key genus, showing both
monoplacophoran and bivalve features, as well as a bit of a feature
sometimes interpreted as a pegma, which would tie it to rostroconchs. We
do not consider it to be a true pegma in our cladistic analysis (in press).
Standard rostroconchs seem out of the bivalve line, probably on their own
dead end branch, though they survived well past the Cambrian.
Monoplacophora is at least one class of mollusks, the ancestral stock for
the classes of snails, clams, squid, etc. Rostroconchia is another class.
David C.
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