[Taxacom] Were bivalves the first molluscs to evolve?

[David Campbell]

Molecular data indicates closer relationship between the extant
monoplacophora and the polyplacophora than between the monoplacophora
and the other univalve/bivalve forms.  Exact relationships between the
various fossil "monoplacophora" and extant ones, and between various
early Paleozoic multi-sclerite taxa and the modern polyplacophora and
aplacophora are unclear, but there are likely early Cambrian
representatives of the polyplacophra/aplacophora/monoplacophora group.
Neither morphological nor molecular analyses have supported
euthyneurans being basal to streptoneurans, nor for cephalopods to
arise within gastropods.  Euthyneurans are well-supported
morphologically, molecularly, and by timing of appearance in the
fossil record as being relatively derived gastropods showing varying
levels of detorsion, not partiallytorted ancestral gastropods.

Molecular resolution of the relationships of major groups in the
Mollusca remains relatively poor, but the extent of genomic coverage
is increasing.

-- 
Dr. David Campbell
Assistant Professor, Geology
Department of Natural Sciences
Box 7270
Gardner-Webb University
Boiling Springs NC 28017