>cladists! There's more to it than a single gene for God's sake! When
> they've looked at all the genes, get back to me. The trigonaceans may not
> even be ancestral to the unionaceans, despite "conventional wisdom." The
> Pachycardiids are probably the most closely related group, but they are all
> extinct. The study you mentioned found Neotrigonia to be the most similar
> among the groups used, but then they could only use living animals -
> pachycardiids would never even enter into their database. It would be like
> a DNA study of Nautiloids -- of course they would find the Coleoids the
> closest because they have no Ammonoids to compare. It's a shoe-horn
> approach to pop taxonomy.

Although I am somewhat sympathetic to your misgivings about cladists, the COI
study is not the only evidence.  Unpublished 18S and 28S data also show
Neotrigonia to be very similar to unionids, although hyriids were not in that
data set.  The morphological study by Graf supported the hyriid-muteloidean
grouping.  Pachycardiids and unionoids do unite with trigonioids in unpublished
morphological cladistic studies, so pachycardiids would be better if we could
get fossil DNA but trigonioids are not bad.  No one has gotten credible
pre-Pleistocene DNA, and even the dubious claims for amber would not help much
for clams.  Certainly there are some COI results that I do not believe, at high
taxonomic levels.

David C.