[Taxacom] How do fossil mollusc taxonomists manage?

John Grehan jgrehan at sciencebuff.org
Thu Sep 24 07:15:26 CDT 2009

The situation Barry describes seems to be quite frequent in the biological world - where morphological characters conflict with each other to the degree that there is not always a clear-cut set of congruent characters to give an unambiguous relationship, or at least one that is not much better than several alteranatives. In this respect the orangutan-human evidence may be a bit of an exception with there being so many derived similarities compared with human-chimpanzee or human-gorilla. 

If recombination of ancestral characters is a major driver of vicariant differentiation (as suggested in panbiogeographic models) then character incongruence is to be expected, and some systematic relationships may never be permanently resolved. Under these circumstances I can understand the attractiveness of molecular similarity when it gives the appearance of a final result.

John Grehan

> -----Original Message-----
> From: taxacom-bounces at mailman.nhm.ku.edu [mailto:taxacom-
> bounces at mailman.nhm.ku.edu] On Behalf Of Barry Roth
> Sent: Thursday, September 24, 2009 1:24 AM
> To: Taxacom
> Subject: [Taxacom] How do fossil mollusc taxonomists manage?
> ... by the seat of our pants, perhaps.  In spite of rampant convergence
> and parallelism in shell form, I find that a close reading of fossil and
> Recent shell morphology often generates compelling evidence for
> identification and taxonomic assignment.  There is often enough
> independent evidence (the thermal and precipitational ranges of Recent
> members of the genera represented in a fossil faunule; the climatic facies
> of associated plant or animal remains; etc.) to make us feel we're on the
> right track.
> I have been encouraged by molecularly based phylogenies to look at, for
> example, family assignments for fossils that might not have occurred to me
> before.
> Not sure this addresses any of the theoretical issues, but it represents
> the practice as she is done.
> Barry Roth
> --- On Wed, 9/23/09, Stephen Thorpe <s.thorpe at auckland.ac.nz> wrote:
> OK, if the argument is this (see below), then it is stronger:
> Molecular evidence applied to extant hominids shows that the morphological
> characters that we have to rely on (for lack of any alternatives) in the
> case of fossil hominids are completely unreliable indicators of
> relationships. But does it really show that?
> Presumably, the case would be similar for molluscs, because conchological
> characters alone are not good for extant molluscs, but it is all we have
> in the case of fossils. So, how do fossil mollusc taxonomists manage?
> Presumably, if you accept that molecular evidence is a priori definitive
> of relationships (BIG assumption), then you need to try to find
> morphological characters that are congruent with the molecular results (in
> the case of extant taxa, and then apply the morphological results to
> fossils). If none can be found, then there is presumably little or no
> point in trying to classify fossils phylogenetically ...
> Stephen
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