fossils as roots

[Una Smith]

Hilary Davis wrote:

>>I'd like to hear some opinions on the use of fossils to root a cladogram
>>or tree.  Is this a valid technique given the possibility that a fossil
>>assemblage for a particular extant group may not be complete?

Curtis Clark wrote:

>Do you mean use a fossil as an outgroup? It should work as well as any
>extant species: if it weren't extinct (a "living fossil"), and you'd have
>no hesitation in using it then, its extinctness shouldn't be an issue.

I think that Hilary is asking about using the fact that a taxon is
extinct as the reason to select that taxon as the outgroup.  For a few
years, this was suggested as an easy way to root phylogenetic trees,
but this method of selecting the outgroup has fallen into disrepute.
As Curtis says, the extinctness of a particular group is not by itself
relevant to the question of the correct root of a given phylogenetic
tree.  Of course, an extinct group may indeed be the best outgroup to
use, but I think you need reasons other than its extinction or age to
support its selection as the outgroup.

        Una Smith               una.smith at yale.edu

        Department of Ecology and Evolutionary Biology
        Yale University
        New Haven, CT  06520-8106

        http://pantheon.yale.edu/~una/