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Paraphyly and sister taxa (was Re: AMARGASAURUS SATTLERI)

Dinogeorge wrote:
< It's difficult (though not impossible) to avoid paraphyletic genera in
<situation. If D. sattleri and A. cazaui are sister groups, then D sattleri
> could indeed be placed in the genus Amargasaurus. But then what happens to
> the common ancestor of Dicraeosaurus and Amargasaurus?

Nick P. wrote...
>Let's worry about that when we dig it up and recognize it as such, OK?
>We only have to name and classify the individuals available for study
>(thank God!).

    But the same problem applies to ALL sister taxa in any given cladogram.
If you have two different taxa, "A" and "B", there are three and only three
possible idenities for their common ancestor:
1. The common ancestor is the same taxon as "A", in which case "A" is
paraphyletic with regard to "B".
2. The common ancestor is the same as "B", in which case "B" is paraphyletic
with regard to "A".
3.  The common ancestor is neither "A" or "B", in which case it is DOUBLY
    Paraphyly is implicit at EVERY NODE where the sister taxa are not
identical, which means at every node except where individual specimens
assigned to the same genus and species are sister taxa.  Unless we are
willing to place every single organism in the same genus and species, there
is no possible system of taxonomy that can eliminate paraphyly.  Taxonomy is
useless WITHOUT implicit paraphyly.

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