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Re: Is *Amargasaurus sattleri* a Parent Clade for Dicraeosaurines?



<Any system of taxonomy is entirely artificial and based on arbitrary
distinctions.  If we in our minds combine organisms that lived at different
times and in different places as a "clade" or a "family" or whatever, we are
creating a concept that didn't exist before we thought it up.>
You are combining two ideas here, uncomfortably.  (Though this is a good
argument, enjoyable.)  First, any 'species' after inception is descended
from another species.  Second, in order to deduce relationship 'we in our
minds' create a system which identifies supposedly significant characters
and uses them  to establish  connections.  The difficulties created by the
limits on our observations do not obviate the reality of descent.

<Even the concept of an individual organism is rather arbitrary since we are
really a huge symbiotic colony of cells...>
Actually, because new species are often created in isolated populations, the
opposite is true.  It is possible that a particular change in a group began
with a single individual.  (This is not the same as arguing for the logical
necessity of unique events, such as feathers arising only once.  Given a
number of initially very similar separate populations, the idea that a
particular change could occur in only one of those populations is difficult
to defend.)

< The system will NEVER be really natural, but it can be made either more or
less useful to us as a means of categorizing...
When a taxonomist puts a genus name in front of all the species in a clade,
the reason he draws the line at thier sister taxa has nothing to do with
common
descent; he sees some specific feature or features in that clade that is not
possessed by the sister taxon that he FEELS is an important distinguishing
point.>
You've drawn the correct logical conclusion from your premises.  The problem
is that modern systems of classification do attempt to draw evolutionarily
significant lines.  That they fail (or cannot demonstrate success) does not
change the intent.
>From what little I know, you seem to be urging a Linnaean approach as
contrasted with an evolutionary approach.
Note that this discussion has not included the question 'what is a
species?'.  However defined, the concern is the systematic linkage of
species.  Any system will distinguish between obviously different animals,
such as birds and salamanders.  When you say:
< If we want a system of classification that reflects phylogeny however, we
need to recognize that they only look unique to us because all the
intermediates are dead.>
you're exaggerating.  They look different to us because they are
substantially different.
We need to decide where to draw species lines between populations (an
arbitrary distinction, but one that can be made) and we need to develop a
means of identifying relationship (an arbitrary logic).  But these arbitrary
decisions do not mean that the subject of analysis (animals are different
and those differences arose from descent with change) is itself arbitrary.