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RE: titanosaurs



Dinogeorge wrote:
 
> This is what I call a "synapomorphy war": a deluge of incredibly minor 
> characters of dubious taxonomic utility thrown together in a torrent 
> purporting to show that a number of specimens belong to the same taxon.

Not sure what you define as "minor".  Whatever their function, these "minor"
characters are the product of evolution.  How can you determine that the
shape of the ulna in titanosaurids is less important than the opisthocoelous
caudals of _Opisthocoelicaudia_ - have you some insight that the latter is
more important to an animal's functional anatomy than the other?  I'm
guessing that you can't.  Therefore, there's no reason to sort characters
into "major" and "minor" based on an intuitive notion of it's importance in
evolution.

> In truth, for any two specimens, there is at most >one< character that 
> is a true synapomorphy for them (and it might not even be in the list 
> of matching characters, such as the above). All the other similar 
> characters are either convergences (because they arose independently in 
> both lineages after they split from the common ancestor) or 
> plesiomorphies (because they were already present in the lineage 
> leading up to the common ancestor). The likelihood of two or more of 
> these characters arising together at >exactly< the same time 
> in the common ancestor species is nil. 

This made very little sense to me.  Maybe I need more caffeine.  Members of
certain clades can be united by a large number of synapomorphies.  Gaps in
the fossil record don't allow us (yet) to resolve the precise sequence in
which these derived features first appeared.  A large number of apomorphies
accumulate in ghost lineages - if fossil discoveries tell us the order in
which these apomorohies appeared, they wouldn't be ghost lineages.

> (I already noted, for example, that non-forking haemal spines is a 
> plesiomorphy for Dinosauria and should not even appear in this 
> particular synapomorphy war.)

Well, it's relevant if this character is present in basal sauropods.  To use
an analogous example, are you proposing that absence of legs cannot be used
as a synapomorphy for snakes because primitive vertebrates (e.g. lampreys,
hagfishes) don't have legs either?  According to your thinking "absence of
legs", being a primitive vertebrate trait, is therefore precluded from being
used in the classifation of higher clades of vertebrates - such as snakes?

"Plesiomorphy" and "synapomorphy" are context-dependent - characters that
are synapomorphic at one level of phylogeny may be plesiomorphic at another
level.



Tim