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RE: Family Nemegtosauridae



 

-----Original Message-----
From: Dinogeorge@aol.com

Since I think putting _Opisthocoelicaudia_ into the titanosaurians is
yet 
another big cladistic mistake (like putting segnosaurians into 
Theropoda)--for much the same reasons that you list (the earliest and
most 
primitive titanosaurians already show strongly procoelous caudals)--let
me 
also add that _O._ would not only be the only titanosaurian with 
opisthocoelous caudals, it would also be the only titanosaurian with
bifid 
neural spines, a character otherwise found in camarasaurids,
euhelopodids, 
diplodocids, dicraeosaurids, and such but not in titanosaurians. These
are, 
as far as I'm concerned, two >major< strikes against considering _O._ to
be a 
titanosaurian, and they outweigh those minor little features that look
like 
titanosaurian features but probably arose convergently several times
within 
Sauropoda.
----------------------------

I find it a little ironic that one of the characters you consider a strike
against the titanosaurian nature of Op., the bifurcation of the nueral
spines, is highly incongruous with other characters. It has almost certainly
evolved on multiple independant occasions. There are at least three,
probably six such instances in sauropod phylogeny. They include the
euhelopodids (which becomes two instances if the family is not monophyletic
and Euhelopus is a somphospondyl), advanced diplodocimorphs and
Camarasaurus. Phuwiangosaurus becomes the fifth if it is correctly placed as
a basal titanosaur and Op. is of cousre the sixth if it is a derived
titanosaurid.
Whereas some of the characters that do link Op. with titanosaurids (or a
subgroup of titanosaurids) such as the shape of the humeral head are not
found elsewhere in the Sauropoda.
I am not entirely convinced that Op. is a derived titanosaurian but I do not
believe that the two cited features are enough to prevent all further
consideration of the hypothesis.

cheers

Adam Yates