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Re: Family Nemegtosauridae



In a message dated 4/8/99 9:15:20 AM EST, TWILLIAMS@canr1.cag.uconn.edu 
writes:

<< In more derived titanosaurids like _Alamosaurus_ and _Saltasaurus_ 
 procoely extends through the mid-caudals too.  According to recent 
 cladistic analyses, _Opisthocoelicaudia_ belongs among these derived 
 titanosaurids.  This means either that procoelous caudals evolved 
 independently among certain titanosaurid genera (not so unlikely in 
 my view, since some basal titanosaurs show anterior caudals that 
 are only mildly procoelous), or that _Opisthocoelicaudia_'s ancestors 
 were procoelous, then reverted to the amphicoelous/platyan condition, 
 then became opisthocoelous.  I still think this transformation is a 
 little improbable. >>

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.