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Re: Family Nemegtosauridae

This reminds me of something which is bugging me.  One thing 
which both Upchurch (1995) and Sereno and Wilson (1998) agree on is 
that _Opisthocoelicaudia_ is a titanosaurid.  But 
_Opisthocoelicaudia_ has opisthocoelous caudals, whereas other 
titanosaurids have PROcoelous caudals (at least for the anterior 
part of the tail).  How did _Opistho.._ get opisthocoelous 
caudals from an ancestor with the exact opposite (procoelous) 
arrangement?  For this to happen the caudals must have passed 
through an amphicoelous/amphiplatyan phase, but this seems improbable 
to me.