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RE: Family Nemegtosauridae
> Might Op. be the
> ultimate expression of this trend? If so the intermediate would have
> opisthocoelous anterior cadauls joined to procoelous posterior cadals
> biconvex vertebrae (or perhaps a short transtion zone somewhere in th
> midlength of the tail).
Except that procoely in titanosaurid evolution BEGAN at the anterior
end of the tail, then progressed more posteriorly to include the mid-
caudals too. If I understand you correctly, what you're suggesting
is some sort of tug-of-war between the anterior caudals
(opisthocoelous) and the posterior caudals (procoelous) over what the
entire tail should be.
Thats sort of what I was suggesting. Think of it more like a progressive
trend of "opisthocoelisation" starting in the neck and heading towards the
tail as you get closer and closer to saltasaurine titanosaurids. I am not
suggesting a tug of war, merely that the opisthocoely is a dominant
phenotype that overprints other types of central morphology (so it wouldn't
matter that procoely began in the anterior tail). We know that the last
sacral - first caudal joint suddenly switched from having the ball on the
posterior face of the last sacral and the socket on the anterior face of the
first caudal to the opposite condition (socket on posterior face of last
sacral and ball on anterior face of first caudal) somewhere deep in
titanosaurid evolution, so why couldn't it happen to the rest of the tail?