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Brain is quite correct. Evolving a hyper-long neck just to low browse is
maladaptive in evolutionary and energetic terms. A very long neck requires a
correspondingly long trachea. The latter dramatically increases the
respiratory dead space, inceasing the energy needed to breath. So evolving a
long neck in order to save foraging energy actually ends up increasing the
energy budget. Add to that the energy needed to grow such a long neck. Only
high browsing explains such extraordinary necks. 

As for Mamenchisaurus, the articulated skeleton in the ground clearly showed
the neck kinked strongly upwards at its base, as in Euhelopus and
Camarasaurus. So it was carried erect, not horizontal. The arms were probably
fairly long too. Mamenchisaurus was giraffe-like, not low slung.