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For those who do not think that elephants rear to feed in the wild, see the
gorgeous photograph that graces the cover of the Nov Natural History. Of
course, elephants do not stand bipedal often, but then they have not a single
adaptation for rearing. Sauropods were all better suited for standing on two
legs because they bore most of their mass on their hindlimbs which were
invariably thicker boned than the fore (even brachiosaurs), and had big tails
as counterweights. 

On a related matter, the digital restorations of sauropod neck posture (Nov
Discover) seem to assume that the nuetral posture (in which the zygapophysis
are in fully articulation and the centra faces are flat to one another) is
the normal neck carriage. Many moons ago I articulated a giraffe-neck, and
the nuetral posture has the neck sloping only about 30 degrees above
horizontal, when the normal posture is 70 degrees. The reason for this is
obvious. If the neck's "nuetral" posture is also close to or at its maximum
vertical reach, and the latter is near vertical, then depressing the neck to
ground level would not be possible. So nuetral posture is at an intermediate
angle that allows a reasonable range of motion in both directions. 

Although the neck base is too poorly preserved to be sure, it is possible
that brachiosaurs has a giraffe-like neck posture. Nuetral posture may have
been subhorizontal, but the well developed, giraffe-like withers suggest that
nuchal ligaments normally held it at 50-60 degrees. Even more interesting are
camarasaurs, mamenchisaurs and euhelopids, in which the neck base was kinked
strongly upwards.