I would add that tamanduas and other anteaters have quite
long forelimbs, whereas those of alvarezsaurs are short and stubby.
I think this is because the anteaters suffer from a constraint: their
forelimbs must be long enough for walking, or at least climbing. The bipedal
alvarezsaurs (and, while I am at it, tyrannosaurs) were able to fully
exploit the mechanical advantage of "reducing the arms to the max" -- this
brings the distal ends of the bones closer to the muscle attachment sites,
and this in turn increases the force that can be applied (at the expense of
speed, which is probably useless in these cases).