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quadrupedal dinosaur locomotion
With regard to quadrupedal dinosaurs and the way their forelimbs move, it
seems to me that there are pretty stringent limits on this imposed by the
trackway evidence. I have heard it suggested that the humeri can deflect
considerably from vertical and the animal still produce trackways such as
we have. Greg Paul has commented on this, and needless to say he is more
familiar with the material than I am. The details of the trackway evidence
should tell us much about the dynamics of limb action. It is not merely a
matter of the spacing of tracks. The application of pressure and the way
the foot twists at different times should tell us a great deal.
Much has been made of the difference in length between the fore- and
hindlimbs of ceratopsians. I don't really see how this forces their humeri
outward. Mammals such as raccoons, armadillos, and anteaters have short
forelimbs yet hold their humeri vertically. It gives their running a sort
of "bouncy" quality. If the details of the shoulder articulation support
the view that the humeri were not held vertically, that's fine, but this
seems to be a bone of considerable contention.
It seems to me that ceratopsians are designed for rapid pivots, as are
stegosaurs. I believe their shoulder articulations reflect this. In the
course of pivoting rapidly in a fixed location, no doubt their humeri will
deflect considerably from vertical. Their shoulder girdles should reflect
this ability to shuffle sideways rapidly and this may be at the root of the
controversy about how their limbs articulated.