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Re: Quadruped theropods?

Graydon wrote:

Well, a terrestrial quadrupedal stance the mass of the body providing a
force vector down, and the structure and muscles of the limb providing a
resisting force between the body and the supporting surface.  This tends
to lead to things like pads on the feet and arrangement of the skeleton
and connective tissue to take loads via passive support as much as
possible.  Those loads, and the main structural loads, have to pass
through the forelimb where (even in sprawling lizards) the stress is
compressive and more-or-less perpendicular to the axial skeleton.

This would still apply during horizontal arboreal quadrupedalism, wouldn't it? Like walking along a tree branch, for example?

The arboreal clambering stance has the mass of the animal hanging from
the forelimb; the resisting force is above the body, and the alignment
of the stress is more-or-less parrallel to the axial skeleton.

I guess I'm not sure what you mean by arboreal clambering. Do you mean suspensory locomotion and brachiation, like in spider monkeys and arboreal apes (perhaps I'm reading too much into the term 'hanging')? In any case, this may not matter, given that I was referring to the use of all four limbs in locomotion (with the front and back limbs in contact with the same substrate at some point) when I called microraptorans and early birds 'arboreal quadrupeds'. In these critters, both the fore- and hindlimbs may have been used to move the theropod both vertically and horizontally in the trees. But this is highly speculative, given that the biomechanics of quadrupedal climbing in theropods are so poorly understood - if indeed they did climb at all.

I'm dubious that these are equivalent locomotor postures.

I won't even try and answer that, given that primatologists still debate the difference(s) between arboreal walking and climbing, and how the changeover occurs from one to the other with increased inclination of the substrate.

Cheers

Tim