Re: Quadruped theropods?

[Graydon <oak@uniserve.com>]

On Mon, May 22, 2006 at 02:23:28PM -0500, Tim Williams scripsit:
> 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 way squirrels move when horizontal?

It sure looks like that; feet under the body.

> >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')?  

I'm thinking of the baby hoatzin 'foot on a branch, foot on a branch,
wing claws hanging from a branch' posture; it's not the basic primate
tree-hugging splat, and it's certainly not brachiation -- amusing though
that mental image is -- since there's none of the necessary shoulder
motions or wrist rotation available, but it is (sometimes) suspended
from the wing claws; you could, with a little care, pick up a baby
hoatzin that had both sets of wing claws hooked over a horizontal dowel,
just like you can pick up a (stubborn) parrot that has its beak clamped
on a similar dowel.

I don't think the hoatzin would *like* this; I've never seen pictures
where the hoatzin didn't have at least on foot firmly placed.  But the
ability to support the body from the wing claws, when the wing claws are
above the head, sure looks like it's there.

Corrections from hoatzin biomechanical experts recieved gratefully, and
all.

> 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 think it's likely that the small forms *could* climb, because most
small animals can climb; how well, and by what means, no opinion.

Agreed that all four limbs were likely involved in tree climbing when it
happened; given the lack of perching feet, I don't see how the forelimbs
*couldn't* have been involved.

> >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.

Can't say as I blame them. :)