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Re: "Flight theory has legs"

On Sat, Jul 12, 2003 at 02:44:14PM -0700, Jaime A. Headden scripsit:
> Graydon (oak@uniserve.com) wrote:
> >This can't be extended to aviform dinosaurs, though; they don't have
> >the basic mammal tree-or-ground-hugging-splat posture.
> Typical scanorial locomotion is not limited to trunk-hugging, but
> extends to branch-walking, canopy-leaping, and so forth.

All of which is very *pedal*, though; those locomotor modes are
extensions of flat surface locomotion.

> Non-sprawling arboreal/scansorial animals include the chameleontid
> lizards. The non-sprawling posture, however, does not neccesarily
> limit the effect of the manual AND the pedal claws on the same
> substrate. A semi-sprawling, semi-hugging posture is possible, as
> first restored by Chatterjee, for a spread-armed, tucked-legged
> position. I restore this to some degree on the Dinosauricon, for
> elucidation (near the bottom of my gallery page).  Another version,
> featuring a vertical featherless *Deinonychus*, is also available
> apart from *Sinornithosaurus* (but the arm position is outdated).

I've seen restorations like those.

I think they're plausible as slow climbing postures, but implausible as
a typical locomotor posture because they involve a very limited range of
the available range of motion in the limbs.  Something that did that a
lot would have different limbs.

> >Until someone finds quadrepedal bird tracks, I think the
> >no-substrate-contact is a pefectly decent assumption.
> No one is suggesting birds could be quadrupedal -- or extant birds, in
> any case (I made a clear distinction in my prior post). Yet. I think

You did, but you're talking about manual ungual substrate contact; there
isn't any difference other than stress load between walking up a tree
trunk and walking on the ground.

> >I am also unable to come up with a hand-claw climbing scheme that
> >wouldn't involve the feet at all; the shoulder anatomy wouldn't let
> >them hang straight down from the hands.
> Well, if orangutans can, who knows.

We can, too, but we have very different shoulder anatomy with lots of
brachiating ancestors in it.

So far as I'm aware, *no* dinosaur has an anatomy that allows them to
put their manual digits above their back and have the unguals face
toward, rather than away from, the midline of their body; I'm pretty
sure that this is actively impossible for multiple reasons in theropod
anatomy.  (non-rotating carpals, plus linear muscle attachment to a long
narrow scapula.)

> However, I don't think anyone suggested that they could. Alos, if a
> bird hung upside down, belly up, the arm position is not impossible
> for this, given claws.

Not impossible, no; plausible?

Anything developing into a flyer or glider from that position has
obliged itself to invert to land, and modern birds don't always get the
simple foot landing correct.

oak@uniserve.com | Uton we hycgan    hwaer we ham agen,
                 | ond thonne gedhencan    he we thider cumen.
                 |   -- The Seafarer, ll. 117-118.