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Re: tiny-armed theropods



2011/10/6 Vivian Allen <mrvivianallen@googlemail.com>:

> How would walking quickly forwards (all the time) help balance a
> cranial CoM? You'd have to constantly be accelerating, otherwise no
> constant balancing force. Most theropods CoM position seems to be
> pretty cranial of the hip, somewhere around where I'd expect the
> stomach and gut to be (paper coming out soon!).

I do not know about the involved physics, and wait for your paper. You
speak in other message about nose-down pitch. Sharks seem to avoid
this in water thanks to their pectoral fins. May not the long feathers
of the forearm "wing" (present at least in maniraptorans) help with
this problem in terrestrial neotheropods? Groups with evidence of
"wings" as oviraptorosaurs, deynonychosaurs and avialians include
members with tails reduced in mass, so perhaps there is a connection
between lack of balance and wings. I acknowledge that this may in any
case (if at all) apply to running instead of fast-walking.

Regarding walking, I think the center of mass is cranial to the foot
when the foot is retracted, but not when protracted at footfall. In
walking, footfall would stop the fall of the frontal part of the body.
I do not know how much time takes gravity to make fall the frontal
part of the body, but footfall and immediate retraction would oppose
to falling forwards, and the faster the walking, the greater the
number of footfalls and the less likely gravity has enough time to
drop the animal to the floor.

Cheers