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Re: Ceratonykus braincase described



No, a triangular arm is not practical. But that would be an extreme
example. I guess I was primarily referring to the upper arm segment.
Moving the deltapectoral crest etc distally is definitely possible. I
have a friend working on a paper (this argument would be a lot easier
if got himself in gear and actually published it sometime this
century) detailing proportional changes to moment arms (gearing)
achieved by doing just that in birds. Moment arms for muscles in
extant crocodylians have been found to migrate distally over ontogeny
(Dodson 1975, Livingstone et al . 2009), and I'm pretty sure similar
phenomena have been seen in mammals.

Relative changes to the moment arm of the biceps group and other elbow
flexors also seems possible:

Fujiwara et al (2011): Topsy-turvy locomotion: biomechanical
specializations of the elbow in suspended quadrupeds reflect inverted
gravitational constraints.
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7580.2011.01379.x/abstract

And relative changes to elbow extensors are obviously possible (olecranons).


On 28 April 2011 11:18, David Marjanovic <david.marjanovic@gmx.at> wrote:
>>  Power has a pretty specific definition:
>

> Moving the muscle attachment sites distally _is not possible_. Vertebrate
> development is constrained; it's not possible to end up with a triangular
> arm. Even in animals with a propatagium, the biceps inserts near the elbow;
> to get the insertion site of the biceps closer to the wrist, all you can do
> is shorten the forearm so the wrist gets closer to the elbow. This is seen
> in tyrannosaurids and alvarezsaurids. To a much smaller degree, it's also
> seen in Neandertalers as opposed to average *H. s. sapiens*, and in the
> latter as opposed to me.
>