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Re: tiny-armed theropods

On 10/7/2011 11:02 AM, Jason Brougham wrote:

To figure out any sort of principles of arm reduction in theropods we'd have to 
do a survey of the distribution of this feature and look for correlates. Others 
have suggested and I agree that there would be several and perhaps innumerable 
different correlates and settings with and in which it happens.

Vivian stated that a low-and-forward CM* relative to the point where forward thrust from the legs is applied to the body implies that a pitch-down moment is applied to the hip-forward body upon acceleration.

The physics seem to me to be unarguable relative to balance-beam bipeds, at least in the qualitative sense, but I am not a physicist. Is there another take?

If Vivian's physics _are_ correct, and the forces non-negligible in magnitude, then ALL balance-beam type bipeds that need competitive acceleration ability have reason to reduce weight on the front-end (forward of CM).

Obvious points -- 1) in general, both herbivores and carnivores need to accelerate well, excepting very large herbivores.

2) events wherein acceleration ability is being maximally exploited are likely critical in the selective sense.

2) reduction of cranial size is limited, perhaps obviated, by the need to acquire and process food.

* "...cranial CoM..."