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Re: tiny-armed theropods



I see, so you are suggesting that all theropods would benefit from reduction of 
their pectoral limbs if they could find a way to do without arms in feeding and 
brooding and whatnot, because it would make them more agile.

It's just that dispensing with the arms' uses must have been impossible for 
many theropods, possibly for a large or even an innumerable set of interacting 
reasons.


On Oct 7, 2011, at 12:24 PM, Don Ohmes wrote:

> 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..."
> 

Jason Brougham
Senior Principal Preparator
American Museum of Natural History
jaseb@amnh.org
(212) 496 3544