Re: How did dromaeosaurs use their arms?

["Matthew Bonnan" <mbonnan@hotmail.com>]

Dinogeorge writes:

> When I walk down the street and want to turn left, I do this by moving my
> feet against the ground, not by flailing my arms against the air. All
> tetrapods that I have ever observed, small and large, from kangaroo rats to
> kangaroos, including birds, do likewise.

Sure.  But the arms, even in a biped animal, help to balance the body during 
locomotion and can assist in turning, changing direction, etc.

Surely pelage, feathers, whathaveyou, help terrestrial animals as far as 
insulation goes.  As far as feathers on the arm, dinosaurs have a femoral 
head that is cylindrical (unlike mammals where it is spherical) and allows 
little abduction/adduction.  They therefore have a stiff-action limb that is 
great for walking/running forward in a straight line, but is less great for 
tight turns just because of the physical geometry of the hip socket and 
femoral head.  Feathers on the arms, as well as a long, stiff tail, may have 
allowed a terrestrial predator with less mobility in its hips to compensate 
for this by incorporating the forelimbs and tail in turning, etc.  And then 
there is always sexual display, etc.

As far as streamlining from feathers, penguins are aquatic birds that use 
the feathers on their bodies to swim more efficiently through the water.

So the presence of feathers on an animal does not necessarily have to 
reflect an arboreal ancestry and could be used for other purposes as well.  
The complexity of the maniraptoran fossil record that is unfolding before us 
makes it difficult at this time to conclude whether maniraptorans are 
descendants of arboreal animals or terrestrial ones.  In either case, the 
presence of feathers on the forelimb does not necessarily preclude a 
terrestrial ancestor and there are other viable scenarios.

Matt Bonnan
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