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Re: How did dromaeosaurs use their arms?
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.
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