Re: Adaptive advantage (was Re: ABSRD BAND on Sinornithosaurus feathers)

["Tim Williams" <twilliams_alpha@hotmail.com>]

Ronald Orenstein wrote:

> At 09:14 AM 16/03/01 +0000, Scott Hartman wrote:
> > Theropods are the anti-scansors.  Their limb proportions and acetabular
> > articulation precludes this type of lifestyle.  [snip]

> I have been pointing out for some time now on this list that this simply is
> not true.  There are living birds that are excellent climbers without
> having to resort to flight to help them do it - even if you leave aside
> specialized trunk-foragers like woodpeckers.

Agreed.  There are claws which, based on their morphology, are specialized 
for climbing trees.  Birds with this type of claw do indeed climb trees.  
However, it does not follow that this type of claw is essential for 
tree-climbing.  Birds *without* claws of this type are still able to climb 
trees.

The shape of the claws in _Archaeopteryx_ suggest that it did use its claws 
to climb trees (Yalden, 1985; Feduccia, 1993).  This does not mean that 
small maniraptoran theropods which did *not* have specialized scansorial 
claws were incapable of climbing trees.

My preferred model for the origin of flight does not claim that small 
maniraptorans were specialized scansors.  They climbed trees when it suited 
them.  All prey capture (including after they had leaped down from trees) 
occurred on the ground.  Hence the retention of long legs and other features 
ill-adapted for scansoriality.


> I see no reason why a long-legged theropod could not have done the same
> thing, if perhaps less efficiently - and remember that even though they
> lacked wings, they could use arms and mouth to grip limbs if necessary.

Exactly.  The long arms, large hands, mobile wrists, and sharp claws (on 
both hands and feet) may have served adequately (but certainly not 
perfectly) in getting these predators up tree trunks.  Small size would help 
too (_Archaeopteryx_-size, or lower).  Chatterjee has suggested that the 
stiff tail was used as a brace during trunk-climbing - this I'm a little 
more skeptical of.


Tim

------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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