RE: Powered Flight Definition (part of RE: Cost in Aquatic Birds (long))

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

Jaime Headden wrote:

> The power flap, or power stroke, in the bird wing, also present in the 
> bats, is a result of the presence of a triosseal canal. I previous 
> indicated this. The reason being the condition is neccessary for the 
> rotation of the humerus about it's long axis as it is being elevated 
> above the horizontal.

Ah, now I know what you're talking about.  Thanks.

> I have wondered about a functional attitude for the brachial
> structures on *Sinornithosaurus* for a while now, [snip]
> Brian Cooley's model shows this off rather well, I think. 

I'm not familiar with Brian Cooley's model - could you elaborate?.

> and I saw *Sinornithosaurus* as a trunk-branch clammerer and
> short-distance leaper. In this fashion, the arms would seem particularly
> suited to assisting, sifaka-like, in leaps across intra-arboreal
> distances.

IMHO, the flaw in this theory is that the hands of eumaniraptorans
(deinonychosaurs and basal birds alike) seem unsuitable for grabbing onto
branches.  See Alan Gishlick's work on the manus of Deinonychus: the locked
phalangeal joints, severely restricted hyperextension, and reduced
opposability of the pollux to the other digits rendered the eumaniraptoran
hand nearly useless for grasping.  (Note: This only applies to one-handed
grasping.  Non-avian eumaniraptorans excelled at two-handed grasping, a
behavior for which they were highly specialized.)

However, perhaps the claws could be used to snag or hook branches at the end
of a short leap.  But this would place enormous stress on the manus,
especially if the hands were used in a brachiation-like fashion to support
the body.  In other words, I just can't imagine small theropods swinging
primate-like through the canopy.  They don't have the forelimb architecture
to pull it off.


Tim

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

Timothy J. Williams 

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

Phone: 515 294 9233 
Fax:   515 294 3163