Re: To Sprawl or Not to Sprawl?...

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

Scott Hartman wrote:

>    It took tens of millions of years to rearrange the forelimbs from the 
> primative theropod condition (e.g. posterior facing glenoid) to a flapping 
> configuration, but you think (nearly?) contemporal sister-
> taxa have totally different pelvic and femoral structures?

Actually, I'm not at all convinced (yet) that _M. gui_ was capable of 
greater lateral excursion of the hindlimb than seen in other theropods.  I 
was very careful to raise this issue as a "working hypothesis" requiring 
further analysis.

> Remember that making the head spherical is only one small step.  You would 
> have to close off the acetabulum, and you would have to totally change the 
> insertation points on the proximal femur.

Modern prosimian-grade primates that live in trees show differences in the 
shape of the femoral head.  In some species (tarsiers, galagos) the femoral 
head is more cylindrical, whereas in others (avahi, sifakas, indri) the 
femoral head is more subspherical in shape.  The latter type of articulation 
allows greater femoral adbuction.  The cylindrical joint is less mobile, but 
apparently provides greater stability when the primate is landing.

> Since the feathers could have been useful without employing them 
> horizontally (e.g. rudders), there is no evidence right now to support the 
> sprawling M. gui idea.

Yes, these "butt-fans" could have been useful as rudders for parachuting or 
gliding theropods.  Also, it may not be necessary for the hindlimbs to be 
laterally-oriented in order to provide a surface capable of
generating lift during aerial leaps or glides.  I think further 
biomechanical analysis is required to establish whether these hindlimb-wings 
could have been used as a gliding surface, and could be held approximately 
coplanar with the forelimb-wings.  Until that work is done, I'm keeping an 
open mind.



Tim

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