Re: The Continuing Story of Gliders to Dinosaurs

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

Philidor11@aol.com said:
> If the disappearance of a muscle were creating significant difficulties,
> wouldn't the muscle stop disappearing?  Doesn't this imply that the bird
> ancestors had already found the flexion/extension alternative before the
> muscle disappeared?  Would flexion/extension be advantageous to a
> non-arboreal animal?

The muscle we were discussing was the Caudofemoralis longus.  Yes, in a 
rush, I've oversimplified things.  This ties in with the decoupling of the 
tail from the hindlimbs that Gatesy has discussed in numerous papers and 
talks.  Essentially, the tail is becoming a separate component, not tied to 
the hindlimbs.  There is a sort of "shelf" made up of the proximal third to 
half of the transverse processes on the caudal (tail) vertebrae in most 
archosaurs with tails.  This "shelf" acts as an anchor for the 
Caudofemoralis longus muscle.  It inserts on a posterior bump on the femur 
called the fourth trochanter.  The shelf and fourth trochanter are well 
developed in most dinos, and even sauropods have a pretty appreciable fourth 
trochanter and tail shelf.

However, maniraptoran dinos, especially the deinonychosaurs and the like, 
have greatly reduced the tail shelf and have almost no trace of the fourth 
trochanter.  Furthermore, the tail in these dinos becomes stiff and rigid.  
The idea is that perhaps the tail began to be used as a rudder to help the 
animal change directions more quickly with proper balance.  Because the C. 
longus muscle appears to have been greatly reduced, the tail is essentially 
"decoupled" from the hindlimb.

Now that you have a tail that is independent of the hindlimb and can be used 
for "steering," one can see how possibly a decoupled bird tail could arise 
from such a condition.  Furthermore, as you become more like a bird and lose 
your counterbalancing tail and the big C. longus muscle, you compensate for 
your balance by shifting your femur forward and bending your knees to put 
your feet below your new center of gravity: hence the bent-kneed bird leg.

I take no credit for these ideas: Gatesy, Padian, Dial, Middleton, and a 
number of others are all currently doing excellent work on this and other 
areas of bird and theropod locomotion.

Hope this helps,
Matt Bonnan


> From: Philidor11@aol.com
> To: mbonnan@hotmail.com
> CC: donosaur@usc.edu
> Subject: Re: The Continuing Story of Gliders to Dinosaurs
> Date: Sun, 26 Sep 1999 18:27:50 EDT
>
> In a message dated 9/25/99 5:30:19 PM Eastern Daylight Time,
> mbonnan@hotmail.com writes:
>
> <<  Loss of the big caudofemoralis longus muscle that pulls the thigh back 
> in
> most dinosaurs would have forced  bird ancestors to use knee flexion and
> extension which would help reorient  the center of gravity over the feet.  
> As
> a result of this, suddently
> arboreal niches open up to once terrestrial animals. >>
>
> If the disappearance of a muscle were creating significant difficulties,
> wouldn't the muscle stop disappearing?  Doesn't this imply that the bird
> ancestors had already found the flexion/extension alternative before the
> muscle disappeared?  Would flexion/extension be advantageous to a
> non-arboreal animal?
> Thanks, Matt.

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