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Re: The Continuing Story of Gliders to Dinosaurs



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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