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Re: Dinosaurs and birds



Scott, would you elaborate a bit on the two quoted points, please? I
note in passing that pterosaurs don't have "a pulley-like supracoracoideus needed to prevent dislocation of the humerus during downstrokes". Is a pulley-like supracoracoideus absolutely required for flight? <<<

And David M. asked about bats too: the problem with phrasing the question like this is that birds have many of their own adaptations to flight, and just because bats or pterosaurs have their own derived solution to a problem doesn't mean that a proto-bird that lacks the derived avian condition is magically exempted from a biomechanical problem. As noted by Baier et al (Nature, 2007), the theropod glenoid capsule is shaped such that it is open on two ends (rather than being a true ball and socket joint). It's orientation in all maniraptorans (including avialans) is such that it cannot prevent dislocation of the humerus during the flight stroke unless their is tendonal and muscular opposition to the movement.

At SVP Baier was quite blunt about the problems that Archaeopteryx would have (modelled during a steady-state glide, not even a downstroke!). In the Nature paper they simply (and uncritically) refer to Archaeopteryx as "volant", while indicating that its acrocoracoidhumeral ligament did not appear to have even an incipient function in opposing the downstroke.

While retaining the same lateral facing of the glenoid, pterosaurs none-the-less have a very different orientation of the glenoid (e.g. http://www.school.pterosaur.co.uk/identify/curved/Pteranoscapcor.jpg) in terms of where the open (non-buttressed) portions of the glenoid are. Bats obviously have radically different mammalian shoulder girdle morphology, and solve the problem of joint stress in a completely different way (e.g. http://www.ucmp.berkeley.edu/vertebrates/flight/bats.html).

Niether of these derived solution that are unique to each group implies that a stem-group bird or early avialan that lacks the _avian_ derived condition can be exempted from these biomechanical problems. In other words, a "pulley-like supracoracoideus" is only necessary for flight if you lack other derived adaptations to solve the biomechanical problem. Bats and (presumably) pterosaurs have them, basal birds that lack the avian solution do not.

Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333

www.skeletaldrawing.com

-----Original Message-----
From: jrccea@bellsouth.net
To: dinoboygraphics@aol.com
Cc: dinosaur@usc.edu
Sent: Wed, 4 Apr 2007 10:06 AM
Subject: Re: Dinosaurs and birds

Scott, would you elaborate a bit on the two quoted points, please? I note in passing that pterosaurs don't have "a pulley-like supracoracoideus needed to prevent dislocation of the humerus during downstrokes". Is a pulley-like supracoracoideus absolutely required for flight?Â
Thanks,Â
JimÂ
Â
----- Original Message ----- From: <dinoboygraphics@aol.com>Â
To: <twilliams_alpha@hotmail.com>; <dinosaur@usc.edu>Â
Sent: Wednesday, April 04, 2007 9:09 AMÂ
Subject: Re: Dinosaurs and birdsÂ
Â
........Outside of the assymetrical feathers (which is a
drag-reducing > adaptation, not a lift-generating one, although the two are related)Â
Â
the lack of a pulley-like supracoracoideus needed to prevent
dislocation > of the humerus during downstrokes, Â


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