RE: vaulting pterosaur launch, questions

[Tim Williams <twilliams_alpha@hotmail.com>]

Scott Hartman wrote:


> Sorry, but VERY. In fact, that was the entire point of my post, that
> the popular image of Archaeopteryx is not consistent with the
> peer-reviewed literature. For example, it's been observed that Archie
> did not have the shoulder mobility of a flapper:
>
> Senter, P. (2006). Scapular orientation in theropods and basal birds
> and the origin of flapping flight. Acta Palaeontologica Polonica.
> 51(2): 305–313.


There are differing opinions on this in the peer-reviewed literature.  For 
example, very recently Dial &c argued that the excursion of the forelimbs 
remained fairly constant during the non-avian to avian transition, in terms of 
its 3D motion.  In other words, motion of the forelimb at the shoulder did not 
change in an *absolute* sense, although its motion relative to the rest of the 
body did change.


Dial, K.P., B.E. Jackson  &  P. Segre. (2008).  A fundamental avian wing-stroke 
provides a new perspective on the evolution of flight.  
Nature 451: 985-989.


Specifically (from Dial, 2008): "The shoulder joint (glenoid) is thought to 
have evolved from a primitive ventro-lateral orientation allowing a 
cranial–caudal excursion (as observed in theropod ancestors) to the derived 
dorso-lateral orientation allowing a dorso-ventral excursion (among extant 
flying birds).  Jenkins suggested the 90° rotation of the glenoid's excursion 
axis relative to the body was to accommodate the derived wing-stroke angle of 
extant birds.  We agree with the character states Jenkins eloquently describes 
and offer a novel perspective about the process underlying the evolutionary 
sequence.  We suggest the orientation of the shoulder joint remained relatively 
fixed in 3D space (in the global and gravitational frames of reference) over 
evolutionary time.  Living ground birds exhibit a slinging of the torso about 
the shoulder (Fig. 3a).  We suggest this same feature allowed proto-birds to 
use a functional wing-stroke (even with proto-wings) aligned to gravity which
 assisted their hindlimbs as they scaled increasingly pitched obstacles, 
allowed controlled flapping descent and powered rudimentary flight in the 
transitional stages leading to level flapping flight (Fig. 1).  In other words, 
the gravity-based wing-stroke did not come about through a long series of 
migrational stages of the forelimb (from ventro-lateral to lateral to 
dorso-lateral): rather, the primitive wing-stroke started in a similar 
orientation as we see it today in hatchlings using their proto-wings."


I don't know who's 'right'; but note that Dial &c's work does not contradict 
Senter's biomechanical work, merely its conclusions in terms of the inferred 
flight abilities (or lack thereof) of _Archaeopteryx_ and other 
non-ornithothoracean birds.   BTW, Senter's study also posits that 
jeholornithids, confuciusornithids, and sapeornithids were also incapable of 
flapping flight, for the same reasons as _Archaeopteryx_. 


Cheers

Tim
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