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RE: Origin of bird flight: ontogenetic-transitional wing (OTW) hypothesis



Michael Habib wrote:


> --Good point. I remain skeptical of the applicability of their model to basal 
> birds. The authors assert that the model is analogous because the juveniles
>  have relatively undeveloped wings, but the model taxa are still gallanaceous 
> birds, which are burst launch specialists. I wonder if the relatively well 
> enlarged pectoralis fraction, with large amounts of anaerobic power, and the 
> very rapid upstroke of burst launch specialists comes into play with regards 
> to 
> the incline running and similar behaviors. 


Yes, and to emphasize that point, it may be Dial &c are arguing that many of 
these aerodynamic behaviors (WAIR included) arose quite deep in theropod (even 
dinosaurian) phylogeny.  At least, that's what I take home after reading this...


     "This hypothesis [OTW] differs from other published accounts in that it is 
flap-based (in contrast to requiring a gliding precursor), involves an 
      aerodynamically functional proto-wing, incorporates both the simultaneous 
and independent use of legs and wings and assumes that a fundamental wing-
      stroke (described herein) was established for aerodynamic function early 
in the bipedal ancestry leading to birds.  Such an evolutionary pathway 
      provides a parsimonious explanation for numerous non-avian theropod 
morphologies (for example, semilunate carpal, delto-pectoral crest, furcula, 
proto-
      wings, symmetrically vaned feathers, long bipedal hindlimbs, etc.) that 
have not been discussed in a synthetic context."


The long bipedal hindlimbs and a deltopectoral crest are primitive for 
Dinosauria, whereas the furcula turns up in basal theropods.  Are the authors 
arguing that all these features first evolved for aerodynamic behavior?  Or am 
I reading too much into their choice of words?   It may be that they're arguing 
that novel aerodynamic features (e.g., proto-wings; symmetrically vaned 
feathers) functioned well together with ancestral characters that originally 
evolved for a non-aerodynamic purpose, such as predation and bipedal locomotion 
- which makes perfect sense to me.  


> Considering that basal birds had a deltoideus-driven upstroke, and possibly 
> less humeral elevation than modern taxa, it may be that the incline-running 
> type mechanics were unavailable to them.


The authors appear cognizant of this, and counter that the orientation of the 
shoulder joint stayed more or less constant ("in the global and gravitational 
frames of reference"), and it was in fact the body axis that re-oriented.  It's 
a novel idea, for sure; but I don't know enough to comment either way.


Cheers

Tim
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