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RE: Origin of bird flight: ontogenetic-transitional wing (OTW) hypothesis
>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
I think they are indeed arguing that, but I'm not sure it is mechanically
supported; that's the issue.
>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.<
That makes sense to me, as well, but I'm not sure it is enough for WAIR type
behaviors, even if it was enough for generating some aerodynamic forces. The
issue is that the authors tacitly assume that juvenile galliform birds are
"near-volant" in the same way that dromeosaurids et al. would have been, and I
am not yet convinced that this is the case. The problem is that WAIR behavior
might actually require a more derived shoulder architecture and power supply
(i.e. pec minor with trioseal canal) to work, because the upstroke probably has
to be rapid.
>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.<
It is an interesting idea, and I think they have a viable observation regarding
how modern birds position themselves relative to flow and wing cycles during
flight. It may also play into the early evolution of flight, as they suggest.
However, it doesn't actually address my concern regarding shoulder structure
and deltoid-driven upstroke mechanics. The orientation of the shoulder, in
their argument, stayed more or less constant with regards to rotation about a
coronal axis. I'm concerned about the position relative to a sagittal axis
(that is, the shoulder may not have been shifted enough in a dorsal direction
in basal-most birds). Even if the shoulder position was viable, there is still
a problem of power output and upstroke speed (which are linked, because a
faster upstroke can deliver more power by reducing the frame time).
To put my concern in phylogenetic terms: Dial et al. make the argument that
WAIR dynamics could have appeared quite deep in the avian phylogeny because it
seems to be plesiomorphic for modern birds. Their argument that this abililty
is plesiomorphic for neornithines is strong, but the extrapolation to Aves as a
whole is much weaker, because there is reason to think that some of the
features required for WAIR show up relatively late in the tree (around the
origin of Ornithurae). That still leaves WAIR plenty of room to influence the
evolution of birds, but leaves it post-dating the origin of flight by a fair
margin (and thus not a viable mechanic for the origin of flight as a whole).
Furthermore, there is the additional confounding problem that many of the more
basal neornithines are burst specialists, meaning that they have a
hypertrophied power supply system for flight, especially with regards to the
upstroke power (but also for the downstroke). If this is related to th
e use of WAIR, then it could be that the ability nests even further up the
tree, closer to the origin of Neornithes.
My concerns could be alleviated (i.e. my counter-hypothesis to Dial et al.
falsified) if the power usage and wing speed required for WAIR were quantified,
and a compelling case were made that basal-most birds could have generated the
required power and upstroke elevation.
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181