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Re: Feather barbs show history of flight

Great thoughts, Tim. I am pleased to finally see a vertebrate paleontology 
paper out that treats primary feather asymmetry as a continuous character! Now, 
when I give my “asymmetry isn’t dichotomous” talks I can at least cite one good 
study on fossil birds that does it right.

My “trees to ground to air” model, presented at SVP in the fall, was my 
personal attempt to set up a transition phase that matched the phylogenies and 
known morphological patterns. The idea is that some semi-arboreal phases might 
have pushed control and weight support, but secondarily less aerial lineages 
might have been where flight stroke evolution mostly took place, under a regime 
that essentially selected for strong burst performance. I don’t necessarily 
think it’s better than the other models out there, but it was received pretty 
well. The general sentiment I got after the talk was there is still a bit of a 
“slow build” versus “fast build” split in the community on the flight stroke 
(that’s anecdotal, though).



> On Feb 17, 2015, at 9:28 PM, Tim Williams <tijawi@gmail.com> wrote:
> Jason Brougham <jaseb@amnh.org> wrote:
>> Wow! Cool paper. Dr. Prum always has impressive contributions. I was hoping 
>> that fossil barb preservation was good enough to do
>> this kind of analysis. This seems to be a solid piece of evidence supporting 
>> what, I sense, has been a rough consensus (at least in
>> the Theropod Working Group): that fully powered flight did not arrive until 
>> around the Confuciusornis node.
> Agree, on all accounts.  Prum's work never fails to impress.  This
> latest is a powerful study that debunks the time-honored assumption
> that asymmetric feathers = powered flight.  Instead, the geometry of
> the feather barbs supports the hypothesis that powered flight did not
> appear until close to the base of Ornithothoraces, and was absent from
> 'stem birds' (_Microraptor_, _Archaeopteryx, __Sapeornis_,
> _Confuciusornis_).  The study favors the few that these theropods used
> some kind of aerial locomotion, but this was passive gliding rather
> than true (flapping or powered) flight.  The findings accord with
> biomechanical studies which indicate that stem birds
> (non-ornithothoracean paravians) were incapable of executing a flight
> stroke, owing to the plesiomorphic orientation of the glenoid.
> I do wonder though if lack of powered flight means that stem birds
> were necessarily 'passive gliders'.  The thing about gliding is that
> this form of aerial locomotion is not exactly 'passive'.  Work on the
> colugo ('flying lemur') has shown that highly refined orientational
> control is essential during the glide, especially to ensure a safe
> landing .  So maneuverability is a critical factor.  The manner of
> pre-flight aerial locomotion used by stem birds might not have been
> arboreal gliding, but some form of terrestrial behavior in which
> orientational control was central.  I think WAIR (wing assisted
> incline running) is off the table, because it requires a flight
> stroke.
> Small mammals are scansorial in the sense that they use their four
> limbs to progress across uneven substrates.  A large terrestrial
> mammal can simply step over a small obstacle.  But a small mammal has
> to climb over it.  Scansoriality in small mammals involves generalized
> climbing, and is conducive to both terrestrial and arboreal habitats.
> (This is why it can be difficult to differentiate scansorial mammals
> from arboreal mammals based solely on osteology - many of the skeletal
> adaptations of the former can be used for climbing and living in
> trees.  Arboreal mammals often have specializations that are extreme
> versions of scansorial characters.  Scansoriality in birds is very
> different to mammals, and represents a highly specialized behavior of
> using trunk-climbing for foraging - as in woodpeckers.  Thus,
> 'scansoriality' is defined differently for mammals versus birds,
> because the flight abilities of birds obviates the need for routine
> climbing.  If stem birds could not fly, then 'scansoriality' in these
> forms would fall within the mammalian context.)
> Here's one hypothesis.  As paravians became smaller (=
> miniaturization), terrestrial progression became more challenging on
> the forest floor.  Unlike mammals, theropods have limited (or at least
> highly proscribed) mobility in their joints.  So it is possible that
> stem birds used their wings to help negotiate these environments
> during terrestrial progression.  The clawed fore- and hindlimbs could
> be used to scramble up obstacles, whereas the feathered limbs and tail
> could be used to facilitate short descents.  Over time, the wings were
> recruited for brief ascents as well - this is where flapping came in.
> (Many modern birds with poor flight abilities use their wings to help
> travel through dense vegetation.)  But this rudimentary flapping
> flight might not have appeared until the base of the Otnithothoraces.
> This scansorial hypothesis for incipient flight in theropods does not
> fit the "ground-up"/"trees-down" dichotomy (which is unhelpful for
> many reasons).  IMHO, this scansorial hypothesis gels with the
> osteology of stem birds, including their lack of arboreal
> specializations (although I'm aware there is disagreement on this
> issue - I side with those who find that non-ornithothoracean theropods
> lacked arboreal adaptations).  No stem bird has features consistent
> with being an arboreal quadruped, which would be necessary if the
> animal was incapable of powered flight and needed to reach the
> branches of a tree to perch.  Alternatively, stem birds might have
> used their wings to glide when running along the ground, to increase
> speed (especially when running downhill, as reported for the modern
> kagu) or to improve maneuverability (sharper turns).  In any case, I
> don't think this study by Feo, Field, and Prum should be cited in
> support of arboreal behavior (including arboreal gliding) in stem
> avians.
> Cheers
> Tim