Re: Feather barbs show history of flight

[Tim Williams <tijawi@gmail.com>]

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