A New Hypothesis for the Origin of Flight?

[Waylon Rowley <whte_rbt_obj@yahoo.com>]

    I just got my copy of GSP's DA a couple days ago,
and I'm blown away by the information and artwork
condensed into this extraordinary book. My hat goes
off to the author. 
    While reading the chapter on arboreality and the
various hypotheses regarding the origins of flight, a
thought struck me. Now that we have all these small
Jehol avepectorans with freakishly long arms (yet
somewhat small primaries) and tails (the new unnamed
Dromaeo-avemorph with contour feathers), we have to
come up with an explanation of their function. Here's
a scenario I thought up:

 When small dinosaurs took to the trees, they     
developed small stiff pin-like feathers on their
forearms, possibly for adding grip to the hand, or
using them like the spines on a mantis arm to catch
insects with. When they moved among branches, they
didn't leap or glide, but simply extended their long
tails behind them to counterbalance their forebodies
when they reached for another branch. Longer arms and
strong muscles would be favored, as would longer,
stiffer, and heavier tails. The retroverted or reduced
pubes and more pneumatic thoracic cavity in some forms
may be an attempt to push the center of gravity
backward, so the legs could fully extend the body
forward, and still remain balanced on the perch. As
they reached continually further, they might flail
their arms, trying to grasp a branch or twig just out
of reach....a precursor to flapping. The further they
reached out, the more flapping would be needed to keep
from tipping over (and hanging upside-down on the
perch is counter-productive). Even if they did manage
to get a good grip, powerful muscles would be needed
to pull the body onto the next branch....a precursor
to the avian flight musculature. A reversed hallux
would be advantageous, because uni-directional toes
can't maintain a good grip if you're metatarsus is
almost in the same vertical plane as the branch you're
on. An arctometatarsalian or co-ossified pes would
also be pretty handy in this situation. Elongate
distal retrices would add more "tail-torque". As soon
as more sophisticated flapping took over, the costly
tail could be abandoned, and true flight could begin.
After all, it's alot easier to build feathers rather
than bone and tissue. So, the tail might shorten,
whereas retrices might continue to lengthen, in which
case you need a strong attachment site, probably a
pygostyle. In secondarily terrestrial Oviraptorosaurs,
the pygostyle would be lost, and genetic duplication
of a proximal caudal "design" would re-lengthen the
tail, effectively masking what was left of the
Dromaeosaur-style tail. The pubes would swing forward,
and feathers would atrophy to more manageable sizes.
What the protoavians were doing in the trees in the
first place is anyone's guess, but I'm betting that
they were insectivores, as the conical teeth and
"pointy" skulls of Archaeopteryx suggests to me.
Microraptor may be in the process of adopting or
abandoning insectivory, and Rahonavis may be moving
toward a more carnivorous diet of small vertebrates,
where the sickle claw could be put to use. Hopefully
this is a convincing (and testable) example of how
flapping could arise, and exactly what the newest
Jehol theropods were doing with their unusually long
tails and arms. So, let's try to tear it apart.

Cheers,
Waylon Rowley 





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