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Re: flight beginnings



James Norton <jnorton1946@gmail.com> wrote:

> The recent postings on this list about bird origins reminded me that I
> should pass along a thought I had about the origins of flight, the
> role of feathers, and forelimb and shoulder girdle motions.  I
> recently watched a slow-motion video of long jump techniques, and was
> struck by how applicable the arm, trunk, and leg motions of human long
> jumpers might be to a bipedal terrestrial predator.  The creature
> could use the extra surface area provided by forelimb feathers to gain
> thrust, height and distance in leaping attacks on prey.  Similarly, I
> have watched men and women at the local gym practice their standing
> vertical leaps from the floor onto 2.5-3 foot high stacks of gym mats.
> Again, the human arm motions are such that the addition of feathers
> would provide additional thrust and increase vertical leap height of a
> theropod predator.  Just a thought or two.



It's certainly an interesting hypothesis.  In my view, it has two
major strengths.  Firstly, it ties the origin of wings to predatory
behavior - in this case, leaping attacks against prey (particularly
large prey).  Secondly, it involves a terrestrial biped (rather than,
say, an arboreal quadruped).  Both are consistent with the ecology
inferred for the maniraptoran ancestors of birds: bipedal terrestrial
predators.

One potential problem with this hypothesis is that it requires thrust
is being generated by the incipient wing (pre-flight wing or
"proto-wing").  Based on the morphology of the shoulder girdle, the
first birds and their ancestors might not have been capable of a
thrust-generating motion of their forelimbs.  But this issue isn't
settled to everyone's satisfaction (though I'm inclined to accept it
for the time being).  It is difficult to reconstruct the orientation
of the glenoid and (hence) the range of motion of the humerus from the
skeletons of early birds and their relatives.

My favored hypothesis is that the first birds (and their maniraptoran
kin) used their feathered forelimbs for control during brief descents,
rather than for ascents (which require thrust, as well as lift).
Predation may have played a key role in promoting the development of
proto-wings, by facilitating descents from large prey (such as if the
predator was flung off the back of a large thrashing herbivore).  Or
for returning to the ground after climbing a tree, such as to catch a
small prey item.

Either way, the skeletons of the first birds and their relatives do
not appear to be suitable for true arboreality.  So the concept of
small maniraptorans spending their lives up in the canopy, and gliding
from tree to tree like modern mammal gliders (colugos, flying
squirrels, etc) appears implausible for any maniraptoran, IMHO.  (See
the works of Gregory S. Paul for a diametrically opposite
interpretation.)  One of the (many) interesting things to emerge from
the recent study of locomotor modes in small mammals by Chen and
Wilson (2015) is that arboreal gliding only evolves in mammals that
are already highly specialized for arboreality; this was not the case
for any non-avian maniraptoran dinosaur.  So, like your model, I
propose a model in which a bipedal terrestrial predator is front and
center.




Cheers
Tim