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Re: [dinosaur] Fwd: Tyrrell's 1884 Albertosaurus skull + early bird liftoffs + near-dinosaurs + larval temnospondyl + more

Does anyone think that Yi was not an arboreal theropod?


From: dinosaur-l-request@usc.edu <dinosaur-l-request@usc.edu> on behalf of Tim Williams <tijawi@gmail.com>
Sent: November 2, 2016 2:41:54 AM
To: dinosaur-l@usc.edu
Subject: Re: [dinosaur] Fwd: Tyrrell's 1884 Albertosaurus skull + early bird liftoffs + near-dinosaurs + larval temnospondyl + more
Ben Creisler <bcreisler@gmail.com> wrote:

> ===
> Earliest birds could take off from the ground
> https://urldefense.proofpoint.com/v2/url?u=https-3A__www.sciencenews.org_article_early-2Dbirds-2Dcould-2Dachieve-2Dliftoff&d=DgIBaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=0fjEj5iweUYD1KTpZapMhxsOY9_dikYqq4RkGaA8rxA&s=iSFjB5PEARJuGznMbwJoZ_7MbmTq5y9JPozCXh7JL78&e=

It's great to see that the 'ground-up' origin of avian flight is
experiencing a revival.  John Ostrom pushed this model quite
vigorously back in the 1970's, based on his work on _Archaeopteryx_:
he interpreted _Archaeopteryx_ whole-heartedly as a terrestrial biped,
and couldn't identify any arboreal characters in the skeleton.
(Ostrom also quite sensibly rejected the analogy between
_Archaeopteryx_ and juvenile hoatzins, which clamber about in trees
using all four limbs, plus their beak.  _Archaeopteryx_ is maladapted
for such hoatzin-like behavior.)  Forty-plus years later, I think
Ostrom was right all along: birds descended from terrestrial bipeds,
not arboreal quadrupeds; and birds did not pass through an arboreal
glider stage on the way to evolving powered flight.

I blame _Microraptor_.  I suspect that the interpretation of
_Microraptor_ as a four-winged ('tetrapteryx') arboreal glider was a
red herring.  This interpretation gave impetus to the view that the
immediate ancestors of birds were arboreal gliders.  But there's
little if any evidence that _Microraptor_ was arboreal.  Sure,
_Microraptor_ and _Archaeopteryx_ (and various other maniraptorans)
might occasionally have climbed trees; but this a far cry from the
true definition of arboreality, which is spending most or all of the
time in trees.  In today's world, all gliders are dedicated to an
arboreal lifestyle, and are not just opportunistic climbers.  (Among
mammals, gliding lies at the extreme end of the arboreal spectrum in
terms of locomotor modes, consistent with gliders having evolved from
mammals that were already specialized for arboreality.)

The idea that rudimentary take-offs evolved long before
sustained/powered flight might seem counter-intuitive. But there's
many reasons why small maniraptorans might have wanted to leap into
the air, without any need to stay there: attacking larger prey;
escaping predators themselves; clearing hurdles on the ground; leaping
onto branches from the ground to access food, etc.  This model also
bypasses the need for WAIR (wing-assisted incline running) as a
primordial flight behavior.  Although WAIR an important behavior in
juvenile gallinaceous birds, the hypothesis that it played a role in
the origin of avian flight is far more controversial.