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Re: Yet more on dinosaur quad climbers

Gregory S. Paul <GSP1954@aol.com> wrote:

> Micraptor clearly had what was needed to be a
> tree specialist -- small size, long limbs for long reach between branches and
> good leaping ability, fairly long grasping fingers and toes tipped with
> big, strongly hooked, sharp tipped claws, well developed airfoils for moving
> aerially about among the trees when any limitations in its climbing abilities
> preclude further quadrupedal progress.

Firstly... These characters in _Microraptor_ might be arboreal - but
then again, they might not be.  Some of these characters ("long
reach", "big, strongly hooked, sharp tipped claws") sound suspiciously
like predatory characters.  This is perhaps not surprising, given that
microraptorines are dromaeosaurs.

Secondly... You studiously avoid mentioning those theropod characters
that are maladaptive to arboreality.  Arboreal quadrupeds typically
have limbs that are highly adapted for moving through a
three-dimensional substrate.  But _Microraptor_ and related theropods
have many characters that are unambiguously maladaptive to life in the
trees.  The maniraptoran semilunate carpal ("swivel-wrist")
constrained movement of the manus to a single plane - in stark
contrast to the highly flexible wrists of arboreal quadrupeds.  The
theropod ankle was not adapted for arboreal climbing; the tarsus
conferred minimal flexibility (still does, in birds).  And as for
those "grasping fingers and toes"... this requires some clarification.
 The long fingers allowed the manus to hook and wrap around objects -
but this is not really the same as "grasping".  There was no digital
opposability, and it's even doubtful whether _Microraptor_ could even
grasp small or narrow objects (like a tree branch) with one hand.  To
make matters worse, the fingers would have diverged during flexion.

> In fact, microraptors were better
> adapted than the best climbing primates for the tree life because they had
> wings.

As a primate myself, I'm offended by this statement.  From the very
beginning, primates were consummately adapted for life in the trees.
Even little _Purgatorius_ had an arboreal-adapted tarsus (SVP 2012).
To say that _Microraptor_ even comes close to primates in the arboreal
stakes is a bold statement indeed.

As for the claim that quadrupedal climbing is incompatible with the
use of the forelimbs in flight... this too is surprising.  The Eocene
bat _Onychonycteris_ might be especially surprised by that statement;
it was a volant animal, and a superb quad climber.  Proto-bats could
climb, and have clear adaptations for that purpose.  Proto-birds and
their relatives (like _Microraptor_) do not.

> Indeed, as others point out, the flight abilities of the winged dinobirds
> reduced the need for, and may have precluded, the evolution of greater
> specialization for quad climbing, since adaptations for the latter could 
> impair
> the aerodyamic function of the wings (which benefits from arm rigidity to
> improve flight in birds).

Again, there are a lot of assumptions packed into that statement.  I
agree that the highly defined motion of the forelimb ("arm rigidity")
would have severely impeded quadrupedal arboreal locomotion.  But to
me that is further evidence that "dinobirds" were not arborealists.
An arborealist doesn't want "rigidity" in its appendages - but that's
exactly what maniraptoran limbs were all about.

> Only if arboreal
> adaptations were consistently low among theropods could we say they were all
> nonarboreal, but the adaptations do vary, so arboreality should have too.

Contra your statement, I would say that arboreal adaptations are
actually low among theropods (non-avialans and basal-most avialans).
In most, they were non-existent.  For me, the first evidence of
arboreality is seen in sapeornithids and maybe confuciusornithids.

> Even if they did not have wings, microraptors could have been reasonably
> good arborealists in an age when they were not going up against primate
> competitors (such clipped dinobirds would probably not make it against primate
> level competition).

Primates might not be invented yet, but there were plenty of other
arboreal mammals around in the Jurassic and Cretaceous.  Some of them
even glided (like _Volaticotherium_).