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Re: Microraptor hanqingi, new species from China.



On Thu, May 24, 2012 at 6:44 AM, Tim Williams <tijawi@gmail.com> wrote:
> K Kripchak <saurierlagen1978@gmail.com> wrote:
>
>
>> Tim... We've discussed this before... and this is not directed at
>> you... but the opportunity to return back to it again has presented
>> itself...
>
>
> No problem.  Because there always seems to be a new paper out on
> Mesozoic birds or the origin of avian flight, the topic of arboreal or
> aerial behaviors in theropods always seems to be current.
>
>
>> Since ornithothoracean theropods ended up being comfortable in the
>> trees, how and why did that happen if non-ornithothoracean theropods
>> weren't becoming comfortable in the trees?
>
>
> It is possible that true arboreality arose relatively late in avialan
> evolution.


"True" arboreality... If we are leaning toward the notion that "true"
arboreality arose with the perching pes, no discussion necessary. It's
what I see as those "pseudo"- arboreal or "near" arboreal critters
that came before (and were often still around)...


> For confuciusornithids and sapeornithids there is
> morphological support for a perching pes.  The hallux was medially to
> posteromedially directed, sat low on the foot, was enlarged (although
> less so in _Confuciusornis_ ), and all pedal unguals were large and
> highly recurved.  These traits are consistent with some perching
> ability.  Later birds went further and swung the hallux into a fully
> posterior position (though derived enantiornitheans and
> ornithuromorphs achieved this in different ways).  But
> confuciusornithids and sapeornithids have all the basic ingredients of
> a perching foot.
>
> I'm quite at peace with the idea that avialans did not become truly
> arboreal until Pygostylia, or thereabouts.  There are also studies
> that have inferred that archaeopterygids and confuciusornithids were
> incapable of powered/flapping flight, but were only capable of some
> form of gliding.  So it may be that both powered flight and arboreal
> behavior arose at around the same juncture: at or just before
> Ornithothoraces.
>
> I'm not saying that a perching foot is the be-all-and-end-all of
> arboreality.  But if you're a theropod, and you want to spend a large
> part of your life sitting on boughs or branches, you need to have a
> way of holding on.  From where I'm standing, _Archaeopteryx_ and
> _Microraptor_ just weren't cutting it in this department.  Neither the
> hands or feet were adapted for grasping branches.


Alright... Microraptorans are considered to have been gliders.
Confuciusnornithids are considered to have been gliders.
Confuciusornithids are considered to have been "true" arboreal because
they had a foot morphology that enabled a degree of perching.
Therefore, Confuciusornithids... gliders... were climbers (had to get
"up" there somehow) and were "true" arboreal since they had perching
feet.  Microraptorans... gliders... *may* have been climbers... and
they couldn't have been arboreal to any degree since the characters
(pleural) they did possess are ambiguous and they lacked the foot
morphology to enable perching?

It's hard not to interpret the perching foot as being the linchpin for
arboreality when it is repeatedly referred too during most discussion.
Sure, they weren't *perfect* hands, or feet, or ungul geometry, or
limb/digit length ratios, or whatever for grasping branches... But why
expect animals in transition to have the perfect anything or fully
functioning whatevers for behaviors or niches they were in the process
of adapting to/being selected for to exploit?

What marks the transition from scansorial to arboreal? Are their
grades or degrees to it? It is behavior-based, morphology-based, or a
combination of both?  Was there ever such a transition?  I'd derive
there was given those Confuciusornithids... again pressing the
question of critters possessing a "lower grade" of
scansorial/climbing/arboreal abilities and behaviors.

>
>> I'll go out on a limb and say behaviors conspired with selection, but
>> we'll need to enlist the services of H.G. Wells' Time Traveler to
>> decipher that mystery. The way I see it,... although Science demands
>> it (and we'd like to have it), an orderly, by the numbers,
>> trait-specific and clearly defined pattern for a transition that was
>> likely being influenced by a symphony of selective pressures across a
>> wide range of species to different degrees with different expressions
>> and over a great expanse of time... just doesn't exist.  It's not like
>> we're discussing something "easy", like changes in centrosaurine horn
>> and boss counts. We're theorizing on the how's and why's a group of
>> animals underwent a radical change in their mode of life, using
>> road-kill smears of the transition stitched with traits of derived and
>> extant forms to trace the path backwards. No wonder there are
>> different points of view based on the same evidence.
>
>
> I agree: it's not easy.  But it's hardly hopeless.  The radical change
> you speak of was almost certainly slow and gradual, with reversals and
> dead-ends in various lineages.  The shift to an arboreal morphology,
> with the hallux recruited for grasping (i.e., effectively opposing the
> other three toes), was no more radical than the transformation that
> went on in the therizinosauroid foot.
>


Reversals... Sounds good to me.

And totally out of left field with this one... but are Gorilla
beringei beringei (mountain gorillas,) which spend most of their time
foraging on the ground, yet have the morphology to be arboreal,
considered to be "true" arboreal?


> I concur that there are limits to what the fossil record can tell us.
> But if we (a) have strong support for the sequence of character
> acquisition in the line leading to birds, and (b) a well-founded
> scientific basis for the function(s) of these characters, then I think
> we're well on the way to reconstructing the origin of arboreal
> behavior in theropods.  Having an evidence-based rather than an
> opinion-based approach certainly helps.
>


My approach is not opinion-based. My approach is (1) questioning our
artificial constructs of (a) and (b) based on a different
interpretation of the available evidence and (2) investigating the
reasoning behind theories built upon said evidence and constructs.


>
>> Such are the reasons for why, in the grand scheme of things, I
>> hesitate to regard particular theropods as not being scansorial or
>> even slightly this side of arboreal because certain traits have been
>> deemed ambiguous for arboreality. Ambiguous to who (or whom)?
>
>
> By "ambiguous" I mean that the connection between these characters and
> arboreality is disputed, or even downright incorrect.  Some of the
> connections have been wholly intuitive: such-and-such character is
> consistent with an arboreal lifestyle.  But when someones takes the
> trouble to scrutinize these "arboreal" characters, the connection is
> found to be wanting.  The link between elongated penultimate phalanges
> and branch-grasping ability is one such example.
>
>
> The distal shift in the hallux, such that the first metatarsal it is
> positioned lower on the foot, similarly may have little or nothing to
> do with arboreality.  The "Raptor Prey Restraint" hypothesis has a lot
> of sensible things to say about how certain predatory adaptations in
> the paravian foot might have pre-adapted it to perching later on.  If
> so, the evolution of the perching foot would be an example of
> exaptation.
>
>
> Other "arboreal" traits described for non-avialan theropods are
> actually non-existent.  For well over a century _Archaeopteryx_ was
> assumed to have a reversed hallux, based on preservation in the
> fossils.  Then someone actually looked at what characters make up a
> reversed hallux, and found these characters to be non-existent in
> _Archaeopteryx_ (or any other non-avian theropod).
>

No, no... I understood what you meant by ambiguous.   "Who or whom"
was insinuating that to us, the characters/traits are ambiguous, but
if the animal put itself up in a tree, they were anything but.


>> Alright.  That dead horse has been sufficiently beaten... again ;-)
>
>
> Expired equines aside... I think in our eagerness to put winged
> theropods in trees, there is the temptation to view many of their
> characters through the lens of arboreality.  In reality, the evidence
> for arboreality in these taxa is very weak.  I personally *like* the
> idea of arboreal microraptorians and archaeopterygids: it's cool.  But
> just because something is way cool doesn't necessarily make it so.
>


Just like everyone else who lays awake critically thinking about this
stuff, I'm drawing my own set of conclusions based on our current
understandings... not just pulling them out of wishes and desires for
coolness. Although arboreal microraptorans are rather cool :-)


>
>
>
>
> Cheers
>
> Tim


V/r,
Kris