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Re: Microraptor hanqingi, new species from China.
K Kripchak <firstname.lastname@example.org> 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
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. 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
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.
> 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.
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.
> 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
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).
> 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.