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Re: Microraptor hanqingi, new species from China.
> Expired equines aside... I think in our eagerness to put
> theropods in trees, there is the temptation to view many of
> characters through the lens of arboreality. In
> reality, the evidence
> for arboreality in these taxa is very weak.
I think it's just an artefact of the fact that crown dino diversity is mostly
passerines. We're familiar with perching birds. But the fact is that even basal
Neornithes do not seem to have been significantly arboreal. Even "perching
ducks" ("Cairinini") are equally well adapted to running as to perching.
This may not hold true for all stem (par)avians. And even being better at
running than at climbing does not rule out some degree of arboreality. And
non-flapping flight virtually requires an elevated starting point of some sort
But altogether, I don't see any reason to presume that in the Cretaceous there
was a lot of true arboreality. Critters that could make it up a tree safely and
down adain just as safely, no problem with that. But critters that spend most
of their lifes in trees? Not many.
Given that cursoriality and incipient arboreality have to coexist for true
arboreality to evolve in a terrestrial lineage, it may be we are looking for
the wrong things. Adaptations to climbing are fairly trivial, but they are NOT
evidence for true arboreality, only for low-level adaptation to a woodland
What is probably more important is *loss* of *cursorial* adaptations. If we
assume that even Archie used elevations (cycads for example) to launch, we
could probably "prove" this. It was certainly able to get up stuff where a
therizinosaur or ornithomimid just couldn't get up on. Same for _Microraptor_,
it was "more arboreal" than those relatives strictly adapted for cursoriality.
But this alone does not make them arboreal taxa. What is more important than
some feature that *could* be helpful in clambering around in woody vegetation
is the *loss* of running adaptations.
Because to lose the ability to get out of trouble by moving horizontally on the
ground, the ability to move horizontally in the air and/or vertically off the
ground must be well-developed enough... anything else would make the local
predators very happy.
So rather than asking "was [some tasxon] adapted to arboreality?", we might
instead better ask "was [some tasxon] *disadapted* to *cursoriality*?"
As a side note, the principal evolutionary breakthrough of the "higher
landbird" radiation (which seems to roughly coincide with the K-Pg extinction*)
seems to have been an extraordinary plasticity of the distal tibiotarsus.
* Though it is not clear if the extinction was the reason for this plasticity
(via increased mutagenesis and founder effects), or the reason for the success
of a pre-existing plasticity (via removing the competition). In any case, it
looks like both the alpha diversity and the morphological diversity of crown
"higher landbirds" is in some way related to how the Mesozoic ended.