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Re: Catch-22

K Kripchak <saurierlagen1978@gmail.com> wrote:

> The chatter in the recent “11th Specimen of Archaeopteryx” discussions
> concerning capabilities and traits (or lack thereof) for climbing (NOT
> “perching”) in theropods/basal birds reminded me that there is still
> an 800-lb gorilla in the room wearing a pink t-shirt that is one size
> too small with "Catch-22" scrawled across it in Air Force Blue...
> There’s an obvious question that still hasn't received good,
> convincing, direct answers...

I'm so sorry about that.  We really try and do our best here.

> Were theropods/basal birds up in the trees BEFORE they possessed the
> MODERN traits we currently use to define a bird as being arboreal?


> If the answer is yes; what were those traits that allowed them to
> climb into the trees?
> If the answer is no; WHY did selection favor characteristics for an
> arboreal lifestyle when theropods/basal birds were cursorial?

That's just it: the question you raised doesn't appear to have a
simple yes/no answer.  If it did, this thread would have ended a long
time ago (to the relief of many, I don't doubt).  There are some
characters pertaining to the pes of taxa such as _Microraptor_ and
_Archaeopteryx_ that *might* have been involved in scansorial or
arboreal behavior.  Characters like a lower hallux, and elongated
penultimate phalanges, for example.  But these characters might
equally have been the product of allometry, or involved in predation.
If so, they still might qualify as incipient arboreal characters, if
they were the raw material upon which more derived taxa (which were
presumably volant) constructed a specialized arboreal pes.

> Whales, before getting wet, didn't just decide to lose their hind
> limbs, sprout flukes, and head for the sea, right?

Right.  IMHO stem-cetaceans provide a good analogy, because raoellids
and pakicetids have both been interpreted as terrestrial quadrupeds
that headed to water only occasionally, such as a refuge, or to feed
on fish.  Raoellids and pakicetids show minimal adaptations to an
amphibious/aquatic lifestyle.  The limb bones of both groups are
pachyostotic ('osteosclerotic'), a trait that has been tied to
increased submersion by these animals, with the heavier bones
providing ballast.  In all other respects, the postcranial skeletons
of these stem-cetaceans are thoroughly terrestrial and cursorial.

To return to theropods, it is possible that the ambiguous scansorial
characters seen in the pes are indeed nascent arboreal characters in
otherwise dedicated  terrestrial/cursorial animals.  I tend to think
so.  But I admit that this assertion comes from viewing these
characters through the prism of what came after.  Arboreality had to
start somewhere, and the elaborate plumage suggests to me that the
paravian integument had a big headstart on the osteology.



> Kris
> PS: Bit of trivia for you literary types/WWII buffs... Anyone catch
> the reason for the "Air Force Blue" reference? ;-)