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Re: Adaptive advantage (was Re: ABSRD BAND on Sinornithosaurus feathers)



I have been pointing out for some time now on this list that this simply is not true. There are living birds that are excellent climbers without having to resort to flight to help them do it...(SNIP)..This is not true either. The Kakapo is a flightless bird that is quite
capable of climbing...(SNIP)...Which reminds me, of course, that hoatzin chicks are flightless arboreal scansors too. <<<

Hoatzin chicks as scansors? That is news to me. I'd be interested in a reference to hoatzin scansorial behavior, as every reference (and nature film) I've seen shows them climbing slowly through the trees. Laborious climbing is arboreal behavior, but is isn't scansorial behavior. Scansors utilize a ballistic phase in locomotion from branch to branch.
The point you raise about lack of knowledge of recently extinct flightless birds is true enough, but (even though I brought it up...) I think the entire issue is a red herring, as is citing birds that exhibit scansorial behavior without opening their wings. All birds have already had millions of years of evolution that pre-adapts them to scansorial behavior, namely they have had to adapt to the environmental three-dimensionality common to flight and scansorial behavior. Since flight is plesiomorphic to all extant groups of avian scansors (obviously), they also all had the safety-net of flight _while_ they were adapting to scansorial habbitats. Finally, while hopping from branch to branch, avian scansors usually use a symetrical gate, as do many terrestrial birds. A symetrical gate greatly reduces the amount of yaw and roll created during locomotion.
Theropods adapting to scansoriality had none of these benefits. There are no known non-avian theropod tracks showing symetrical hopping gates, so suggesting that arboreal non-avian theropods did would be speculative at best. (I am assuming mainstream theropod phylogeny here, rather than BCF, which changes this arguement some.)
Moving the center of mass away from a substrate makes torosional forces harder to counter, and theropods simply don't show adaptations to counter those forces. They could have shortened their limbs, but in fact the opposite trend is true of progressively more bird-like theropods. Birds are a bad model for theropods in trees, and all other scansors are even worse.


Further, larger rails are quite capable of getting into some trees without flying, by walking up sloping limbs.<<<

I really don't object to small theropods getting into trees, I object to them being well enough adapted arboreal scansors to have the optertunity to evolve gliding behavior.


I have repeatedly suggested that an UPWARD leap to snatch prey from leaves or thin branches (eg insects, frogs, lizards could have been in the repertoire of early proto-flyers, and birds certainly do this today.<<<

I agree that this is the most likely way that flight would evolve in an arboreal scansor, but coelurosaurs consistently evolve characteristics that would hinder this tye of behavior.


I wrote: >>> Energetic demands dramatically favore this interpretation of the insect version...(SNIP)<<<

To which you replied:
Falling out of a tree while trying to snatch a prey item in mid-leap can be pretty catastrophic too.<<<

That was a typo. I meant to say: Energetic demands dramatically favore this interpretation _over_ the insect version, implying that _terrestrial_ hunting of large tetrapods is more prone to catastrophic failure than _terrestrial_ predation of insects.
Of course, arboreal insect predation is also prone to catastrophic failure, but no one has shown that there is an imediate benefit to theropods with arm fringes in this type of behavior, while there is to terrestrial vertebrate hunters.



Another post suggests that:
The shape of the claws in _Archaeopteryx_ suggest that it did use its claws to climb trees (Yalden, 1985; Feduccia, 1993). This does not mean that small maniraptoran theropods which did *not* have specialized scansorial claws were incapable of climbing trees.<<<

Previous studies (such as Feduccia's) are misleading, in that they did not consider raptorial claw types when making their comparison. Jason Kenworhty's talk in Mexico City showed that the claws of Archaeopteryx are more like those of raptorial birds than those of climbing or perching birds. The reversed hallux of Archeopteryx is too small (and has too small of flexor tubercles) to have been much use during perchng, and was more likely used for manipulation of small prey. There continues to not be any positive evidence of arboreal adaptation in Archeopteryx.


To sum this up, I realize that animals are very adaptable, and that small theropods may well have been able to climb trees. But all evidence surmised from the aquisition of derived traits points to increased cursoriality in coelurosaurs, including Archeopteryx. This is why I took the time to develop an aerodynamically sound hypothesis for the evolution of flight within terrestrial cursors. Just because my hypothesis is aerodynamicallt sound doesn't mean it happened historically, and we may yet yet find derived arboreal theropod scansors, (Microraptor is certainly not one), but at the moment the fossil record doesn't seem to support the idea.

Scott
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