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

At 12:54 AM 17/03/01 +0000, Scott Hartman wrote:
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.

I was using "scansorial" in the sense I know it, which simply means "climbing".

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.

I disagree here. This is a genuine, known gait that allows an animal to get around in trees without flying, and the "safety-net" need not be a problem. Tree Kangaroos can jump 80 feet out of a tree to the ground without harm, for example.

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.)

If the gait was used in trees, why would it leave tracks? The difference between walking and hopping is really quite minimal; some birds do both, and even among those that don't the difference is trivial; most Savannah Sparrows hop, but the Ipswich Sparrow, which is only subspecifically different, walks.

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.

Have you ever analysed the gait of birds like guans in trees? Or couas, or birds of paradise? This is a very sweeping statement!

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've been through this one before, but I don't see that gliding behaviour is a necessary precursor to flapping flight anyway (If you mean long-range gliding as opposed to simple parachuting). If there was an advantage for them to get from one branch to another, or to jump safely from a branch to snatch a prey item, anything that added control or stability to that action could be selected for.

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.

Such as?

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.

There may very well be, if it gives even a momentary parachuting ability that would permit the animal to return safely to its original, or a nearby, perch.

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.

You don't need a reversed grasping hallux to sit in a tree. Tree ducks do this quite well. And nobody ever looks at guans, which do not so much perch as run along the upper surfaces of large limbs.


Ronald I. Orenstein Phone: (905) 820-7886
International Wildlife Coalition Fax/Modem: (905) 569-0116
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