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



The "climb-leap-flap" arboreal model is not too different from the
"run-jump-flap" cursorial model of Caple, Balda and Willis. Incipient
flight structures on the ends of the arms were suggested by this trio as stabilizers during lunges at flying insects. As well as preventing the proavian from toppling forward during these brief aerial forays, the "proto-wings" helped control the direction of the leap - and later (as the flight surface expanded) the duration of the leap as well. The
downward-and-forward movement of the arms used for these lunges was the
precursor to flapping flight.
I like this idea, proposed nearly 20 years ago (and it is consistent with the arrangement of feathers in _Archaeopteryx_ and _Caudipteryx_.) But, personally I have objections to the "ground-up" evolution of flight. I am very comfortable with small theropods being semi-arboreal, so I tend to favor a "trees-down" origin of flight - the initial stages being assisted by gravity, rather than in defiance of it. I also don't think _Archaeopteryx_, or its flightless ancestors, were suited for snatching insects out of the air.<<<


I think somebody presented something about this at SVP... It certainly seems intuitive that small theropods were hanging around in trees. What isn't often addressed is that being able to climb a tree (as I am, for example) isn't enough. In fact, all gliding tetrapods are arboreal scansors; critters that leap from branch to branch (or branch to trunk, etc). Theropods are the anti-scansors. Their limb proportions and acetabular articulation precludes this type of lifestyle. Archeopteryx takes these problems to truly amazing proportions, and was ill equipped for leaping about in trees.
Citing birds (with grossly similar hindlimb morphology) as examples of facultative scansors is misleading. Birds probably could not function as scansors without the safety net of flight being present. Notably, even though there have been small flightless birds, none of them were arboreal scansors.
I agree that insect predation seems an unlikely driving force for the evolution of flight, which is why I hypothesiszed that in fact predation of vertebrates resulting in a short-duration ballistic phase was a more likely pursuit (pun intended) for the antecessors of birds.
Energetic demands dramatically favore this interpretation of the insect version, since success with larger prey items far overshadows the cost of the behavior. Additionally, catastrophic failures (lack of stability resulting in being trampled by your prey) are far more likely when hunting vertebrates, providing a stronger selective impetus than hunting insects.
Finally, and perhaps most important, the behavior I've described is consistent with that associated with the best supported outgroups of phylogenetic analysis, specifically dromeosaurs and (to a lesser extent) troodonts.
The filamentous branching structures creat more effective grad, and as I showed at SVP this year, drag imparts imediate benefits in terms of stability to this type of predation (mathmatically this is not radically different fromthere is previous models of terrestrial insect predation) An imperfect feather can still intertwine to become a rudimentary aerofoil, and once any amount of lift is created, there is smooth gradation in evolutionary morphospace from inception (assisting in roll and yaw controll) to the ability to increase the altitude and duration of leaps onto prey.
Clearly more research is necessary, but I'm happy to say that the stage of feather evolution represented by Sinornithosaurus nicely matches predictions made by my hypothesis.


Scott

And it's on the long path to theory status...
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