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Re: Adaptive advantage (was Re: ABSRD BAND on Sinornithosaurus feathers)
"run-jump-flap" cursorial model of Caple, Balda and Willis. Incipient
The "climb-leap-flap" arboreal model is not too different from the
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
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
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)
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.
And it's on the long path to theory status...
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