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To clarify matters, by sit-and-wait predation I meant creatures that either
sit and wait for something to come by, or very slowly creep up on it, and use
special, high velocity body parts such as projectile tongues or unfolding
forelimbs to reach out and touch their prey.
The idea that the folding arms of avepectoran dinosaurs evolved for
snap-action predation was not absurd, but was always weak in view of their
absence in running predators, and the lack of need for such organs in
runners. Only if such a system was present is an animal that clearly had no
other evidence of a flight heritage would such an explanation be viable. It
was an ad-hoc attempt to try to explain how such a system evolved in
predators widely thought to have no flight heritage. If these dinosaurs all
flew or had flying ancestors then the problem disappears because the folding
action simply evolved to tuck up the wings like an F6F on an Essex class
carrier (cue in the Victory at Sea theme).
Note that absence of a folding arm does not necessarily refute a flight
heritage, since some flightless birds cannot fold their arms and lack the
semi-lunate carpal. It is interesting that derived therizinosaurs have a less
well developed wrist folding system than the basal examples. Another one of
those reversal issues that vex cladistics.
Although I suspect that basal birds had a better developed wing elevating
system than Nick thinks, he is correct that it was not as good as in modern
birds and this is an argument against the Dial hypothesis.