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Re: Fw: Dinosaurs and birds

David Marjanovic wrote:

1) is not a problem when we're dealing with bipedal saurischians -- the forearms cannot rotate and are fixed in such a position that the palms face each other. This is still the condition in birds. The flapping motion (in the widest sense) is thus the normal state of affairs (the plesiomorphy) for saurischians (actually a larger group, Archosauria at least) and has not been modified on the way to birds; retentions do not need to be explained.

Just a slight clarification, which in no way contradicts what David just said. Maniraptorans modified the basal archosaur mode of forelimb motion to a much more specialized degree. The wrist was converted into a 'swivel', courtesy of the semilunate carpal element, which allowed the manus to be folded (flexed) against the lateral side of the forearm in a manner homologous to wing-folding in modern birds. The maniraptoran forelimb was extended in a highly coordinated manner, such that the humerus would be protracted outward and forward, and the manus swung forward and inward.

All this makes the maniraptoran forelimb "pre-adapted" to the execution of a flight stroke, more so than the basal saurischian (or archosaurian) condition.



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