Re: Fw: Dinosaurs and birds

[Tim Williams <twilliams_alpha@hotmail.com>]

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.

Cheers

Tim

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