Re: Ah ha! That's where therizinosaurs came from

[Jason Brougham <jaseb@amnh.org>]

> 
> I have long been pointing out that all flightless "theropods" with 
> extensive flight adaptations are likely to be secondarily flightless because 
> ... the situation parallels that of ratites etc. 

On that subject I have another question:

Question 3,
What about furculae?

When crown group birds become flightless they quickly lose their furculae, or 
retain them as splint - like, unfused clavicles.

In Dinosaurs of the Air Mr. Paul wrote that Coelophysoids were an early 
ancestral stock to birds. Coelophysis has a  furcula that  some may regard as 
virtually identical to the furcula in dromaeosaurs like Velociraptor, though 
Paul sees the latter as secondarily flightless. Thus, in this scenario, the 
furcula passed through the radical modification of the pectoral anatomy, and 
possibly even the development of the supracoracoideus humeral elevation system, 
without changing. Otherwise the furcula returned to its ancestral state.

If we can say that the similarities in the shoulder anatomy between ratites and 
dromaeosaurs are evidence that supports flightlessness, then we must say that 
differences are evidence that rejects flightlessness, mustn't we? Because if we 
say that some differences don't matter then we can say that some similarities 
don't either.

Moreover, if basal birds kept giving rise to diverse flightless forms over and 
over again, shouldn't at least some of them, perhaps out of  random diversity, 
or perhaps more likely in the most derived fliers, have lost their furculae 
upon becoming flightless?

Mr. Paul, how do you account for this departure from the pattern in flightless 
crown group birds? Is it possible that the furcula is an exception to the rules 
of thumb about the suite of features that indicate flightlessness?

Mr. Paul will likely respond that the furcula is not an important flight 
feature, since modern volant parrots also lost their furculae. But if we can 
say that volant and flightless animals can have the same feature for different 
reasons (the feature here being the lost furcula) then we can say the same 
about all of the features that make non-avian maniraptorans appear to be 
secondarily flightless.