Re: semilunate carpal

["Tim Williams" <twilliams_alpha@hotmail.com>]

Dinogeorge wrote:

> I once watched a mourning dove protect her nestlings by slamming a 
> voracious
> California jay with her wings: She was using the flight stroke as a 
> defensive
> weapon. Give her more primitive archaeopteryx-wings with claws instead of 
> the
> wings she now has and a few million years of evolution, and she could 
> easily
> give rise to a lineage of velociraptor-like animals with predatory-stroke
> killer arms.

The thing is, continued use of the wing as a defensive (or offensive) device 
is not likely to be selected for in a volant bird.  Using the wing to wack 
an harrassing opponent might be used on occasion (like your dove vs jay 
example), but it's going to wreak havoc on the integrity of the wing.

> There's no way that a Velociraptor's arms would have been any good for
> flying. How could you possibly think that they were a step >toward< flight 
> in
> such an obviously non-volant animal (particularly since there were
> contemporary flying birds)?

I'm not saying _Velociraptor_ is the ancestor of birds.  (Yikes, did I 
really need to point that out?)  Velociraptorines and birds share a common 
ancestor, and that ancestor used the predatory stroke (sensu Gauthier and 
Padian) - the lunging, forward-and-downward motion of the forelimb and the 
inward movement of the manus.  Velociraptorines retain the primitive 
raptorial function of this motion in theropods.  The ancestors of birds used 
this predatory stroke, and it was transformed into the flight stroke as the 
feathers along the forelimb became incrementally more aerodynamic.


> The trouble with evolution is that the characters
> and features that we see in the fossil record don't come with little signs
> attached to them saying things like, "I am evolving into a wing," or "I am
> evolving from a wing."

I guess this is where phylogeny comes in to the picture.  The sequence in 
which different behaviors were evolved can be inferred from the acquisition 
of characters associated with these behaviors.


> It is extremely unlikely that the avian wing
> evolved as a random collection of features that at some point in time all
> miraculously came together into a unified structure perfectly adapted to
> flight --like being dealt a royal flush as your first poker hand--without
> already having some kind of flying function.

Poppycock!  (And I mean that in the nicest possible way :-) )

Only *one* feature needed to refined in order to turn the ancestral 
maniraptoran raptorial forelimb into a flapping wing - aerodynamic feathers. 
 This was the Ace which allowed theropods to fly - little maniraptorans 
already had the rest of the cards in their hands.

The skeletal apparatus of dromaeosaurids required to work the predatory 
stroke is *almost exactly* the same as _Archaeopteryx_ used to execute the 
flight stroke.  Apart from the sternal keel (which is actually not present 
in most _Archaeopteryx_ specimens) name one substantive difference between 
the skeletons of _Archaeopteryx_ and a samll dromaeosaurid (such as 
_Microraptor_ or _Sinornithsoaurus_).


> The Big Picture of dinosaur
> evolution--the one that makes the most sense to me, anyway--is that birds
> evolved by incremental improvements from small arboreal diapsids, and on 
> the
> way scads of archosaur and dinosaur lineages and groups branched off this
> evolutionary line, forever forsaking arboreality to become larger, heavier,
> and more cursorial animals.

All that's missing in this intuitively attractive, well-thought-out, 
explicit hypothesis is the fossil evidence to back it up.


Tim


---------------------------------------------------------------------

Timothy J. Williams

USDA-ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163




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