Re: Paedomorphosis ( Re: BARYONYX' CLAWS )

["Matthew Troutman" <m_troutman@hotmail.com>]

<< I wish you'd stop calling "swimming" flying. It doesn't help your 
argument a bit. These are birds that have exapted their flight anatomy 
for the purpose of swimming through water. In becoming more efficient 
swimmers, they lost the ability to fly through the air. You noted in a 
previous post that there may have been a transitional stage in which the 
wings were used both for flying through the air and swimming through the 
water. With this I can agree. Likewise with small, arboreal theropods, 
there was probably a stage in which the forelimbs were used for grasping 
and climbing and for some kind of flying. In later theropods, the 
grasping uses prevailed; in later birds, the flying uses prevailed. Why 
is this incredibly simple, obvious, and natural idea so difficult for 
you to accept?>>

     I'm not calling swimming flying. Just because cetaceans swim does 
not mean they fly. However, where the swimming birds differ from 
cetaceans, swimming birds had volant ancestors and their wings still 
take on a function that is similiar to the flight stroke so they are not 
"flightless" in the sense of ratites. They still are essentially fiers. 

This incredibly simple, obvious, and natural idea is so hard for me to 
accept because the submarine fliers still use their forelimbs for the 
same functions as flight ( creating thrust, moving the body ). In your 
transitional pseudo-phylogeny the small arboreal climbing/gliders would 
not use their forelimbs for the same functions as flight ( creating 
thrust, etc ). The majority of the work would have to be done with the 
hindlimbs. And while this is happening, paedomorphosis would have to 
occur for the animal to become completely flightless and the forelimbs 
to be freed from their flight motions. And since there is no evidence of 
the paedomorphic trends seen in flightless birds in theropods, this 
scenario is unlikely.

<<All you're saying here is that theropods would have made lousy 
swimmers (if they tried to use their forelimbs for this). With this, I 
would probably agree, too.>>

    No. Retaining volant characteristics in penguins, auks, and 
plotopterids is essential for their type of swimming or flight. Now, if 
in theropods, they needed to use their forelimbs for predation : why 
wouldn't they lose flight the typical avian way ( paedomorphosis ) and 
tweak their forelimbs to work the way it does in phorusrhacids ? ( in 
which flight was lost the typical avian way and the forelimbs was 
modified ).

>>The basic theropod shoulder girdle is utterly different from the 
>>penguin, auk, and plotopterids. And the penguin, auk, and 
>>plotopterids still use the same  basic flying motions.

<<Of course the basic theropod shoulder girdle is different. The avian 
shoulder girdle had not yet evolved when "basic theropods" (whatever 
those are) diverged. The "basic theropod" ancestor probably used its 
wings for grasping as well as for some kind of flying (certainly not 
powered, flapping flight, but perhaps a kind of fluttering--better than 
nothing for those moments of free fall when living in the trees).>>

     Grasping is not an adaptation that comes easily from the avian 
flight apparatus. It goes against the basic movements of the avian 
forelimb. In phorusrhacids the forelimb has been remodeled and changed 
so the flight apparatus can be made into a grasping organ. Nothing of 
the sort was done in theropods, whose forelimbs were of conversative 
design. Lets look at the avian flight apparatus ( this would apply for 
"primitive" fliers too ) : 

The avian flight apparatus has two main muscles, the M pectoralis ( 
downstroke ) and the M. supracoracoideus ( upstroke and humeral rotation 
). Plus the M. biceps branchii ( pulling limb forward ) and the dorsal 
elevators ( pulling the humerus back ). Now flight is impossible, even 
for primitive fliers, without lift, thrust, and special forelimb 
mechanics. For the wing to be exapted for grasping the M. biceps 
branchii would have to be hypertrophied ( because the motions of 
grasping are basically where the limb is pulled forward ) and the M. 
pectoralis and M. supracoracoideus would have to either atrophy or 
change their function to work with the M. biceps branchii. Since this 
features are not seen in theropods ( features along these lines are seen 
in penguins because thrust is the main component of their flight ) it is 
unlikely that they came from flying ancestors. 

>You're making me ramble.

   Rambling is good. It makes people think.

MattTroutman


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