NO SECONDARILY FLIGHTLESS THEROPODS II

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

     In the replies to my first point the constant counterpoint was that  
the ancestors of these theropods were more "primitive" than modern 
birds. 

To this point let me first point out that the *constant*,*consistent*, 
and *only* cause of flightlessness in Aves is paedomorphosis. This is 
the easiest way for a flying animal to lose flight. Darwin argued for 
"disuse" of the forelimbs in flight. The shortcoming of this theory is:

A) There is no selective factor other than paedomorphosis that can 
explain the atrophy of the forelimbs.

The same would hold for early "dino-birds" with dual forelimb functions. 
For an animal to become flightless the limbs and the flight apparatus 
have to be atrophied. 

( Note: For theropods with short forelimbs such as allosaurs and 
tyrannosaurs, the word atrophy does not apply since the forelimbs are 
still robust and strong. ) 

Now if a hypothetical "dinobird" that has volant abilities goes to a 
teresstrial existence, it would probably still stay a volant animal. 
Why? The only reason for a teresstrial animal to go flightless is if 
paedomorphosis occurs. There is nothing to do with the dual function of 
the forelimbs, because they still are essentially wings. 

Feduccia, in his Archaeopteryx claws study, showed that Archaeopteryx is 
still essentially a bird. It shows several features only seen in and 
characteristic of birds. It is still essentially a bird because it has 
feathers and can fly. It has been said that it is possible for 
dromaeosaurs and kin to be descendents of gliders. This is unlikely 
since the glenoid faces ventrally and the forelimb cannot be held that 
far outwards, so it is not a descendent of a glider. 

Ron Orenstein noted that if some theropods were descendents of basal 
birds like Archaeopteryx, then the paedomorphosis argument against this 
issue does not work because the animal does not have the myriad of 
flight adaptations that more derived fliers have. Then it would be hard 
to tell whether the animal is secondarily flightless or not. The problem 
is, that the animal is still a flying animal and many features are 
needed to gain flight. So the animal is still essentially a modern bird 
and the same processes apply to it. ( Let's look at this issue, even 
Archaeopteryx has a posteriorly facing coracoid, a heart-shaped ulnarae, 
a prominent biceps turbercle, and a furcula that could bear large flight 
muscles ). 

Also brought up by Orenstein is the problem of the shift between 
"non-flight" and "flight". An animal that is inbetween these two phases 
that goes "flightless" would be indistuinguishable from a normal 
flightless dinosaur. However, the problem with that scenario is that it 
assumes that flight characteristics were acquired in the transition from 
non-flier to flier. This may not be the case. Various flight 
characteristics may have evolved for arboreal or other purposes. What 
this suggests is the transition from non-flight to flight may not be 
that gradual and paedomorphosis still would apply.

Regarding penguins. Penguins evolved from petrels or petrel-like birds 
that dove into the water. Over time they became accomplished swimmers 
with their forelimbs and they still "fly" in essense. Understood?

MattTroutman

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