Re: NO SECONDARILY FLIGHTLESS THEROPODS

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

<<   Its also worth noting that secondarily flightless birds dropped out 
of flying when they had already become much more modified for flight 
then _Archaeopteryx_. The adaptations of a secondarily flightless 
immediate descendant of _Archaeopteryx_ (Or a close relative) and the 
secondarily flightless descnedant of of neornithine bird SHOULD differ, 
because they started out with different material. >>

    Why? The basic flight architecture is still there in Archaeopteryx. 
It has hypertrophied forelimbs, scapulacoracoid with an acute angle, low 
interclavicular angle, etc. Though it may the triosseal canal and a 
prominent bicipital crest, it is nonetheless still a flier with features 
similiar to modern birds. A flightless descendent of an 
Archaeopteryx-like animal would still have to become flightless through 
paedomorphosis. There's no getting around it, no matter how basal a 
flying animal is, the process by which it becomes fligthless is most 
probably paedomorphosis. 

<< In the case of forelimb size neornithines can't use them for much but 
flying, because they are too modified along those lines, so a 
secondarily flightless descendant would be expected to have atrophied 
wings. However, _Archaeopteryx_ still has unfused fingers and claws, so 
a descendant (like the dromeosaurs) might still be able to get some 
functional use out of them if it stopped flying.>>

      No matter how much the forelimb is able to function is irrelevant. 
Phorusrhacids have a functional forelimb that is some atrophied (though 
was intensely modified ). No matter if the animal takes up a seperate 
function of the forelimb, the ( for lack of a better word ) urge to fly 
is still there. Paedomorphosis is the factor that makes all birds 
flightless. The way it does this so consistently is that it takes most 
all function from the forelimb and feathers so the animal is sort of 
forced into flightlessness ( phorusrhacids appear to have reevolved some 
function to their forelimbs, which accounts for their remodeled 
structure ). Phorusrhacids give a good example at how a functional 
forelimb can evolve in a secondarily flightless animal. The movements of 
the flight stroke are essentially controlled by two muscles : the 
M.pectoralis ( downstroke ) and M. supracoracoideus ( upstroke ). The 
flight stroke is very steorotyped in structure. If an animal secondarily 
evolves function into the forelimbs the forelimb has to be remodeled. 

MattTroutman

  

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