Re: NO SECONDARILY FLIGHTLESS THEROPODS

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

<<I very much appreciate Matt Troutman's thorough account of the 
paedomorphosis issue.  To me, the strongest reason for being cautious 
about the secondarily flightless argument is, as Matt says, the apparent 
absence of characters suggesting that the supposedly secondarily 
flightless dinosaurs nest closer to modern birds than does 
Archaeopteryx.  Without such evidence, and without anatomical characters 
unequivocally linked to flight (and considering the argument about the 
function of Archaeopteryx's wings, I cannot imagine what such characters 
would be!) present in these supposedly-derived forms or in a clear 
ancestor to these forms, it would seem to me that the "secondarily 
flightless" argument must fail on grounds of parsimony alone (and I'll 
leave others to argue about the Alvarezsaurids, which Matt did not 
include in his analysis).>>

     Thank you for agreeing on that point.

<<However, I am not sure that the paedomorphosis argument is as 
compelling as Matt makes out in itself.  All of the flightless birds he 
discusses - things like ratites, Thambetochen and the phorusracoids - 
are clearly derived not just from Aves but from highly-evolved birds 
(Neornithes, if I can still use that term).  For such birds, the kind of 
anatomical changes necessary to produce flightless forms may well be 
most readily achieved through paedomorphosis - in effect, since chicks 
are flightless, the easiest way to stay flightless is to stay a chick 
all your life.>>

    Nice summarization of my point. Thanks. 

<< I am not sure that this would necessarily follow for a 
secondarily-flightless animal derived from a creature like 
Archaeopteryx, which had made far fewer anatomical "commitments" towards 
modern bird-dom (if you see what I mean), or that you would notice the 
difference if it did - a re-evolved flightless form would, I think, be 
far more like its flighted ancestor if that ancestor had a minimum of 
flight-related adaptations.  The trouble is, of course, that since the 
shift from "non-flight" to "flight" was presumably gradual, a descendant 
of an animal plucked from midway along this evolutionary pathway might 
not be distinguishable from one branching off earlier or later.  In 
other words, at that early stage "secondary flightlessness" might in 
fact amount to very little.>>

     I don't think that "commitments" have anything to do with the 
issue. I think that flight in birds would be impossible without certain 
characteristics : hypertrophied forelimbs, pectoral musclature, 
assymetric feathers, folding forelimbs, lightened weight. I think that 
Feduccia in 1993 showed appropriately showed that even though 
Arhcaeopteryx is morphological "primitive", it is nonetheless a bird. 
And since it still is a bird, the same processes apply for it and its 
relatives. Though the shift was probably gradual, essentially the 
creature would still be a bird and the same processes might apply. A 
creature plucked from a gliding existence to a teresstrial or whatever 
existence would not apply as a flier. But for a fliers, the easiest way 
to get flightless is to stay a chick.

<<Which brings me back to a point I have made before - all this 
speculation about ecological and behavioural adaptations (including the 
very regrettable - IMHO -"ground-up/trees down" debate which I think is 
both unrealistic and unresolvable) simply confuses the issue of actual 
relationships.  Yes, it has an application to determining convergence, 
but only if you can be sure you know what the characters you are looking 
at are really for, and you can't do that below a certain level of detail 
in extinct animals with no known modern representatives.  The question 
should not be "are oviraptorids (or whatever) secondarily flightless" 
but "what are their relationships".>> 
   
     I agree.

MattTroutman

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