Re: NO SECONDARILY FLIGHTLESS THEROPODS

[Ronald Orenstein <ornstn@inforamp.net>]

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).

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.  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.

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".  Once you clear
that up, the first question may become a lot easier to answer, or at least to
speculate about.
--
Ronald I. Orenstein                           Phone: (905) 820-7886
International Wildlife Coalition              Fax/Modem: (905) 569-0116
1825 Shady Creek Court                 
Mississauga, Ontario, Canada L5L 3W2          mailto:ornstn@inforamp.net