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Re: CNN: how first birds flew



At 03:04 PM 5/7/99 -0400, Larry Febo wrote:

>I`ve always thought of flight as
>involving very specialized structures and precise motion of the wing
>structure.

As various authors have shown, the refinement of the modern flight wing of
birds took a series of stages in bird history.  Development of structures in
the wrist, in the relative proportions of the forearm bones, of the manus
(the alula, etc.), and so forth did not happen all at once.  Yes, the modern
bird wing is a highly specialized structure, but earlier bird wings were
less specialized.

>I can see how the degree of movement becomes "locked in"
>genetically, as a bird has little reason to stray from this confined motion.
>It`s wings have one main purpose, that being...to produce lift for flight.

Grrr...  That is part of the point of the Burgers & Chiappe article: that
thrust is ALSO a major component of the bird wing.

>For theropods to have developed similar restricted motion in anticipation of
>future use for flight we all know is impossible. As for this motion being
>"useful" for a predatory grab...OK, I guess it could have been (after all,
>these creatures did survive). But, to consider this restricted motion as
>somehow being optimal for dealing with prey, that I don`t see.

Curious?  Why not?  Praying mantis forelimbs are restricted to certain types
of motions, cats to others, and other animals which use their arms to grab
prey in others still.  Also, remember natural selection does not find
optimal solutions: it selects (on average) the better working solutions from
among those available variants.  There are a lot better ways of building a
panda hand or a giraffe drinking posture than what natural selection produced. 

>I would think
>that with all the time theropods had to exist as cursorial organisms since
>their Triassic precursors, they would have developed freer joints, with less
>restricted range of motion,...more "ideal" for catching and manipulating
>prey.

Well, they DID.  The wrist might be more restricted, but elongation of the
manus, and greater degree of movement at the shoulder and elbow do form a
new and greater volume in which the arms could move.

Consider this scenario.  I'm not saying it is correct, but I am offering it
as a possibility:
I) Ancestral dinosaurian carnivores with moderate length arms, wrists
without much "folding" ability in the manner of birds.
II) Selection favors variants with a longer reach (longer arms, longer hands).
III)  Selection favors variants among long-armed dinosaurs with the best
ability to fold and tuck the arms.
IV)  Having evolved the fold-and-tuck structures, variants with greatly
elongated arms can develop and still use their forelimbs to sieze prey or to
climb with, without the arms getting in the way while running.

Although this is all a just-so story, it matches the pattern we see in
theropod evolution (I being basal theropods and ceratosaurs, II being basal
tetanurines, III being basal avetheropods (with small semilunate carpal
block) such as carnosaurs, _Coelurus_, _Scipionyx_, tyrannosaurids, and
maybe ancestral ornithomimosaurs, and IV being long-armed maniraptorans.

In this sense there is a sort of feedback: evolution of longer arms produces
conditions in which folding becomes favorable; evolution of folding produces
conditions in which greatly elongated arms are possible without interfering
with running.

>The fact that they did not,...I would attribute to their, in fact,
>being secondarily flightless forms, restricted in their arm motions due to
>prior genetic "commitments" to the specific function of flight. And I think
>this development of secondarily flightlessness happened many times to yield
>the various theropod types. Of course, this is just my opinion, but I really
>see it as having happened this way.

Fair enough.  However, as I said in the last posting, opinion of a model and
fitness of a model are two separate issues.

>On the other hand, I see little or no
>evidence showing it couldn`t have happened this way,

But can you provide evidence why that model should be favored over any other
model, without resorting to "I like this model better" or "I can't imagine a
better way it could have happened"?  If so, you can suggest ways to test
between two (or among more) models.

>and as to why it isn`t
>even considered as a possibility by some I find even harder to understand.

But it *is* considered a possibility by some others: the fact that people
are discussing it shows that.  However, it may not be the best supported
scenario at present, but that could change if new evidence were presented in
a form other than "I think that this model makes more sense to me."

In a very real sense, Burgers & Chiappe's paper is a similar type of
situation.  Although the "ground-up" model has been supported by many
people, it has primarily been supported on phylogenetic rather than
biomechanical or aerodynamic grounds.  In fact, some people have claimed (in
papers or talks) that it would be phyiscally impossible to develop flight in
this manner.  What Burgers & Chiappe have done is attempt to show that, in
fact, it is *not* impossible to generate early flight in this condition.

The study does not "prove" that flight started from the ground up: as we
just reminded some of our students defending their Senior Theses, you don't
"prove" things in Science.  However, you can create models to test your
hypotheses, and the Burgers & Chiappe paper does just that.  It would have
been entirely possible for the model they produced to generate results
showing that it was in fact physically impossible to begin flight under
these parameters.  Instead, they values they produced are consistent with
running flight.

So, in the future, it might be productive to start thinking of ways to model
and to test the various flight origin scenarios rather than simply saying "I
can't understand why people believe this".

COULD various traditional maniraptorans groups like oviraptorosaurs or
dromaeosaurids or troodontids be secondarily flightless?  Damn straight.
Would it be consistent with the evidence?  Sure thing.  However, the
evidence is currently ALSO consistent with the alternative (that these forms
lie outside flying dinosaurs, and that the basal maniraptoran forelimb
morphology evolved for purposes other than flight).

What we need now are explicit methods to support or reject one or the other
scenario (for all the groups, and for each individually).  One way is
phylogeny: current published or presented cladograms do not place
troodontids, dromaeosaurids, or oviraptorosaurs within known fliers.  That
could change in the future, with new or reinterpreted data.  Other ways
include biomechanics, taphonomy, etc.: at present they are either equivocal
or unstudied.  So, we have some potential evidence that these non-avian
maniraptorans lie outside known fliers, and no unequivocal physical evidence
that they are secondarily flightless.  Thus, the current prefered choice
would be that they never were flying.  However, *IF* someone were to present
compelling phylogenetic evidence for placing them within known fliers, or
*IF* someone were to present compelling evidence that they possessed some
physical features which unequivocally could be considered demonstrating
secondary flightlessness, then that would become the prefered hypothesis.

At present, most workers do not feel that the anatomical features cited by
various authors (Greg Paul first and foremost) are unequivocally secondary
flightless features.  That could change in the future, through
reinterpretation of the evidence or discovery of new features.  Because of
this, and because current phylogenetic analyses do not favor placing any of
the three main candidates (troodontids, oviraptorosaurs, dromaeosaurids)
within the _Archaeopteryx_ + ornithothoracine clade, the idea that the these
three non-avian clades were primarily flightless is the best supported one.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661