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Ostrom Symposium - Part 4



I've been away in Washington for a few days (and I'm off to Florida in a
week) so these reports have been getting (and will continue to be)
intermittent.  Never-the-dagboned-less, as Pogo used to say:

On to day two.  The first session in the morning dealt with the contentious
subject of feathers.  Alan Brush gave an outline of feather evolution.  He
suggested that feather variety has been great since the earliest appearance
of feathers, so that they must have evolved their structure, complexity,
and a great deal of variety very rapidly.  All of the chemical,
developmental, and cellular mechanisms for feather evolution and
development, with the exception of a single gene shift and one change in
the follicle structure, already existed in a wide range of precursors.  The
evolutionary step that permitted the development of feathers was, as Alan
put it, a machine that could turn out a highly adaptable structure, in
assembly-line fashion.  Thus the same follicle can produce different
generations of feathers throughout the life of an individual, starting with
juvenile down, proceeding on to adult down, contour feathers or whatever
else ends up in the adult animal.  Thus even an early feathered creature
like Caudipteryx had the genetic mechanism to produce such advanced feather
structures as bristles or filoplumes.  Brush illustrated his talk with
slides showing the a developmental process by which a feather develops from
a hair follicle.  This is easier to illustrate than to describe, so I am
not going to attempt to do so here.  Besides, I believe this information is
available in print already, and as Alan reads this list, he could no doubt
supply interested people with references (assuming that you can't wait for
the proceedings).

Mary Ellen Schweitzer then turned our attention to the feather structure
that actually appears in the Chinese fossils.  I must say from the outset
that this paper was very cautious indeed, confining itself entirely to
description without drawing any conclusions as to what the structures we
were looking at really were.  Given the amount of speculation flying around
in this particular field (if you'll pardon the pun), I found this extremely
refreshing.

In it is a truism that feathers are made of keratin.  However, the story
isn't that simple.  The feathers of birds are made only of beta keratin, a
protein unknown in mammals.  The beta keratin in feathers is further
distinguished from other beta keratins by a deletion.

Schweitzer has found that immunological tests on feathers of modern birds
express only beta keratin.  When she tried the same test on the fibres
associated with Shuvuia, she got the same result (as I am not really
knowledgeable about the procedure, I am not sure if she was able to
determine whether the beta keratin involved was the special type found in
bird feathers - can anyone enlighten me on this?).

Most of her paper dealt with her examination of the fibres of
Sinosauropteryx and the rectrices of Caudipteryx (she has not yet been able
to examine any Confuciusornis material).  Basically, the Caudipteryx
rectrices, when examined at the electromicrographic level, showed complex
branching filaments that interacted with each other (as I noted above,
Schweitzer scrupulously avoided saying that this result was consistent with
these structures being like modern feathers).  As you might expect,
Sinosauropteryx showed less in the way of organization (though I wonder
whether this is in part due to the difference between a rectrix and a
contour "feather"; this is one reason why I would e very interested to see
her compare Sinosauropteryx with Confuciusornis, whose integumentary
structures are a lot more reminiscent of Sino's skin fuzz (at least
superficially).

Zhonghe Zhou provided the only paper by a Chinese scientist.  Zhou examined
Confuciusornis, especially with respect to its flying ability.  The
coracoid in this bird is strutlike, and shows a condition somewhat more
derived than in Archaeopteryx, but it is stuill short and broad, lacking
the triossial canal that in modern birds allows the tendon of the
supracoracoideus muscle can act as an elevator of the wing.  You might
think that this would mean that Confuciusornis could not take off or fly.
You might be right about takeoff, but, strangely enough, a pigeon with the
tendon of the supracoracoideus severed can actually fly (using the deltoids
as wing elevators, as was explained in a later presentation), though it
cannot take off.  And if a pigeon could do that, so could be early birds
with their less developed coracoids.  To me, this suggests that
Confuciusornis (and Archaeopteryx) must have had to gain height to fly
(Confuciusornis has long, slender wings which do not seem designed for
ground starts - similarly(?), I have observed myself that long and
narrow-winged bats of the genus Molossus have to gain considerable height
to take off, and if released on a tree trunk will hitch themselves up
thirty feet or so before launching into the air).

For me, one of the most satisfying papers was given by James Hopson.  It
set to rest, for me, the (as I have always conceived it) non-question of
whether Archaeopteryx was arboreal or terrestrial.  Hobson examined the
feet of a wide range of birds, and noted an interesting difference in the
structure of the third digit.  In ground-living birds, the proximal phalanx
of this digit is longer than the distal phalanx.  This is reversed in
climbing or arboreal birds, a difference that becomes even more striking if
you look at individual bird families rather than a a wide range of taxa.
Thus, in the family of birds containing the American Orioles, which are
arboreal, and the meadowlarks, which are terrestrial, the difference in
foot structure is very obvious.  Hopson showed that using modern birds as a
sample, you can get a very high predictive value out of the ratio of the
bones in the third digit that relates quite well to whether or not the bird
is ground-living or arboreal.  So what about Archaeopteryx?  Archaeopteryx,
as I would have expected from the very beginning for what is obviously a
basal and highly generalized animal, falls out midway between arboreal and
terrestrial forms on a principal components analysis.  In other words,
Archaeopteryx could probably get along in the ground or in trees, whichever
happened to be available at the time.  I would remind anyone who is about
point out to me that the Solnhofen area did not have trees that this means
extremely little, as we do not know whether Archaeopteryx-like animals were
found in other areas where there were forests and tall trees.  There
certainly are many living groups of birds that have both forests and open
country representatives that are closely related to each other.

Looking at other basal birds and related animals, Hopson found that the
foot of Rahonavis has the proportions of a ground dweller, Confuciusornis
comes out somewhat more arboreal in its structure  than Archaeopteryx,
Sinornis and Iberomesornis  end up as arboreal perchers, and the
alvarezsaurids  come out as ground-living creatures, as does Patagopteryx.
The non-avian theropods, when examined by this method, show trends towards
ground-living and cursoriality, but the more primitive ones are less well
marked in this way.  Thus Coelophysis actually comes out rather like
Archaeopteryx.

The proportions of hind limb elements also give some separation between
ground-living and arboreal forms, especially in groups like hornbills and
pigeons, which have both fully arboreal and strongly terrestrial species.
Interestingly, the various specimens of Archaeopteryx do not cluster
together in this way, the three largest specimens coming out among arboreal
pigeons, while the two smaller ones that Hobson examined appear to be more
terrestrial, again suggesting that we are dealing with a pretty generalized
animal (or, perhaps, complex of animals).

The next paper was a substitution, given by a graduate student whose name
escapes me.  He talked about the use of the forelimbs in maniraptoran
dinosaurs, including early birds, and in particular the use of the four
limbs in prey capture.  I have already alluded to this paper in my posting
on feathers as fossilized behavior, which I'm glad to see has started a
little discussion here.  The speaker pointed out that although the second
digit, if it had feathering equivalent to that in modern birds, would have
given rise to the remiges along its length, the third digit would have been
free and could have been used to subdue prey.  He compared be possible
hunting method of Velociraptor to a cat using its forelimbs to catch prey,
following up with a killing byte (or in the case of Velociraptor,
presumably a killing slash with the claw of the hind foot).  The evidence
that Velociraptor used its arms in this way is best shown by the famous
specimen of Velociraptor locked in combat with a Protoceratops.  The
speaker compared this method to that used by wolverines for killing
caribou, which are much larger animals than themselves.  I have already
gone into some detail, I should think, on why I am skeptical that an animal
that used its forelimbs in this way to would have had long, well-developed
remiges, and for reasons already expressed by another poster I do not think
I will spend much time going into the idea that Velociraptor might have
used these feathers as blinders.  One interesting comment, though, was that
using the arms in a forward lunge to capture prey actually imitates the
downstroke of a flying bird, suggesting that the two behaviors are at least
homologous.
--
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@home.com