Ostrom Symposium - Part 2

[Ronald Orenstein <ornstn@home.com>]

On to Part Two of my recollections of the Ostrom Symposium, helped along by
whatever notes I could scribble down at the time.  I appreciate Tom Holtz
for filling in some of the genuine science, though I am sorry if he thought
my asking five questions over the course of two days amounted to hogging
the microphone.  Well, if you don't ask, you don't know, and at least I
didn't ask how small cursorial dinosaurs could avoid tripping over
vegetation or why flying birds hadn't lost their legs altogether (I had
better say that the last question was asked by John Ostrom himself, and I
admit I'm still not altogether sure what he was driving at).

Anyway, on to Larry Martin's paper, which was called "Are there any flying
dinosaurs?".  No prize for guessing what he thought the answer to that
question was.  Larry did make some quite valid points, one of which is that
interpretation of morphological characters is only as good as your
description of them in the first place, but I must say that although I
found his paper entertaining, I did not find it altogether convincing.
Larry insisted that in order to derive birds from dinosaurs you need to
have a morphological character that clearly shows common ancestry.  I
thought we had lots of those, but Larry raised the specter of convergence
once again, with a reference to sabre-toothed cats (a point that was later
rebutted by pointing out that the marsupial and placental sabre-tooths were
far more different from each other than dromaeosaurs are from basal birds).

Larry is a great believer in good posture, a point that he emphasized with
his restorations of Archaeopteryx  as some sort of flying squirrel, capable
of embracing a tree trunk with its arms and legs as it climbed.  This idea,
which certainly raised eyebrows in the crowd (though I suspect many of them
were not encountering it for the first time), is based on the Larry's
rather controversial restoration of Archaeopteryx hip structure, and his
view that the absence of the superacetabular shelf in birds is a
fundamental difference between them and dinosaurs, making it impossible for
them to adopt to the horizontal body posture, using the femur as a fulcrum,
the seen in theropods.  Of course, Phil Currie pointed out later that many
dinosaurs also lacked the shelf, and also have the laterally facing glenoid
fossa in the pectoral girdle that Larry considers to be unique bird
feature.  I am not sure, personally, why Larry is surprised that a flying
bird, which certainly uses its limbs in a novel manner, should have some
different arrangements of the pelvic and pectoral girdles to reflect that,
but what do I know?

Larry, of course, thinks that Caudipteryx is a bird.  He showed drawings of
the skulls of Caudipteryx  and Confuciusornis, and remarked that they were
very much alike (without going into details, and I admit I can't see much
resemblance myself).  He also noted that they were contemporaries, although
I understand that there is some question as to whether the beds  that
yielded the Caudipteryx specimens are indeed contemporaneous with those
that turned up so many skeletons and feathers of Confuciusornis.  In one of
his few concessions to the general flow of opinion, he did agree that
alvarezsaurids are most like ornithomimids.

I had better say that I am not trying to make fun of Larry here; I both
like and respect him, and it would certainly have been a much duller and
less challenging symposium if he had not been there.  Though I have been
cheering for the dinosaur ancestry myself for some time, I found myself
almost regretting that, in the end, he had failed to convince me.

Phil Currie got down to the subject that had brought most of us there - the
skeletal anatomy of the Chinese specimens themselves.  Parenthetically, I
must say that if I was disappointed in anything about the symposium it was
a lack of focus on these unique fossils - I would have liked to hear
something about, for example, the ecology and taphonomy of the site.

Anyway, Currie pointed out the similarity between the "halo" effect of the
feathers of Confuciusornis and the integumentary structures (whatever they
are) of Sinosauropteryx - noting that nobody has questioned the identity of
the former, though they have not been subjected to close scrutiny!  Nobody
has suggested, for example, that the imprints in Confuciusornis represent a
fringe bordering the animal rather than a covering over the entire body -
similarly, there would be no reason to suppose that the Sinosauropteryx
structures are confined to the midline, even if there were not clumps of
them visible over the ilium as well.  He showed slides indicating that
these structures appear thicker at the base, with longer and thinner ones
extending further out, a condition that also seems to prevail in
Protarchaeopteryx.  Currie interprets this as representing a thick base
breaking up into a distal plumulaceous portion, indicating a simple
branching structure - though I am not sure I saw this in the slides.  He
also stated that there is no clear evidence that a "liver" is visible in
the specimens or that the front edge of any organ can be made out - a
conclusion disagreed with, to put it mildly, by Ruben the next day.

The unique Protarchaeopteryx specimen is broken in the middle (no, it isn't
a chimera - the matchup is very good).  It appears to have a short tail,
ending in, of course, real feathers.  Its arm is quite long relative to the
femur, and no arm feathers have been preserved (Caudipteryx, by contrast,
has very short arms).  It has large, serrated premaxillary teeth; its other
teeth are serrated too, but are smaller and similar to those in Archaeopteryx.

There may, by now, be four specimens of Caudipteryx.  This animal has long
feathers on its tail and arms, and downy feathers on the body and the base
of the tail.  Its teeth are non-serrated, and its dentary is almost
identical to those in oviraptorids.  Its vertebrae are typically
coelurosaurian, and its glenoid faces backwards (a point Larry Martin will
have to deal with if he continues to insist that it is a bird).  The
oviraptor similarities extend to the ilium; even its short tail is
approached by some members of that group.  Other non-birdlike features
include: an ischium like oviraptors and coelurosaurs (and unlike
Archaeopteryx or enatiornithines), a primitive femur; and an extending
process on the astragalus, with a process otherwise found only in
oviraptors.  In short, neither Caudipteryx nor Protarchaeopteryx fall
within the Avialae.

Luis Chiappe turned our attention to basal birds themselves - now
consisting of some 45 known genera, representing a tremendous increase in
our knowledge since John Ostrom first examined Deinonychus.  THere may be
as many as 1000 specimens of Confuciusornis, by the way!  Chiappe's matrix
now covers 169 characters in 24 taxa.  Chiappe has recently redone his
analysis, and although he does not seem ready, as yet, to embrace an
ornithomimid alliance for alvarezsaurids, he now agrees that they fall
outside the clade involving Archaeopteryx and its descendants (by contrast,
Rahonavis falls inside the clade, as a sister taxon to birds possessing
pygostyles).  Nevertheless, he says that a sister-group relationship
between alvarezsaurids and birds (Avialae) is supported by adaptations for
cranial kinesis and teeth.  Alvarezsaurid vertebrae are "very unlike" those
in ornithomimids, and Chiappe holds out the possibility that further data
might seat the group back among the birds (or not, depending).  We need
more information about basal alvarezsaurids, particularly the South
American forms.

Chiappe went on to discuss Confuciusornis.  This bird was originally placed
as a sister-group to the enantiornithes on the basis of its elongate
coracoids, but this character fits all birds except Archaeopteryx (the same
is true for the pygostyle [Rahonavis excepted] and pleurocoels in the
thoracic vertebrae  [does Rahonavis have these?]).  Thus the argument for
enantiornithine relationships is poor.  Chiappe accordingly considers
Confuciusornis as a sister taxon to all other pygostyle-bearing birds,
enantiornithes included, within a clade called Pygostylia.

The discussion section that ended the first day, with many of the questions
directed at Larry Martin's paper.  As I noted, the acetabular shelf issue
came up, with Paul Sereno noting that tyrannosaurs also lack much of a
crest here (though they presumably had a horizontal body posture) while
Hans Dieter Sues added that in modern birds, cartilage fills the area where
the crest should be in dinosaurs.

Paul Sereno was asked about Unenlagia, which has similarities to both
dromaeosaurs and Avialae, but the remains are poor and their interpretation
unclear.  Peter Dodson raised the issue of Protoavis; Sereno replied that
he considered it a composite, but added that its braincase is that of a
derived theropod.

In a provocative question, the moderator asked Phil Currie, Larry Martin
and Louis Chiappe what it would take to convince them that their theories
of bird origins were wrong.  Currie replied (and Chiappe agreed) that it
would take the discovery of material showing a genuinely viable
alternative.  Larry Martin plumped for a clearer character analysis,
remarking that elongate penultimate digits, for example, are widespread in
tetrapods and that anterorbital subsidiary fenestrae also occur in some
crocodilians.

I asked Larry (as Tom, no doubt, cringed in his chair) if he could suggest
any character in birds, not found in dinosaurs, that was not part of the
locomotor mechanism and would be unlikely to have arisen by a shift from
the dinosaur condition.  He brought up the pattern of tooth implantation
and replacement, and the shape of the teeth in Caudipteryx (though he
downplayed the issue of serrations), noting that birds were much more
similar to crocodilians in this area than to dinosaurs. Phil Currie
responded, however, by saying that troodontid tooth patterns are unlike
those in other dinosaurs, but resemble the condition in crocodilians and
birds.  One Chinese dromaeosaur tooth, he noted, is birdlike, with a
constriction between the crown and the root (Larry told me later that he is
uncertain that this tooth is indeed from a dromaeosaur).

One interesting historical footnote from the floor:  Thomas Huxley was not
the first to suggest a link between dinosaurs and birds.  Huxley apparently
cited the work of a German embryologist named Gegenbauer (?), who suggested
the connection after his study of chick embryos, which he saw as quite
dinosaur-like.  Does anyone out there know more about this?

Anyway, that concluded day one, except for another visit to the Peabody to
commune, through the glass, with the fossils that were the mute cause of
all this fascinating fuss.

More to come (though I'll try not to repeat things Tom and others have
already posted).



--
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