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The Origin of Bird Flight: Roundtable Report
Introduction
Our charge during this roundtable disussion was to consider
evidence, old and new, regarding the origin of flight in birds.
This topic, one that many of us approach with what seems to be
a genetically programmed fascination, had been broached numerous
times in both the morning contributed paper session
and during the previous two roundtables. To initiate discussion,
Sankar Chatterjee kindly agreed to share an illustration summarizing
several competing hypotheses regarding the transition
from nonflying to flying forms. The ensuing hour focused on
four general questions/themes: (1) were theropods capable of
climbing; (2) what can claws tell us; (3) what are the limitations
of the cursorial theory; and (4) do we have the right perspective?
My first priority in this record is to reproduce the ideas and
thoughts expressed by the participants and, whenever possible,
to do so by reporting the conversation verbatim (from audioand
videotapes made with the knowledge and cooperation of
the participants). In some instances for clarity, I made editorial
alterations that are not intended to change the meaning of what
was said. I took the liberty to rearrange the order of some comments
to group them within logical topical headings.
The Roundtable Discussion
1. WERE THEROPODS CAPABLE OF CLIMBING?—Pondering
the various scenarios for the origin of flight, Larry Martin
asked Gregory Paul, "Do you think these (theropod) dinosaurs
were good climbers or not? I would not have thought so from
some of your reconstructions." Paul responded, "A good analogy
would be a jaguar. If I were being chased by a jaguar, the
jag could catch me on the ground. If I ran to a tree, the jag
could climb the tree and catch me there as well. Jags are about
the same size as the dromaeosaurs. The jaguar scenario suggests
the situation for dromaeosaurs. I think they were very
good mnners, but I also think they were good climbers, as is
tme for many of these small theropods. My theory is that there
G.E. Goslow, Jr., Department of Ecology and Evolutionary Biology,
Brown University, Providence, Rhode Island 02912, United States.
G.E. Goslow, Jr.
341
exists a group of small arboreal theropods from the Triassic or
perhaps Jurassic that we have not found yet, because they will
be very hard to find, that were good climbers. That's where you
get Archaeopteryx from as well as some of the Cretaceous
theropods. I agree with Sankar Chatterjee that overlapping
fields of vision and large brains are not necessary for flight because
pterosaurs do not have large brains, nor do insects. Nor
do pterosaurs have overlapping binocular vision, but primates
evolved these things in trees. [There] are other suggestions
[that] these theropods were climbing; they had raptorial hands
and three-toed feet with reversed hallux trackways, which suggests
they could wrap this stucrure around. Even Tyrannosaurus
has a reversed hallux trackway, so yes, I would agree that
many of these small theropods could be semiarboreal forms."
"So you do not have any problem with these forms being arboreal?"
asked Martin. "No, I would agree, the arboreal hypothesis
is far superior," responded Paul.
Steve Gatesy raised a cautionary note regarding tracks and a
reversed hallux by adding, "We are finding in the Triassic
Greenland forms what we are calling a tetradactyl trackway,
where we have shown that a 'reversed-hallux' trackway can actually
be made by a form without a reversed hallux by plunging
the foot into the substrate in a certain way that the toe is not really
reversed anatomically. We must be careful about looking
for perching feet in Triassic forms from trackways."
Paul expanded further about theropod design by commenting
on their shoulder architecture and by referring to a set of reconstructive
drawings he provided for the participants. "There
have been some misconceptions about the shoulder girdles of
dinosaurs. Quadmpedal forms, of course, walked with forelimbs
outstretched to the ground and the limbs under the body.
In a lot of the theropods, for example Syntarsus and other Cretaceous
forms, the shoulder glenoid faces laterally so that the
humems can be brought out laterally. When I manipulate the
humems in the shoulder of Syntarsus, I can extend the humems
laterally and slightly dorsally as I have illustrated. The range of
motion in these theropods is very similar to that of Archaeopteryx;
there is very little difference. Not until later birds do we
see the glenoid facing dorsally so that the wing can be brought
higher up over the back. This ability to position the humems