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Debate Over the Wings of Pterosaurs


Disagreement over the wing structure of extinct flying reptiles has led to a
lacerating scientific debate between paleontologists, the outcome of which could
oblige artists and film makers to begin redrawing the terrible pterosaurs of
fact and fancy.

Important issues, including the precise relationships between dinosaurs,
pterosaurs and birds, could eventually be affected by the debate. But for the
moment, attention centers on the fossil remnant of a single reptile about the
size of a crow that flew over what is now Kazakhstan 156 million years ago.

Given the erroneous scientific name Sordes pilosus, meaning "hairy evil spirit,"
this little pterosaur had a pointed beak lined with needle-sharp teeth, and a
long, flexible tail that may have helped it maneuver.

Paleontologists no longer believe that Sordes pilosus had hair, but its original
name has stuck. The creature had long, membranous wings, in which an elongated
string of bones equivalent to those in the fourth finger of a human hand served
as the supporting structure. The animal probably lived on fish, but it is not
known whether it could dive beneath the surface of the lakes where it hunted.

The latest volley in the debate over pterosaur wings was fired in a report in
current issue of the British journal Nature. The article argued that pterosaurs,
flying reptiles that were contemporaries (and relatives) of the dinosaurs, had
fleshy membranes extending from their wing tips along their bodies all the way
to their hind feet. The authors said a membrane extension, called a uropatagium,
bridged the space between the animal's ankles, giving it a dive brake or flap
useful for maneuvering in low-speed flight, but severely hampering the
pterosaur's locomotion on the ground.

The authors of the study, Dr. David M. Unwin and Dr. Natasha N. Bakhurina, a
married couple who are paleontologists at the University of Bristol, England,
said that the fossil evidence suggests pterosaurs were very agile flyers, but
as ungainly as grounded bats when crawling.

The two scientists said that the fossil wing membrane they studied contains
two kinds of fibers once thought to be the remains of hair, but which are now
believed to be stiffeners that evolved in the pterosaur wing to improve its
aerodynamics. Unwin and Bakhurina found long, rigid fibers in the outer sections
of the pterosaur's wing, the part that needs stiffness in flapping flight, and
thin, curly fibers closer to the animal's body, where more flexibility would
have been needed.

The scientists concluded that all pterosaurs, not only the small Sordes pilosus,
probably had much more extensive wing membranes than those depicted in
traditional restorations of these animals.

The Sordes pilosus Unwin and Bakhurina studied was excavated in the 1960s from a
rich Jurassic period fossil bed near Karatau, Kazakhstan, by Soviet
paleontologists. The stone in this deposit is very fine-grained, and the details
of fossils embedded in it, even the outlines of fleshy membranes, have been
superbly preserved.

Dr. A.G. Sharov of the Paleontology Institute of the Soviet Academy of Sciences
reported the results of his study of Sordes pilosus in 1971, but investigations
of this unusual fossil by other scientists have continued. Unwin, a British
paleontologist, was conducting research in the Soviet Union in the 1980s when he
met and married Bakhurina, who was also investigating pterosaurs. When the
couple moved to England, they took some of the Kazakhstan fossils with them,
including the specimen they described in Nature.

An opposing interpretation of pterosaur wing structure has long been argued by
Dr. Kevin Padian of the University of California at Berkeley. Padian believes
that pterosaurs had walking gaits like those of modern birds, and that their
hind legs were unencumbered by linkage to wing membranes or by bat-like
uropatagiums tying their hind feet together.

Padian believes that the hind limbs of pterosaurs more closely resemble those of
birds and dinosaurs than they do those of bats, and that since birds walk erect
on two feet, pterosaurs did, too. For one thing, he said in an interview, the
top of a pterosaur femur, or thigh bone, is set off at an angle of 90 degrees to
its hip bone - an arrangement that would have made movement much more natural if
the animal's legs extended directly downward from the body than otherwise. Also,
he said, the fibia leg bone of the pterosaur is much smaller in relation to its
tibia leg bone than is the case with crocodiles and lizards - typical crawling

"So the reconstruction of Sordes pilosus proposed by these guys," Padian said,
"would effectively dislocate the animal's hind limbs."

Padian said that neither he nor some of his colleagues have been given a chance
to examine the contentious fossil directly. He said that despite claims that
Sordes pilosus had a wing membrane between its hind feet, "how do we know that
this specimen isn't a kind of road-kill mess - a three-dimensional animal
squashed into two dimensions? What Unwin interprets as wing membrane might be
some other soft tissue displaced by being mashed."