One of the most successful orders of animals, the flying reptiles known as pterosaurs, thrived for 135 million years and diversified into some 85 species, some as small as sparrows, some the size of airplanes.
Although more than 3,000 specimens of pterosaurs have been collected from every continent except Antarctica, and although scientists have been studying them since 1784, when Cosimo Collini, a former secretary of Voltaire, found the first one, pterosaurs remain an evolutionary mystery."I'm afraid the only thing on which all paleontologists can agree," said Peter Wellnhofer of the Bavarian State Collection for Paleontology in Munich, "is that the pterosaur was a creature that could fly. Beyond that, we don't agree on much."
This was the thrust of discussions by scientists who attended a meeting of the Society of Vertebrate Paleontology in New York.
The biggest mystery of the pter-o-saurs is their origin. No fossil has been found that can be identified as an immediate ancestor of the pterosaurs.
By contrast, the evolution of birds from dinosaurs is marked by several intermediate creatures that paleontologists have found, including a primitive bird called archaeopterix - with a skeleton almost identical to that of certain dinosaurs but covered with feathers of advanced design that clearly enabled it to fly.
Another kind of missing link between reptiles and birds came to light at the paleontology meeting, where American and Chinese paleontologists displayed photographs of a recently discovered dinosaur fossil that has traces of feathery down along its backbone, tail and sides. It is clearly a dinosaur of the compsognathus type, but the down looks like primitive feathers.
No such intermediate animal has ever been found bridging the evolutionary gap between reptiles and pterosaurs, however.
"It's as if pterosaurs sprang full blown from the head of Zeus," said Robert Bakker of the Casper Museum in Casper, Wyo.
The pterosaurs eventually went into a decline that lasted several million years, according to Wann Langston Jr. of the Texas Memorial Museum in Austin, and then died out about the same time as their reptile cousins, the dinosaurs, 65 million years ago. There is little agreement as to why.
Among the many subjects of disagreement is the question of whether pterosaurs walked with an erect stance, like that of birds, or alternatively, dragged themselves along the ground like bats. Large issues are related to this question, including the origin of flight. If early pterosaurs were able to run like modern birds, they might have developed the ability to leap into the air and make short flights, eventually achieving true flight. But if they sprawled in the manner of bats, they are more likely to have begun flying by gliding between trees or parachuting to the ground.
The wings of pterosaurs were unlike those of either birds or bats. A bird wing consists mainly of fairly rigid feathers attached to the animal's forearm and second finger, while a bat wing is a thin membrane supported by its equivalent of the second through fifth fingers. The pterosaur wing was a membrane stretched between a very elongated fourth finger and the animal's trunk.
But how the membrane was attached is a point of disagreement. Some paleontologists argued that the main part of the membrane, the brachiopatagium, extended all the way to the hind feet, thereby making the hind feet very clumsy on the ground and necessitating a crawling stance. This drawback might have been made still more troublesome if another membrane, called the uropatagium, spanned the space between the hind legs, adding lifting surface to the pterosaur's wing.
One way to examine this issue depends on the analysis of fossil tracks left on wet sand, but the interpretation of possible pterosaur trackways spurred many disagreements at the meeting.
Martin G. Lockley of the University of Colorado at Denver and David Unwin of the University of Bristol in England presented images of footprints they identified as those of pterosaurs, in which two separate types of marks could be discerned: those that might have been left by a "pes" or hind foot, and those left by a "manus" or the equivalent of a hand at the front edge of a pterosaur wing.
Trackways discovered recently in England, France, Spain and Utah were almost certainly those of pterosaurs. Moreover, the impressions of the animal's "hand" are deeper than those of its hind feet, perhaps implying that it threw more of its weight on its forearms than its hind feet as it crawled. Some tracks appeared as parallel scratches that might have been made by pterosaurs swimming in very shallow water, with their feet sometimes scratching along the underlying mud.