Flat-footed fossil of former flyer.

Before birds evolved, winged reptiles called pterosaurs ruled the skies. These ancient aerialists were clearly adept fliers, but paleontologists cannot agree on how they moved while on the ground. Some researchers envision pterosaurs as crawling awkwardly on their hands and feet, while others argue that they ran swiftly on their toes, as an ostrich does.

Now, with the discovery of a well-preserved fossil in northeast Mexico, paleontologists have caught pterosaurs flat-footed. The foot bones of this 180-million-year-old specimen from early in the Jurassic period contradict the idea that pterosaurs ran on their toes, says James M. Clark of George Washington University in Washington, D.C. Clark and his colleagues describe the fossil in the Feb. 26 Nature.

The Mexican find, a new species of the genus Dimorphodon, is a primitive pterosaur with a long tail and a wingspan of 6 feet. The researchers discovered it in 1985, but excavating the fossil at the field site took 3 years and freeing the bones from the surrounding rock required many more years of work in the laboratory.

The fossil is unusual because the bones of the foot are whole and attached to each other as they were in life, providing evidence of how the bones moved when the animal walked. Clark and his coworkers observed that the joint between the toes and foot could not have flexed enough to make possible the digitigrade movement, or toe-walking, seen in birds and some dinosaurs. "They had to be flat-footed. They had to walk on the soles of their feet," says Clark.

Their conclusions contradict part of the swift-pterosaur hypothesis put forward 15 years ago by Kevin Padian of the University of California, Berkeley. Padian calls the new observation interesting but says that researchers must examine the other bones of the foot to rule out digitigrade motion in pterosaurs. "You can't really explain locomotion from looking at only one joint," he says.

Clark and his colleagues contend that pterosaur toes could grasp in much the same way as the feet of perching or climbing birds do. This finding agrees with theories that pterosaurs arose from tree-dwelling reptiles rather than from running ground dwellers. The new findings do not, however, resolve whether pterosaurs were swift or slow or whether they walked on four limbs or two, he says.