Freed to Fly Again
Through CT scans that can show sub-millimeter details, an imprisoned fossil reveals its secrets
Four Legs and Wings
At a meeting of the Society for Vertebrate Paleontologists, one of the scientists who had tried to prepare the fossil mentioned the problem to Alan Walker, a paleontologist at the Pennsylvania State University. Walker suggested that Fraser bring the specimen to the Center for Quantitative Imaging at Penn State to see what the micro-CT scanner there could do with it.
The specimen was difficult to scan because the slab containing the fossil was thin and flat. Explains Tim Ryan of the imaging center, who worked on the specimen, "CT scanners don't particularly like flat, oblong things. They much prefer cylindrical sorts of objects. It was a test of the scanner and our ingenuity that we were able to get decent data from it."
Ryan placed the slab inside a plastic tube and filled the space around the slab with florist's foam to create a cylindrical object. In the computer, Ryan was able to subtract digitally both the matrix and the foam, leaving only images of the fossilized bones.
What he and Fraser saw was an extraordinary creature about 25 centimeters long. It was a long-necked, four-legged reptile with wings. Fraser, Olsen, Alton C. Dooley, Jr. (the assistant curator of paleontology at the Virginia Natural History Museum) and Ryan named the new species Mecistotrachelos apeoros, Greek for "long-necked soarer," in the June 2007 issue of the Journal of Vertebrate Paleontology. This might be the first time that a new species has been named entirely on the basis of micro-CT images of specimens that cannot otherwise be seen.
The narrow, pointed skull of these specimens is only about two centimeters long and the neck is more than five centimeters long—unusual proportions compared with the relatively short-necked lizards of today. More startling are the bizarre, delicate ribs of Mecistotrachelos. In the 2003 specimen, these are draped along the upper spine like the ribs of a half-closed umbrella. The first rib, the one closest to the animal's head on each side, looks normal, but the next eight are exceptionally long. Some ribs are about seven centimeters—twice as long as the entire hind limb from hip to ankle.
The shape and arrangement of these bones indicate that Mecistotrachelos spread its ribs out sideways to support folds of skin that became gliding wings. Special muscles that attached to the heads of the second and third ribs manipulated the angle and curvature of the wings. These features suggest that Mecistotrachelos would have been very adept in the air.
The muscles for controlling the wings may have been especially important because of Mecistotrachelos's long neck. Many long-necked birds, such as herons, hold their necks in an S-shape when they fly, a position that helps keep the head and neck—and the rest of the body—stable in flight. Mecistotrachelos didn't have enough flexibility in its long neck to achieve this posture, so it must have glided with its neck outstretched despite the risk of crashing.
The bones of the hind foot of Mecistotrachelos are hooked for perching on vegetation, and its teeth suggest an insect diet. Although the fossils were found in what was once the deepest part of the lake, Fraser is certain they did not swim. "There is not a single aquatic adaptation in their skeletons," he says. "It seems likely that both specimens were blown off course and out over the lake, where they sank into the fine mud on the bottom and were preserved."