What flew, but was not a bird?

Had a wingspan of 40 feet, but weighed less than 130 pounds?

Belonged to an order of reptiles that endured for 135 million years and died out about 64 million years ago?

Answer: Quetzalcoatlus northropi -- one of the flying reptiles known as pterosaurs -- named after the Aztec god who took the form of a feathered serpent.

A reconstruction of the pterosaur, believed by scientists to be the largest flying animal ever to live, is a dramatic addition to the Museum of Natural History's Dinosaur and other Fossil Reptiles Hall, which opens to the public Friday.

Quetzalcoatlus -- the world's only life-size reincarnation -- is poised above the balcony devoted to flight, in a "slow banking dive to the left, his left wing tucked in a little more, his right wing more extended," says paleontologist Jessica Harrison.

The story begins in the early '70s, when Douglas A. Lawson, then a student at the University of Texas at Austin, discovered a number of wing bones from an unknown species of pterosaur in Big Bend National Park in West Texas.(The bones were later determined to be the remains of Quetzalcoatlus northropi.) These and a second group of fossil remains -- the scattered bones of at least a dozen smaller animals -- were the basis of the reconstruction.

A team of modelmakers at the Smithsonian's Office of Exhibits Central and paleontologist Harrison -- who had flown to Texas to study, measure and photograph the fossils -- and experts from Yale and the University of Texas pieced together what could be known about Quetzalcoatlus.

"You are interpolating between a number of fossil remains," says modelmaker Jim Reuter. "You pull together every bit of information about the animal, coupled with the best guess as to what the animal had to do to survive, filled in with what similar types of animals surviving today have to do."

The preliminary working models included a small-scale clay and wire model made by artist John Gurche, and a second, more detailed wood and fiberglass model by Harrison and model shop supervisor Walter Sorell. These were evaluted by pterosaur experts, including Smithsonian fossil reptile curator Dr. Nicholas Hotton.

"When we were happy with the model and had incorporated the corrections suggested by the experts," says Harrison, "we began the process of blowing it up, enlarging it, solving technical problems." (Among them: The model had to be light but very strong, with a weight limit of 500 pounds; the finished version weighs about 140 pounds.)

It is "essentially an artistic process, based on certain fossil remains and certain interpretations made by scientists," says Reuter. "If scientists ever come up with a full skeleton of this pterosaur, we'll find that we aren't very far off, if at all."

Reuter created the fierce, predatory eyes by pouring clear resin into a rubber mold, placing a layer of black construction paper at the base of the mold for the pupil, then painting the iris with plexiglass colors and adding small particles of combed red felt to imitate the capillaries. His head incorporates a wrinkled skin and a serpent-like tongue.

"There were," says Bruce Hough, "months of work in the wings. They were the hardest things to create and they make the whole thing really plausible." The wing bones used a three-wire truss system; balsa wood was glued onto it, then carved to conform to the configuration of bone and flesh. A very thin coat of fiberglass was applied to seal and strengthen the balsa.

The wing membrane, supported by a wooden platform while it was being constructed, was built out of modeling clay, reinforced with strips of wood used to produce the air-foil shape. The approximately 1/2-inch layer of clay was carved to a flat contour, then detail -- to emulate the long, thin fibers that reinforced the pterosaur's wing membrane and probably prevented it from bending -- was drawn in the soft clay. The clay was then used as the mold for the fiberglass.

The pterosaur's dark and light synthetic fur, its recreaters admit, is the result of some guesswork. They wanted it to be "plausible but not outlandish." Modelmaker Tree O'Donnell spent hours dyeing, matching, clipping, glueing and stitching fine bits and strips of fur.

And when -- after over a year of work -- the pterosaur was finished?

"We all felt a sense of accomplishment," says Bruce Hough."We developed a feeling for it."

Steeping themselves in the lore of Quetzalcoatlus northropi, he says, made the project come to life.

"Otherwise," he says, "it's just a chunk of steel and fiberglass."