A clawed wonder unearthed in Mongolia.

Mongolian and U.S. researchers have found a 75-million-year-old, bird-like creature with a hand so strange it has left paleontologists grasping for an explanation.

Called Mononychus, such fossil specimens display several anatomical features characteristic of avians, leading the discoverers to classify the animal as an early bird. While other researchers question whether to perch Mononychus in the same family tree as birds, paleontologists agree this creature had a bizarre set of front limbs. Mononychus had stubby powerful arms that each ended in a large single claw, which the animal might have used for digging. But that arrangement seems at odds with its long legs, apparently built for running.

"This is one of the most unusual things that i have ever seen," says one of its discoverers, Mark A. Norell of the American Museum of Natural History in New York City.

Paleontologists from the Mongolian Museum of Natural History in Ulan Bator unearthed the first specimen of Mononychus in 1987. Last year, in a joint project, researchers from the two museums discovered two additional specimens during an expedition. Later, a fourth fossil turned up among bones collected in 1922 during a Mongolian expedition by other American Museum researchers. Norell and others describe the animal in the April 15 NATURE. Among its avian characteristics, Mononychus had a keeled sternum, which in modern birds provides a broad surface for anchoring the flight muscles. But the tiny forelimbs of Mononychus would not have permitted flight, Norell says. Mononychus also showed other important avian developments, such as a bird-like pelvis and a shrunken fibula, also common to modern avian species.

The researchers say the Mononychus specimens are particularly important because the bones were preserved without being crushed. In most fossils of early birds, the delicate bones have been flattened by overlying sediments. The Mononychus finds provide a three-dimensional representation for paleontologists to study.

Norell and his colleagues have raised two possible theories to explain the relationship between this animal and all birds, including the 147-million-year-old Archaeopteryx, the earliest known bird. In one scenario, the new-found animal belongs to a lineage of flying birds that had lost that ability by the time of Mononychus. In the other possible interpretation, Mononychus represents the relict of a bird line that never flew. In this case, flight must have evolved first with ancestors of Archaeopteryx and then, independently. among the forebears of modern birds.

While other paleontologists hail the new discovery, they remain unconvinced that Mononychus fits in the same phylogenetic category as Archaeopteryx and all later birds. Paul Sereno of the University of Chicago notes that Mononychus had arms built much like those of digging animals. Because moles and other diggers have keeled sternums and wrists reminiscent of birds, the classification of Mononychus becomes difficult, he says.