Fossil Remains Bolster Dinosaur, Bird Link

WASHINGTON — Fossil remains discovered in Argentina appear to support the theory that birds descended from dinosaurs, scientists announced Tuesday.

More than 20 bone fragments extracted from a green sandstone hill in the Patagonia region come from a previously unknown ancient critter whose pelvis resembles both dinosaurs and birds and whose shoulder is uncannily bird-like. The flightless creature stood nearly 4 feet tall, was about 7 1/2 feet long and ran on two legs.

Paleontologist Fernando E. Novas of the Museum of Natural History in Buenos Aires and his colleague Pablo F. Puerta named the theropod Unenlagia comahuensis, meaning “half bird from northwest Patagonia” in Latin and the language of the local Mapuche Indians.

Novas said he first mistook the remains for a turtle. But once he got the bones back to his desk in Buenos Aires, he “became astonished, looking at the structure,” Novas said at a news conference announcing the discovery at the National Geographic Society, which helped fund the expedition.

Novas describes the discovery in Thursday’s issue of Nature. In an article accompanying the Novas piece, Lawrence M. Witmer of Ohio University called Unenlagia “a true mosaic, begging the question of where to draw the line between what is, or is not, a bird.”

Most paleontologists believe dinosaurs evolved into birds, based on the many similarities between the bone structure of creatures such as the fierce deinonychus and ancient birds such as archaeopteryx. But finding fossils that stand at the midpoint between the two groups has eluded paleontologists.

The new find “fills in a major piece of that gap,” said Mark Norell, chairman of the department of vertebrate paleontology at the American Museum of Natural History in New York. It is one of a number of newly discovered fossils being prepared around the world that will further prove the dinosaur-bird link, he said.

It is the Unenlagia’s oddly configured shoulder that fascinates paleontologists. The dinosaur’s short arms hung down from its shoulders, and the joint’s orientation meant that the animal’s limbs could move forward, back and inward to grasp. The creature’s shoulder was tilted outward, however, giving a completely different range of movement that included up-and-down flapping.

That range of motion might have helped the speedy dinosaur keep from falling as it zipped about, Novas suggested, in the same way that humans wave their arms for balance while surfing or skating.

Novas was careful to point out that the new find could not itself be a direct ancestor of modern birds, because the Unenlagia lived just 90 million years ago--about 55 million years later than archaeopteryx, the earliest known bird. Instead, Novas said, the new discovery was “a late survivor of a critical evolutionary stage” from which archaeopteryx and other birds sprang.