By Will Dunham

WASHINGTON, June 18 (Reuters) - About 2 million years ago, in lowland tropical forests of what is now China, lived an ancestor of the giant panda that was very similar to the famed vegetarian bear except about half the size, scientists said.

Writing on Monday in the Proceedings of the National Academy of Sciences, the scientists described a complete skull and teeth of a pygmy-sized panda, shedding light on the origins of this unique, bamboo-munching bear.

The skull is about 60 percent of the size of a modern giant panda's and it has similar anatomical features, suggesting a similar lifestyle, they said.

The extinct bear, named Ailuropoda microta, is the earliest known member of the panda lineage, said Russell Ciochon, a University of Iowa anthropologist and one of the researchers.

"It's basically a miniaturized version of the living panda," Ciochon said in a telephone interview.

"Before, we only had teeth of this pygmy giant panda, just some isolated teeth. So we knew it existed, but we had no idea what the skull was like."

The giant panda, known for its black and white markings, is considered the most specialized of any of the eight species of bears alive today. It is a carnivore that lost its hankering for meat and evolved into a committed vegetarian, scientists said.

THE ONLY VEGETARIAN BEAR

The skull indicates the panda lineage has been evolving for millions of years completely separated from other bears. While some other bears will eat berries and other vegetation, the panda is the only fully vegetarian one.

Although it is one of the most recognizable animals on the planet, the giant panda is endangered, teetering on the edge of extinction mostly due to habitat loss and encroachment from people. They live in a few upland bamboo forests of China, isolated in rugged mountainous terrain.

It is estimated there are about 1,600 left in the wild.

The pygmy panda was about three feet (1 meter) long, compared to the modern giant panda at up to 6 feet (2 meters) long.

The creature lived in a moist, lowland tropical forest habitat that had bamboo as a dominant plant, the researchers said. It lived alongside two remarkable extinct creatures: the elephant-like Stegodon and the largest ape ever, Gigantopithecus, 10 feet (3 metres) tall and weighing up to 1,000 pounds (450 kg).

The researchers believe the ancient panda ate bamboo shoots based on wear patterns on the teeth and specialized muscle markings on the skull that are evidence of heavy chewing. The evolution of this dietary specialization likely took millions of years to perfect, they said.

Bears appeared in the Miocene Epoch, which ran from about 25 million years ago to 5.5 million years ago.

The fossilized skull was found by Chinese researchers Changzhu Jin and Jinyi Liu in a limestone cave in southern China in 2001.

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