Specialized adaptations to bamboo feeding include strong jaws and robust teeth for grinding down tough plant material.
Modern giant pandas also have a "false thumb" that allows them to strip the leaves off bamboo stalks. Ciochon is hopeful that additional fossils will reveal whether the pygmy giant panda had the same unusual adaptation.
"It is very much needed to eat bamboo," Ciochon said. "If this was a committed bamboo eater early on, one would expect [the false thumb] adaptation was evolving."
Another species, the raccoon-like animal known as the red panda, also has a false thumb. But genetic studies have shown that the two "pandas" are not closely related, with only the giant panda classified among the bears.
Mauricio Antón, of Spain's National Museum of Natural Sciences, in Madrid, has studied the separate evolution of the false thumb adaptation in red and giant pandas.
Antón said the new skull "is an important discovery that fills a gap in our knowledge of the evolution of the lineage leading to the extant giant panda."
Scientists have long debated the exact classification of pandas because of their mix of bear- and raccoon-like features, Antón said.
"The intermediate features of the new fossil further confirm the giant panda's place in the bear lineage, completely separated from the red, or lesser, panda."
A Pygmy Among Giants
The discovery of the pygmy giant panda fossil shows that body size in the panda lineage has shifted back and forth over evolutionary time. A more recent ancestor, living less than a million years ago, was larger than the giant pandas of today.
Pandas have also shifted their range in response to the enormous environmental changes over the past three million years.
"When the [past] ice ages were occurring in northern China, the distribution of bamboo moved southward, as did the giant panda," study co-author Ciochon said.
The moist tropical forest favored by the pygmy giant panda was also home to other early relatives of creatures alive today.
The diminutive panda lived side-by-side with the stegodon, a 13-foot-tall (4-meter-tall) ancestor of mammoths and elephants. (Related: "Thai Rice Field Yields Treasure Trove of Mammal Fossils" [January 3, 2006].)
Also present was Gigantopithecus, a giant ape two to three times larger than the modern gorilla.
Today only a few thousand giant pandas persist in the wild, in upland bamboo forests isolated by rugged mountains in China's Sichuan Province.
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