A new trove of mammal fossils found high in the foothills of the Himalayas suggests that Tibet may have been a harsh, cold testing ground where woolly rhinos and other big mammals developed their cold-climate cool well before the Ice Age began.
Among the treasures from the Zanda Basin is the oldest fossil yet found of a woolly rhino, dating from about 3.7 million years ago, says vertebrate paleontologist Xiaoming Wang from the Natural History Museum of Los Angeles County. The complete skull and several neck vertebrae of the now extinct animal represent a new species, Coelodonta thibetana, Wang and his colleagues report in the Sept. 2 Science.
“It’s an important find,” says vertebrate paleontologist Donald R. Prothero of Occidental College in Los Angeles, who studies the evolution of mammals, including woolly rhinos. The paper is the first evidence for the idea that the lineage of the iconic Ice Age giant, and maybe other species, had already evolved cold-weather adaptations instead of developing them as the big chill set in 2.8 million years ago.
When the Ice Age did come, descendants of the early Tibetan rhinos could have moved gradually off the plateau to lower altitudes throughout Eurasia. Wang and his colleagues note that fossil locations of three later woolly rhino species fit the pattern of a lineage that diversified down and out of the high plateau. “Tibet as a special environment probably is the cradle of some of the cold-adapted species of the Ice Age,” Wang concludes.
That’s a reasonable scenario, Prothero says. “I don’t think it’s been suggested before because there was no evidence for it.”
Wang and his colleagues found such evidence in the Zanda Basin, which sits 3,700 to 4,500 meters above sea level and is surrounded by even higher peaks. “It’s like working on top of Mount Whitney,” the highest peak in the contiguous United States, Wang says.
Toward the end of the field season in August 2007, Wang himself was exploring late in the afternoon a few miles from his colleagues when he stumbled onto a big piece of fossilized bone. Recognizing it as a vertebra, he swung his hammer at a promising spot — and uncovered a hint of what he calls the biggest discovery of his career. “Here is a tooth of a woolly rhino revealed by one single whack of my hammer,” he remembers.
Subtle characteristics identified the skull as a more ancestral species than later, Ice Age woolly rhinos. The Tibetan rhino already had such adaptations as a forward-leaning, flattened horn convenient for sweeping aside snow while foraging for food. The research team estimates the skull came from a mid-sized rhino about the size of today’s Indian and black rhinos. The fossils did not include hair, so paleontologists can only speculate about how woolly it might have been.
In addition to the newly named C. thibetana, the basin has also revealed other cold-adapted mammals whose descendants might have spread during the Ice Age, such as blue sheep and the Tibetan wild ass.
Some Ice Age mammals “probably came from high elevations and were pre-adapted to cold, but I wouldn’t want to say everything followed this pattern,” Prothero cautions. “In the real world nature is always more complex.”