Woolly rhinos, some of the hairiest beasts to roam Ice Age landscapes, may have had a surprising ancestral home. The discovery of a number of fossils indicates that these iconic beasts may have evolved in the frigid environment of the Tibetan Plateau more than 1 million years before global cooling allowed the creatures' descendants to spread throughout much of northern Eurasia. The findings upend the notion that many if not all of the cold-adapted creatures roaming Ice Age landscapes evolved in the high Arctic.
Ice ages have struck North America and northern Eurasia every 100,000 years or so since about 2.8 million years ago. Many of the large creatures roaming the landscape during these cycles, including woolly mammoths, mastodons, and saber-toothed cats, mysteriously went extinct about 10,000 years ago, as the latest episode of global cooling waned. But when and where many of those megafauna originally evolved has been an even larger mystery, says Xiaoming Wang, a vertebrate paleontologist at the Natural History Museum of Los Angeles County in California.
In the new study, Wang and colleagues uncovered a variety of fossils—including a skull, a jawbone, and a couple of neck vertebrae in the Himalayan foothills along the southwest Tibetan Plateau. The plateau is often called "the roof of the world" because its 2.5-million-square-kilometer area—the largest and tallest in the world—has an average elevation that exceeds 4500 meters (14,800 feet). Based on the age of the sediments surrounding the fossils, which was estimated using the magnetic characteristics of the rock as well as the other fossils entombed therein, the researchers say the fossils belong to a new species of woolly rhino that roamed the region about 3.7 million years ago. The team has dubbed the rhino, which was about the size of its modern kin but covered with shaggy fur to help preserve its body heat, Coelodonta thibetana, or "the pit-toothed creature from Tibet."
Previous studies suggested that, at the time, the global average temperatures were as much as 3°C warmer than they are today. Also, Wang says, northern continents weren't covered with massive ice sheets that characterized the ice ages. Despite the warmth of the era, however, the Tibetan Plateau was about as cold and snowy as it is today, with an average temperature around 0°C and wintertime extremes sometimes dropping below -10°C.
The woolly rhino had several features that helped it survive the harsh Tibetan environment, the team reports online today in Science. For example, the size and shape of the bony bump where the rhino's horn attached to its snout suggests that the horn had a flattened cross section, not a conical one like modern rhinos. That flattened profile, a shape also seen in later species of woolly rhinos, allowed the horn to be used to sweep snow from the ground and uncover low-growing vegetation. When the ice ages came along and harsh conditions spread to lower altitudes, C. thibetana and its descendants were evolutionarily primed to take advantage and expand across northern Eurasia, Wang and his colleagues contend.
The researchers also unearthed the fossils of more than two dozen other species at the Tibetan field site, including extinct species such as three-toed horses and modern-day species such as the snow leopard and the chiru, also known as the Tibetan antelope. Because several of these creatures were known across a larger area during the recent ice ages, the researchers suggest that the Tibetan Plateau may have been their evolutionary cradle. Nevertheless, Wang notes, none of the fossils unearthed represent the woolly mammoths or mastodons so familiar to many, which suggests that those creatures may have evolved elsewhere.
The new fossils "are quite fantastic," says Pierre-Olivier Antoine, a paleomammologist at the University of Montpellier 2 in France. A Tibetan origin of the woolly rhino "is quite surprising," he adds. Previously, scientists had suspected that the closest kin of woolly rhinos lived on the southern slopes of the Himalayas in Pakistan and India, but the newly described C. thibetana is obviously a much closer relative, he says.
Scientists have previously suggested that many Ice Age-adapted mammals arose in the high Arctic, especially in the harsh conditions of a land bridge that joined northeastern Asia to what is now Alaska during ice ages, when sea levels were as much as 100 meters or so lower than they are today. However, "[a]n origin for the woolly rhino in Tibet, where high altitude imposed a regime of cold climate and open vegetation, makes perfect sense," says Adrian Lister, a vertebrate paleontologist at the Natural History Museum in London. "We know from today's species that they move up and down mountains in accordance with climate change and that many are now moving upwards to escape global warming," he notes. "It seems perfectly reasonable that a similar thing could have happened in reverse, over longer time scales, in the past."