'Evolutionary cradle' of Ice Age mammals in Tibet
A woolly rhinoceros fossil dug up by a team of Chinese and American palaeontologists on the Tibetan Plateau is believed to be the oldest specimen of its kind yet found. The extraordinary find has led researchers to hypothesise that some giants such as woolly mammoths, great sloths and sabre-tooth cats may have evolved in highlands before the Ice Age.
The woolly rhino lived some 3.6 million years ago in the Pliocene period - long before similar beasts roamed northern Asia and Europe in those regions' ice ages.
A team of geologists and palaeontologists - led by Dr Wang Xiaoming, from the Natural History Museum of Los Angeles county, and Qiang Li, of the Chinese Academy of Sciences' Institute of Vertebrate Palaeontology and Palaeoanthropology - uncovered a skull and lower jaw of the new species in the Himalayas in 2007.
Their study has been published in the current issue of Science. Funds for the research came from the National Natural Science Foundation of China, the US National Geographic Society and the US National Science Foundation.
The team says the existence of this ancient rhino supports the idea that the frosty Tibetan foothills of the Himalayas were the evolutionary cradle for these later animals.
'It is the oldest specimen discovered so far,' Wang said. 'It is at least a million years older, or more, than any other woolly rhinos we have known. 'It's quite well preserved.'
The rhino was found in Tibet's Zanda Basin in 2007. The area is rich in fossil beds, and this specimen was unearthed along with examples of extinct horse, antelope, snow leopard, badger and many other mammals.
The plateau is often called the 'roof of the world' because its 2.5 million square kilometre area - the largest and highest in the world - is on average 4,500 metres above sea level.
The rhino is classified as a new species: Coelodonta thibetana.
Wang and colleagues said it displayed some very primitive features compared with its counterparts that lived through the later great glaciations of the Pleistocene epoch. The Pleistocene is the geological era between about 1.8 million and 10,000 years ago, in which modern humans appeared and the northern hemisphere experienced the Ice Age.
Judging from marks on the skull, the creature's horn, which has not survived, would likely have been quite flat and leaning forward.
This might have allowed the animal more easily to sweep snow out of the way to get at vegetation, a useful behaviour for survival in the harsh Tibetan climate, the team said.
'We think it would have used its horn like a paddle to sweep the snow away,' Wang said.
Even though no impressions of hair were found, based on rhino hairs preserved in permafrost in Siberia, the researchers believed this rhino would have been covered with long hair much like a modern yak.
The 3.7-million-year-old skull would have belonged to an animal that weighed 1.2 tonnes to 1.4 tonnes, Wang said. That is close to modern rhinoceroses and about 10 per cent smaller than the woolly rhinos found a million years later during the Ice Age.
This discovery by Wang and his colleagues has substantiated scientists' long-held notion that some mammals adapted to the global cooling well before it happened.
Scientists 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 the sea was as much as 100 metres or so lower than today.
Although the extinction of Pleistocene beasts such as woolly mammoths and rhinos, great sloths and sabre-tooth cats had been intensively studied in recent years, much less was known about where these giants came from and how they acquired their adaptations for living in a cold environment, the researchers said.
The argument made in the Science paper is that they may have acquired the adaptations on the plateau. 'The Tibetan Plateau may have been another cradle of the Ice Age giants,' Wang said in the paper.
The rhino accustomed itself to cold conditions in high elevations and became pre-adapted for the future Ice Age climate. When the Ice Age eventually arrived about 2.6 million years ago, the cold-loving rhinos simply descended from the high mountains and expanded throughout northern Asia and Europe.
'When this rhino existed, the global climate was much warmer and the northern continents were free of the massive ice sheets seen in the later ice ages,' he said.
'Then, about a million years later, when the Ice Age did hit the world, these Tibetan woolly rhinos were basically pre-adapted to the Ice Age environment because they had this ability to sweep snow. They just happily came down from the high altitude areas and expanded to the rest of Eurasia.'
However, the Los Angeles-based researcher conceded that many more fossil finds would be required to underpin the Tibetan hypothesis.
Andy Currant, an expert on the Pleistocene at London's Natural History Museum, said this was not straightforward in the case of woolly rhinos, and good specimens can sometimes be hard to come by. 'Woolly rhino were preyed on by spotted hyenas and they were eaten pretty thoroughly: the hyenas liked the bones,' he said.
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 snow leopard and chiru, also known as 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.
'Cold places, such as Tibet, the Arctic and Antarctic, are where the most unexpected discoveries will be made in the future,' Wang said. 'These are the remaining frontiers that are still largely unexplored.'