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An evolutionary 'cradle' for ice-age giants

Artist's reconstruction of the new species of Tibetan wooly rhino. Wooly rhinos use their flattened horns as a snow sweep to reveal covered vegetation, a critical adaptation to survive in snowy conditions.
[Image by Julie Naylor]

The frosty highlands of the Tibetan Plateau may have been an evolutionary "cradle" for the wooly rhinos and other shaggy, cold-hardy creatures that roamed North America and Eurasia during the last ice age, a new study suggests.

It's generally been thought that the huge "megaherbivores" of the Pleistocene evolved from less cold-tolerant ancestors in North America and Eurasia, developing adaptations to chilly conditions as the climate cooled there.

But Tao Deng of the Chinese Academy of Sciences in Beijing and colleagues now describe a new species of wooly rhino from the Zanda Basin in the foothills of the Himalayas in southwestern Tibet, which shows clear adaptations to a cold, snowy climate. For example, these animals had flattened horns that were probably useful for clearing off snow-covered plants.

By analyzing the fossil's age and its physical features, the authors conclude that this rhino, Coelodonta thibetana, was a relatively primitive ancestor in the wooly rhino family tree, compared to its counterparts in the Pleistocene Epoch, 1.8 million to 10,000 years ago. The most recent ice ages took place during the Pleistocene.

This finding suggests that the rhinos first adapted to the cold Tibetan Plateau well before climate change occurred in other areas, and that C. thibetana was thus ready to expand into the rest of Asia as the climate cooled. In their study, Deng and colleagues similarly describe several other large animal fossils from the Zanda Basin, including a snow leopard, blue sheep and Tibetan antelope, that were adapted to the cold. This region may thus have been the springboard for a variety of cold-hardy species that expanded their ranges during the Ice Age.

This research appears in the 26 August issue of the journal Science.