2 September 2011

New woolly rhino rewrites Ice Age mammal history

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The fossilised remains of a new species of woolly rhino have been unearthed in the Himalayan Mountains of Tibet and can provide important clues to the evolution of Ice Age giants.
Woolly rhinos

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

Coelodonta thibetana,

Digital composite photo of the skull and lower jaw of Coelodonta thibetana, holotype. Credit: Xiaoming Wang

As global climate cooled and cold habitats expanded, the ancestral woolly rhino descended to northern latitudes, and eventually became one of the most successful Ice Age megaherbivores. Green area is the ultimate distribution of the most advanced woolly rhino, Coelodonta antiquitatis, that lived in the mammoth steppes during the terminal Ice Age. Numbers in circles refer to locations of successively more advanced and geologically younger woolly rhinos. Credit: Science/AAAS

LONDON: The fossilised remains of a new species of woolly rhino have been unearthed in the Himalayan Mountains of Tibet and can provide important clues to the evolution of Ice Age giants.

This is the first woolly rhino to be found in the high altitude Zanda Basin region, and is older than other known species. The discovery challenges the idea that mammals like woolly rhinos and mammoths evolved to deal with the cold climate of the last Ice Age, but instead slowly adapted in cold climates in places like Tibet well before the Ice Age began.

In light of this new fossil, “people must now think more carefully about how animals evolved in response to climate change”, said Xiaoming Wang from the Natural History Museum of Los Angeles, who made the discovery which was published today in the journal Science.

New beginnings

The woolly rhino (Coelodonta thibetana), along with other ‘megaherbivores’ such as the woolly mammoth are iconic mammals of the Ice Age, which began about 2.8 million years ago. As the atmosphere cooled and large sheets of ice covered much of North America and Eurasia, it was thought that these mammals living there adapted to the cold by evolving features such as a shaggy coat.

But the new fossil is around 3.7 million years old, suggesting that these animals may not have evolved from less cold-tolerant ancestors in North America and Eurasia in response to climate change, but instead evolved first in cold areas such as the Himalayas before spreading to lower ground as the Ice Age took hold.

Wang was alone when he found the fossil in 2007. “I found a piece of its neck laying on the ground, and further digging revealed the rest of the skull and jaw,” he said. It then took his team a week to excavate it. “We did not go in the Zanda Basin expecting to find woolly rhinos, but as is often the case in palaeontology, it is all about chance and serendipity,” he said.

The team only realised the fossil was a woolly rhino when they took it back to the lab. The remains included a full skull and jawbone, which the team were able to analyse to determine the age of the fossil and physical features of the rhino, such as the position of its horns.

Nature’s snow-plough

They found that as well as having long woolly fur, the rhino had a flattened, paddle-shaped horn which would have been used to sweep away loose snow in order to find food.

“One of the special features about the woolly rhino is its compressed horn that is leaning forward in front of the nose tip,” said Wang. He added that the compressed, flattened shape of the horn would have provided a greater surface area for scooping snow, and the forward orientation of the horn permitted the rhino to easily lower its head to sweep snow without straining its neck.

The features of the Tibetan woolly rhino were also relatively primitive compared to other known specimens. Taken with the fact that the fossil is also older, the findings strongly suggest that these animals evolved to cope with cold climates in Tibet before spreading out during the Ice Age.

Cold adaptations

The same might be true for other Ice Age mammals too, said Diego Álvarez Lao, a palaeontologist from the University of Oviedo in Spain. “This new idea could also allow us to understand the possible origin of the cold adaptations in other large herbivores, such as the woolly mammoth or the musk ox.”

The new scenario presented by Wang’s team is “simple but convincing”, said Jin Meng from the American Museum of Natural History in New York. By adapting to the cold weather on the high-altitude plateau, “these large herbivores were thus well prepared in their mountain camp and were ready for life in the low land but high latitude in northern Eurasia when the Ice Age kicked in,” Meng said.

“While they walked down from the mountains and headed northward during a period of one or two million years, new and more specialised species were given rise. This study shows a new direction to understand how the megafauna in the northern part of the world evolved and how some species survived in the Ice Age,” he said.

The team also uncovered extinct species of three-toed horse (Hipparion), Tibetan bharal (Pseudois, also known as blue sheep), chiru (Pantholops, also known as Tibetan antelope), snow leopard (Uncia), badger (Meles), as well as 23 other kinds of mammals.

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