Modern rhinoceroses are descendants of even larger creatures, estimated to have weighed 15-20 tonnes (16-22 tons) and stood almost 5 meters (16 feet) at the shoulder. Their heads were probably as high as giraffes' while weighing two to three times as much as an African elephant. The discovery of one of these leviathans on the edge of the Tibetan Plateau has helped fill gaps in their family tree.
Most people lucky enough to encounter a member of the genus Paraceratherium (the largest rhino genus) would not recognize them as rhinos, since they lacked horns, usually considered the defining feature of being a rhinoceros. Such minor matters are of little importance to paleontologists, however, who can spot the true identity under the skin.
Examples of Paraceratherium have been found across large areas of Asia, but have usually been so fragmentary it has been hard to determine which represented common species and which were something different.
A new Paraceratherium skull, mandible, and vertebrae from another member of the same species have been found in the Linxia Basin, on Tibet's north-east edge, detailed in Communications Biology. They date from 26.5 million years ago, and have been named P. linxiaense. Dr Tao Deng of the Chinese Academy of Sciences and co-authors consider their new discovery's closest relative to be P. lepidum, a slightly smaller species living at the same time. However, they argue it “has a tight relationship” with P. bugtiense, the only species from which we have had enough fossils to describe with confidence.
P. linxiaense's location and features allow the authors to propose a line of descent from earlier, smaller but still giant rhinos, whose relationship to P. bugtiense had been unclear. The connection also tells us something about the timing of the Tibetan plateau’s rise.
Back in the Oligocene (33.9-23 million years ago) the plateau, and indeed the whole of Asia, was very different to today. India's collision with Asia was recent, so the plateau was not as high. Meanwhile, much of what is now central Asia was still covered by the Tethys Sea.
The relationship between P. linxiaense and P. bugtiense indicates there were ways to get from Pakistan to China for creatures such as these at the time. Paraceratherium evolved in China, the authors argue, before crossing the incomplete plateau to achieve its most famous form in Pakistan.
“The Tibetan region likely hosted some areas with low elevation, possibly under 2000 meters (6,600 feet) during Oligocene,” the paper concludes, “And the lineage of giant rhinos could have dispersed freely along the eastern coast of the Tethys Ocean and perhaps through some lowlands of this region.”
Like other Paraceratherium, the authors think P. linxiaense lived in open woodlands where it could browse the tops of trees.