The giant rhinoceros, Paraceratherium, is considered the largest land mammal to ever live and is found primarily in Asia, particularly China, Mongolia, Kazakhstan, and Pakistan. However, how this genre spread across Asia has long been a mystery. A new discovery has just shed light on this process.
Professor DENG Tao of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and his collaborators from China and the United States recently reported a new species Paraceratherium linxiaense sp. nov., which offers important clues to the dispersal of giant rhinos across Asia.
The study was published in Communications biology June 17.
Fossils of the new species include a completely preserved skull and mandible with their associated atlas, as well as an axis and two thoracic vertebrae from another individual. The fossils were recovered from Late Oligocene deposits in the Linxia Basin in Gansu Province, China, located on the northeastern border of the Tibetan Plateau.
The phylogenetic analysis gave a single tree of the most parsimonious, which places P. linxiaense as a derived giant rhinoceros, in the monophyletic clade of the Asian Oligocene Paraceratherium. Within the Paraceratherium clade, the researchers’ phylogenetic analysis produced a series of increasingly derived species – from P. grangeri, through P. huangheense, P. asiaticum, and P. bugtiens– ending in P. lepidum and P. linxiaense. P. linxiaense is at a high level of specialization, similar to P. lepidum, and both are derived from P. bugtiens.
Adaptation of the atlas and axis to the large body and long neck of the already characterized giant rhinoceros P. grangeri and P. bugtiens, and was developed in P. linxiaense, whose atlas is elongated, revealing a long neck and a superior axis with an almost horizontal position for its posterior articular face. These characteristics correlate with a more flexible neck.
The western giant rhinoceros of Pakistan comes from the strata of the Oligocene, representing a single species, Paraceratherium bugtiens. In contrast, the rest of the genre Paraceratherium, which is distributed over the Mongolian Plateau, northwestern China and the area north of the Tibetan Plateau to Kazakhstan, is very diverse.
The researchers found that the six species of Paraceratherium are sisters of Aralotherium and form a monophyletic clade in which P. grangeri is the most primitive, followed by P. huangheense and P. asiaticum.
The researchers were thus able to determine that in the Lower Oligocene, P. asiaticum dispersed westward to Kazakhstan and its descendant line spread to Southeast Asia as P. bugtiens. In the Upper Oligocene, Paraceratherium returned north, crossing the Tibetan region to produce P. lepidium in the west in Kazakhstan and P. linxiaense to the east in the Linxia basin.
Researchers noted the aridity of the Late Oligocene in Central Asia at a time when South Asia was relatively humid, with a mosaic of wooded and open landscapes. “The tropical conditions of the late Oligocene allowed the giant rhino to return north to Central Asia, implying that the Tibetan region was still not erected into a high altitude plateau,” said Professor DENG .
During the Oligocene, the giant rhino could obviously disperse freely from the Mongolian plateau to South Asia along the eastern coast of the Tethys Ocean and possibly through Tibet. The topographical possibility that the giant rhino crossed the Tibetan area to reach the Indo-Pakistan subcontinent in the Oligocene can also be supported by other evidence.
Until the Late Oligocene, the evolution and migration of P. bugtiens at P. linxiaense and P. lepidum show that the “Tibetan plateau” was not yet an obstacle to the movements of the largest land mammal.
This research was supported by the Chinese Academy of Sciences, the National Foundation of Natural Sciences of China and the Second Global Science Expedition to the Tibetan Plateau.
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