西藏札达盆地发现的最原始披毛犀揭示冰期动物群的高原起源

冰期动物群长期以来已被认识到与更新世的全球变冷事件密切相关,其中的动物也表现出对寒冷环境的适应,如体型巨大、身披长毛,并具有能刮雪的身体构造,以猛犸象和披毛犀最具代表性。这些令人非常感兴趣的绝灭动物一直受到广泛的关注,它们的上述特点曾经被假定是随着第四纪冰盖扩张进化而来,即这些动物被推断可能起源于高纬度的北极圈地区,但一直没有可信的证据。本研究根据来自西藏的新化石材料证明,冰期动物群的一些成员在第四纪之前已经在青藏高原上演化发展。冬季严寒的高海拔青藏高原成为冰期动物群的"训练基地",使它们形成对冰期气候的预适应,此后成功地扩展到欧亚大陆北部的干冷草原地带。这一新的发现推翻了冰期动物群起源于北极圈的假说,证明青藏高原才是它们最初的演化中心。

Ancestral Woolly Rhino from the Zanda Basin in Tibet, China Suggests Origin of Ice Age Megaherbivores in High Plateau

A new Pliocene mammal assemblage is reported from a high altitude basin in western Himalaya, including a new species of the woolly rhino. Ice Age megafauna have long been known to be associated with global cooling during the Pleistocene and their adaptations to cold environments, such as large body size, long hair and snow-sweeping structures, are best exemplified by the woolly mammoths and woolly rhinos. These traits were assumed to have evolved as a response to the ice sheet expansion. The known fossil record suggests that the woolly rhino evolved in Asia, but its early ancestry remains elusive. The new middle Pliocene （~3.7 million years ago） woolly rhino, Coelodonta thibetana, from the high-altitude Zanda Basin at the foothill of the Himalayas in southwestern Tibet occupies the most basal position of the Coelodonta lineage and is the earliest representative of the genus. As the Ice Age began about 2.8 million years ago, the Tibetan woolly rhino descended, through intermediate forms, to low altitude, high latitude regions in northern Eurasia, and along with the Tibetan yak, argali, and bharal, became part of the emerging Mammuthus-Coelodonta fauna in the middle to late Pleistocene. The new Tibetan fossils suggest that some megaherbivores first evolved in Tibet before the beginning of the Ice Age. The cold winters in high Tibet served as a habituation ground for the megaherbivores, which became pre-adapted for the Ice Age, successfully expanding to the Eurasian mammoth steppe.