古近纪/新近纪之交青藏高原陆地生态系统的重大转折

Significant shift in the terrestrial ecosystem at the Paleogene/Neogene boundary in the Tibetan Plateau

新生代古近纪/新近纪之交是地球生命环境演化史上的重要节点,生物界总体面貌更趋近现代.青藏高原的隆升对该地区的陆地生态系统产生了重大影响,最终形成现代高原冰冻圈环境与生态体系.通过研究近年来高原腹地伦坡拉、尼玛盆地古近纪-新近纪沉积中产出的大量动植物化石,发现青藏高原生态系统在古近纪/新近纪之交经历了由热带、亚热带生态体系向高原型生物群落的重大转折.以上地点渐新世地层中的鱼类、植物和昆虫化石证据表明,高原腹地在26~24 Ma仍为温暖湿润的低地,来自印度洋的暖湿气流还可深入藏北.这一时期动植物以攀鲈和棕榈为代表,不仅反映热带、亚热带气候特征,并且表明当时盆地可能的最大海拔仅有2300 m左右.自中新世开始,高原陆地生态系统整体上向现代型过渡.裂腹鱼开始出现,并自此向特化等级演化,至上新世出现高度特化种类.早中新世植被以北温带落叶阔叶树种占优势,同时出现大量针叶树,草本植物进一步发展,反映气候已具温带特征.哺乳动物在早中新世出现适应温带森林的近无角犀等,而以披毛犀为代表的寒冷适应性冰期动物祖先出现于上新世.青藏高原生态系统这一重大转折与高原主体在早中新世隆升到接近3000 m高度所产生的降温效应相关,同时也受到全球气候转凉的影响.

The shift from the Paleogene to the Neogene represents an important timeline in the history of life on Earth:A point when the biotic realm approached that of nature today.The uplift of the Tibetan Plateau in the Cenozoic Era has imposed profound influence on the evolution of the terrestrial ecosystem by creating a sustainable life system in a freezing climate.A reconstruction of this epic scene on the plateau relies on fossil discoveries.Based on a recent study of numerous wellpreserved fossils from the Paleogene and Neogene deposits of the Lunpola and Nima basins in the central Tibetan Plateau,we recognized therein,for the first time,the turnover of the Late Oligocene tropical or subtropical ecosystem,and the subsequent transition toward a plateau-type biotic assemblage during the Early Miocene epoch.The fossil biota consisting of fishes,insects and plants from the Upper Oligocene suggests that in 26-24 Ma,the hinterland of the Tibetan Plateau was a warm and humid lowland nourished by tropical moisture from the Indian Ocean,which would have been able to reach northern Tibet then.This biota,represented by climbing perches and palms,shows typical tropical or subtropical climatic patterns and maintained a paleoelevation no higher than 2300 m in the depositional areas.In the Early Miocene,the Tibetan ecosystem underwent comprehensive transformation into the one we recognize today.Primitive snow carps,which are endemic to the plateau today,emerged and,in an'Ascending with the Modifications'mode,evolved into more and more specialized species all the way up until the Pliocene epoch.The vegetation of the Early Miocene was dominated by temperate broad-leaved forests mixed with abundant coniferous trees and booming herbs,hence showing a cool climatic setting.Some mammals adapted to the temperate forest,e.g.,Plesiaceratherium(a kind of extinct rhinos),appeared at the center of the plateau during the Early Miocene,and gave way to the ancestors of the Ice Age mammals,e.g.,the woolly rhino of the Pliocene epoch.Such a dramatic transformation of the Tibetan ecosystem around the Paleogene and Neogene boundary was due to the cooling-down effect accompanying the rise of the main body of the plateau to ca.3000 m and the global climate evolution toward icehouse conditions during the Cenozoic.