西藏聂拉木亚汝雄拉晚中新世化石及其古环境意义

Late Miocene fossils from Nainnainxuonla,Nyalam,Xizang,and their implications for paleoenvironment

西藏聂拉木亚汝雄拉南坡保存有较厚的新生代达涕古湖盆沉积物,并产出晚中新世晚期福氏(垂鼻)三趾马(Hipparion(Cremohipparion)forstenae)化石,是研究青藏高原喜马拉雅地区新生代隆升与环境变迁的重要区域之一。目前对喜马拉雅山脉代表性新生代沉积盆地的地层与古脊椎动物化石的研究工作主要集中在札达和吉隆盆地,对达涕古湖盆的研究程度仍很低。2020年,由中国科学院古脊椎动物与古人类研究所组织的第二次青藏高原综合科学考察研究古生物分队,对聂拉木亚汝雄拉地区开展中-新生代地层与古生物调查,在新近纪地层中采集到一小批脊椎动物化石,包括鲤科咽齿、犀类、有蹄类和鼠兔类。其中,鼠兔类化石归属于艾氏拟美兔(Bellatonoides eroli),这是该属在青藏高原地区的首次发现,指示其产出层位的生物地层年代不会晚于晚中新世早期(灞河期)。根据哺乳动物化石,目前可从亚汝雄拉剖面中识别出两个不同的层位和年代,即上部湖滨相地层产福氏(垂鼻)三趾马,其年代为晚中新世晚期(保德期);下部湖相地层产艾氏拟美兔,其年代可能为晚中新世早期(灞河期)。此前艾氏拟美兔发现于土耳其安卡拉(Ankara)和我国内蒙古中部地区中中新世晚期-晚中新世早期的地层中,通常与灵长类及其他森林型哺乳动物共同出现;福氏(垂鼻)三趾马分布于西藏吉隆和山西保德晚中新世晚期地层中,被认为适应于开阔生境,海拔分布不超过2900m。化石证据显示聂拉木达涕古湖盆在晚中新世早期至晚期的古海拔可能比现代高度低了近2000m,古植被类型存在从晚中新世早期的森林到晚中新世晚期的森林草原环境的转变,整体上晚中新世亚汝雄拉的古环境远比现代要适宜得多。

The southern slope of the Nainnainxuonla Mesa,Nyalam,Xizang(Tibet)preserves thick Neogene sediments of the Dati Paleolake Basin,which is considered crucial for studying the Neogene uplift and environmental changes in the Himalayan region.Remains of the late Late Miocene Hipparion(Cremohipparion)forstenae have been found in the Nainnainxuonla section.Although significant work has been done on the stratigraphy and vertebrate fossils of representative Neogene sedimentary basins in the Himalayas,with emphasis on the Zanda and Gyirong basins,research on the Dati Paleolake Basin remains limited.In 2020,the paleontology team of the Second Comprehensive Scientific Expedition to the Qinghai-Tibet Plateau(QTP),organized by the Institute of Vertebrate Paleontology and Paleoanthropology,Chinese Academy of Sciences,investigated the Mesozoic-Cenozoic strata and paleontology in the Nyalam region.A small number of vertebrate fossils were discovered from the Nainnainxuonla section,including cyprinid pharyngeal teeth,limb bones of rhinoceroses and hipparion limb bones,and lagomorph cheek teeth.Among these fossils,the lagomorph fossils belong to Bellatonoides eroli,representing the first discovery of this genus in the QTP and indicating an age no later than the early Late Miocene.Two distinct stratigraphic levels and ages have been identified in the Nainnainxuonla section based on mammalian fossils.The upper fluvial facies strata yield H.(C.)forstenae of late Late Miocene(Baodean)age,while the lower lacustrine facies strata yield B.eroli,possibly of early Late Miocene(Bahean)age.During the late Middle to early Late Miocene,B.eroli was typically found at the strata in Ankara,Turkey and central Inner Mongolia,China.It was often associated with primates and other forest-dwelling mammals.H.(C.)forstenae,which is believed to have adapted to open habitats,is distributed in the late Late Miocene strata of Gyirong,Tibet,and Baode,Shanxi.Its altitudinal distribution does not exceed 2900 meters.Based on these fossil evidence,it is reasonable to infer that the Nyalam Dati Paleolake Basin had a paleo-altitude nearly in 2000 meters lower than its current elevation during the early to late Late Miocene.From the early to late Late Mionce,the paleovegetation type changed from the forest environment to the forest-grassland with quite open environment.In general,the paleoenvironment of Nainnainxuonla during the Late Miocene was much more favorable than it is today.