NO HIGH TIBETAN PLATEAU UNTIL THE NEOGENE

The late Paleogene surface height and palaeo-environment for the core area of the Qinghai-Tibetan Plateau (QTP) remain critically unresolved, with isotopic and palaeontological proxies yielding widely divergent outcomes ranging from a wet subtropical lowland barely ~1 km a.m.s.l. to an arid cold plateau in excess of 5 km. The anatomy of palms (family Arecaceae) renders them intrinsically susceptible to freezing, imposing upper bounds on their latitudinal and altitudinal distribution. Here we report the first discovery of well-preserved fossil palm leaves from lake sediments ascribed to the late Paleogene (Chattian, ca. 27.82-23.03 Ma) within the Lunpola Basin (32.02°N, 89.46°E), central QTP, at a present elevation of 4655 m. Detailed study shows this to be a new species, Sabalites tibetensis T. Su et Z.K. Zhou sp. nov. Climate tolerances of its living relatives evidence a cold month mean temperature above 5.2 °C. Combined with model-determined palaeo-terrestrial lapse rates, this shows a high plateau cannot have existed in the core of the QTP in the Paleogene. Instead a deep palaeo-valley system, whose floor was <2.3 km a.m.s.l. bounded by (>4 km) high mountain systems, formed a topographically highly varied landscape. This finding challenges prevailing views on crustal processes, monsoon dynamics and the origins of Asian biodiversity.