Author: Tao Jun-rong and Du Nai-qiu
J Integr Plant Biol 1987, 29 (6): -.
Collected from the Lawula Formation of Markam County, Xizang (Tibet), there are fossil plants, of fourteen species: a pteridophyte (Equiieturn sp.), a gymnosperm (Abies) and angi- osperms (Betula, Carpinus, Alnus, Corylus, Ulrnus, Sorbus, Cornus). The analysis of fossil pollens shows that Pinus, Abies, Quercus, Alnus, Juglans, Tilia, Lonicera, Chenopodium, Polygonurn, Arternisia, Polypodiaceae Loxogrammaceae, Dipteris, Selaginella, Microlepia, Leiotriletes also existed in the same stratum. The elements of the Lawula flora are mainly deciduous wood plants forming broad-leaved deciduous forests in warm-temperate regions or in subtropic mountainous regions at about 2100–2900 m above sea leavel, but the bearing bed is 3700 m. The fossils indicate that this region must have uplifted by about 1000 m since the Neogene. The uplift is less than that in Namling area of central Xizang. Judging from fossil records of the Late Cretaceous and the Tertiary, it may be speculated that the genus Betula appears relatively early in the geological time, and was prosperous in the Neogene. Based on the distribution of these plants and their ecological requirements, the region was then warm and near a lake. Betula mankongensis Tao sp. nov. Description: Leaves ovate-elliptic, about 4 cm long (preserved part) and 2.4 cm wide, acuminate at the apex, cuneate, slightly asymmetrical at the base, biserrate with teeth broad-cuneate. Petioles stout, 0.9 cm long. Venation pinnate, craspedodromous, midvein slightly curv- ed; secondary veins alternate, 9-paired, diverging from the midvein at an angle of about 45–50˚, bending near the end of teeth; tertiary veins at a right angle to the secondary ones, slightly flexed in the middle and anastomosing each other.