Fossil Woods From Miocene Xundian County Yunnan And Their Palaeoenvironmental Implications

Numerous of liglified fossil woods were uncovered in Nanfeng Coalmine, Xianfeng basin, central Yunnan, southwestern China. Six taxa were identified:Tsuga nanfengensis, Pinus uniseriata, Pinus nanfengensis, Pinus sp., Abies xundianensis and Prunus sp.. Palaeoenviroment of the fossil site during late Miocene was also discussed.The fossil woods were charactered as following:Tsuga nanfengensis:Growth rings distinct, transition from earlywood to latewood gradual. Bordered pits in the radial and tangential walls of tracheids uniseriate, torus extensions distinct. Rays commonly uniseriate. Both normal and traumatic resin canals absent. Cross-field pits piceoid and cupressoid.Pinus uniseriata:Growth rings distinct, transition from earlywood to latewood gradual. Bordered pits in the radial and tangential walls of tracheids usually uniseriate. Axial parenchyma absent. Uniseriate and fusiform rays present. Ray tracheids usually in one marginal row. Horizontal walls of ray tracheids smooth. Both axial and radial resin canals present, epithelial cells thin-walled. Cross-field pits taxodioid and occasionally cupressoid.Pinus nanfengensis:Growth rings distinct, transition from earlywood to latewood gradual. Bordered pits in the radial and tangential walls of tracheids commonly uniseriate. Axial parenchyma absent. Uniseriate and fusiform rays present. Ray tracheids predominantly in two marginal rows. Horizontal walls of ray tracheids weakly dentate to smooth. Both axial and radial resin canals present, epithelial cells thin-walled. Cross-field pits pinoid and taxodioid.Pinus sp.:Growth rings distinct, transition from earlywood to latewood gradual. Bordered pits in the radial and tangential walls of tracheids predominantly uniseriate. Axial parenchyma absent. Uniseriate and fusiform rays present. Ray tracheids in 1-2 marginal rows. Horizontal walls of ray tracheids smooth, rarely pitted. Both axial and radial resin canals present, epithelial cells thin-walled. Cross-field pits window-like.Abies xundianensis:Growth rings distinct, transition from earlywood to latewood gradual. Bordered pits in the radial and tangential walls of tracheids predominantly uniseriate. Axial parenchyma absent. Rays usually uniseriate, ray tracheids absent. Cross-field pits taxodioid. Resin canals absent.Prunus sp.:Growth rings simi-distinct, wood semi-ring-porous or diffuse-porous. Vessels commonly solitary, usually in radial pattern, tangential diameter 30-80 μm; helical thickenings distinct; perforation plates simple; intervessel pits alternate. Axial parenchyma sparse. Fibers with simple to minutely bordered pits. Rays uniseriate and 2-8-seriate. Vessel-ray pits similar to intervessel pits.Combined with previous studies, Xianfeng basin (and also some other areas in Yunnan) was warmer and wetter in late Mionce than today, and the monsoon (especially winter monsoon) was weaker. The change of temperature (especially change of TAR (temperature annual range)) was an important reason leading to the shrinkage of distribution areas of Tsuga from central Yunnan. Pinus uniseriata and P. nanfengensis might adapted the change of environment in 2 different ways. Pinus uniseriata might adapted the environmental change by changes of its structure and physiological functions, while P. nanfengensis might by changes of its distribution areas. On the basis of the fossil wood species, it could be suggested that it was a mountainous environment with distinct altitude difference during late Mioncene in the fossil site. The fossil woods like Abies sp. might be carried down from relatively high elevation areas and then fossilized.