Researches on Plants from the Lower Devonian Xujiachong Formation in the Qujing District, Eastern Yunnan

Fossil plants Hedeia sinica Hao et Gensel 1998, Huia gracilis Wang Ct Hao2001 and Guangnania cuneata Wang et Hao are described from the Lower Devonian Xujiachong Formation,the Qujing district, eastern Yunnan, China. They contribute to our knowledge of the flora in thisdistrict. Based on the occurrence of common plants (Hedeia, Huia, Guangnania and Zosterophyllumaustralianum) and their horizons, it is proved for the first time that the mid-lower assemblage ofthe Xujiachong flora is comparable with the Early Devonian ((late) Pragian) Posongchong flora ofsoutheastern Yunnan and the upper Baragwanathia flora of Australia. In view of this fact and thestratigraphic sequence, the mid-lower part of the Xujiachong Formation is considered to be of (late)Pragian age. Through comprehensive analyses of plant, bivalve and fish assemblages and thelithology, the upper part of the Xujiachong Formation is dated to he of early Emsian age. TheXujiachong flora belonged to the northeastern Gondwana palaeophytogeographic unit during the EarlyDevonian. The early Emsian assemblage of this flora inherited the characters of the Posongchongflora and also exhibited the endemic elements. Belonging to different phytogeographic units, theXujiachong and Posongchong floras differ from the coeval flora of the Laurussian palaeocontinent.The Xujiachong flora is comparatively simple in composition, while the Posongchong flora hasabundant and highly diversified endemics. The disparity between them has resulted from the differentenvironments.

Two Species of Zosterophyllum from South China and Dating of the Xujiachong Formation with a Biostratigraphic Method

ZosterophyUum longa sp. nov. is reported from the Lower Devonian Pingyipu Formation, Jiangyou District, northern Sichuan of China. This new plant has vertically elongate sporangia differing from all known species of Zosterophyllum, demonstrating morphological variations in Zosterophyllophytina. New materials of Zosterophyllum yunnanicum from the Lower Devonian Xujiachong Formation, Qujing District, eastern Yunnan of China show that fertile axes of this species are anisotomous or pseudomonopodial or K-shaped in branching. Sometimes fertile lateral axes are very short and bear an axillary axis at the dividing point. Distinct thickening occurs along the whole margin of a sporangium, with sporangial lobes undeveloped. The evolution of ZosterophyUum is tentatively discussed. A biostratigraphic method of Gerrienne and Streel is introduced and applied to dating of the Lower Devonian Xujiachong Formation, which is considered as of the late Pragian-early Emsian.