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 et Hao 2001 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 this district. Based on the occurrence of common plants (Hedeia, Huia, Guangnania and Zosterophyllum australianum) and their horizons, it is proved for the first time that the mid-lower assemblage of the Xujiachong flora is comparable with the Early Devonian ((late) Pragian) Posongchong flora of southeastern Yunnan and the upper Baragwanathia flora of Australia. In view of this fact and the stratigraphic 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 the lithology, the upper part of the Xujiachong Formation is dated to be of early Emsian age. The Xujiachong flora belonged to the northeastern Gondwana palaeophytogeographic unit during the Early Devonian. The early Emsian assemblage of this flora inherited the characters of the Posongchong flora and also exhibited the endemic elements. Belonging to different phytogeographic units, the Xujiachong and Posongchong floras differ from the coeval flora of the Laurussian palaeocontinent. The Xujiachong flora is comparatively simple in composition, while the Posongchong flora has abundant and highly diversified endemics. The disparity between them has resulted from the different environments.