An Albertia-Dominated Plant Assemblage From The Lower Triassic Dongchuan Formation In Lubei,Eastern Yunnan,Southwest China

The end-Permian mass extinction had caused a dramatic vegetational changeover on Earth.The Cathaysia Flora,one of the four major phytogeographic provinces of the late Paleozoic,changed from lycophyte-,sphenophyte-and fern-dominated vegetation in the late Paleozoic to seed plant-dominated in the early Mesozoic.Thus,the Early Triassic flora plays a critical role in the understanding of plant evolutionary history.In this thesis,2350 fossil plant specimens collected from three beds(F1,F2,F3)in the uppermost of the lower Triassic Dongchuan Formation at Lubei section,Huize County,Yunnan Province,Southwest China are systematically studied.Fourteen genera with forty-two species(two new species)of plants have been recognised in the current fossil assemblage,including lycophytes(2 species of 2 genera,4.76%),sphenophytes(14 species of 2 genera,33.33%),pteridophytes(2 species of 2 genera,4.76%),peltasperms(5 species of 3 genera,11.90%),cycadophytes(1 species of 1 genus,2.38%),ginkgophytes(1 species of 1 genus,2.38%),conifers(2 species of 1 genus,4.76%),cones(4 species,9.52%),gymnosperm seeds(5 species of 1 genus,11.90%),and 6 problematica types(14.29%).Albertia and sphenophytes are the dominant elements in the current plant assemblage in terms of richness and diversity.Except for a few endemic species,the current fossil plant assemblage has a similar floral composition to those of the nearly coeval Eurasian floras,i.e.the Buntsandstein floras in western Europe,and the Heshanggou flora in North China and the Lingwen flora in South China.Comparative analysis indicates a significant latitudinal distinction in floral composition occurred in the Early Triassic: conifers dominated the low palaeolatitudinal floras,whereas lycophytes,sphenophytes and pteridosperms dominated the mid palaeolatitudinal floras,and lycophytes and ferns predominated the high palaeolatitudes.Thirteen insect damages are found in six plant specimens,and are assigned to four distinct damage types(DTs).Both insect frequency and damage diversity are much lower than the damage found in the Buntsandstein floras in Germany and the Anisian flora in Italy.The low frequency and occurrence of insect damage are likely caused by :1.the interaction between plants and insects was still in an initial recovery stage after the end-Permian mass extinction;and 2.Albertia and sphenophytes,the dominating elements,were not the main target plants of herbivourous insects at the time.The current Lubei fossil assemblage represents a critical radiation stage between the Cathaysian Flora and the succeeding Yipinglang flora.This study,sheds new light for the iconic conifer Albertia,and the floral evolution in Southwest China during the Early Triassic.