A new plant assemblage from the Middle Triassic volcanic tuffs of Pingchuan, Gansu, northwestern China and its paleoenvironmental significance

Abstract

A new fossiliferous horizon from the Middle Triassic Dingjiayao Formation, Pingchuan District, Gansu Province, China is described. The plant assemblage preserves a diverse community of horsetails, ferns and seeds of unknown systematic affinity. Ferns are the most diverse elements of the assemblage and are also ecologically dominant, while horsetails contribute a smaller component to floristic diversity and have a lower abundance. Here, five fern taxa are assigned to Marantoidea, Symopteris, Neuropteridium, Cladophlebis and Sphenopteris. Also, six species belonging to three horsetail taxa are described from this assemblage, which are assigned to Equisetites, Echinostachys and Neocalamites. Additionally, remains of doubtful botanical attribution comprise Carpolithus seeds. Some of these groups are recorded from the Middle Triassic of China for the first time. The preservation of these fossils suggests that the assemblage was buried autochthonously or para-autochthonously by volcanic ash fall. Fossilization of these specimens occurred rapidly after the specimens embedded in the volcaniclastic sediments. Also the plant assemblage indicates a local riparian environment in a warm, humid climate.