Phytostratigraphy and Flora of the Late Triassic Deposits (Southern Mongolia)

Triassic continental plant-bearing sediments are limited in their distribution in Mongolia, with the majority occurring in the southern region. The Noyon syncline, located in Umnugovi Province, represents a key stratigraphic section composed of Middle-Upper Permian, Lower Triassic, Middle-Upper Triassic, and Lower Jurassic deposits. Among these, the molasse deposits of the Noyonsoum Formation (Middle-Upper Triassic) are distinguished not only by their unique geological characteristics but also by the presence of fossilized plants and animals. Consequently, they constitute an important subject for studies of stratigraphy, sedimentary processes, and paleoecosystem reconstruction. The Noyonsoum Formation has traditionally been assigned to the Middle-Upper Triassic; however, detailed studies refining its age have been limited. In 2008, 2020, and 2023, paleobotanical field investigations were conducted by our research team in various parts of the formation, yielding significant results. Notably, the majority of plant fossils were discovered during the 2023 fieldwork. Taxonomic analysis identified a plant assemblage comprising nine species belonging to seven genera, including representatives of horsetails, ferns, and ginkgoaleans. For the first time, characteristic Late Triassic (early phase) fern taxa, such as Danaeopsis marantacea, D. angustipinnata, and Lepidopteris toretziensis, as well as Sphenobaiera sp., were identified from the Noyonsoum Formation. These findings provide robust paleobotanical evidence supporting the assignment of the formation to the early stage of the Late Triassic, which represents the principal outcome of this study. The dominance of horsetails and ferns, typical of river channels, floodplains, and swampy environments, together with the presence of peltaspermalean seed ferns adapted to subhumid conditions, suggests that a warm and humid climate with pronounced seasonal variability prevailed in the region.