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The Lopingian (upper Permian) in Southwest China yields diverse marattialean plants; however, most of these have been recorded as isolated fragments of fronds, pinnules, and stems. Marattialean Qasimia specimens were recently collected from the upper Permian Xuanwei Formation of Yunnan Province, Southwest China. The well-preserved synangia, sporangia, and prominent in situ spores permitted the emendation of the genus Qasimia and the establishment of the species Q. yunnanica sp. nov. This plant possesses alternately or sub-oppositely arranged fertile pinnules with abaxially attached synangia. The synangia are sessile and bivalvate, with a length nearly half the width of the pinnules. A synangium valve consists of approximately 20 long ellipsoids and laterally fused sporangia. In situ spores of the Punctatosporites type are monolete, 97 × 74 μm on average, with a microverrucate/microgranulate to vermiculate sculpture; these represent the largest in situ spores of this type. The sporangial wall has at least two layers of polygonal/elongated cells. The exine comprises a perine, sexine, and nexine. The characteristics of the marattialeans with bilaterally symmetrical synangia were analyzed based on a comparison of eight related genera in the late Paleozoic and Marattia after the late Paleozoic. The Qasimia -type synangium represents the earliest occurrence of bivalvate synangium, which is a typical feature of modern marattialeans. Although the origin of the bilaterally symmetrical marattialean synangia remains elusive, marattialeans with this type of synangia first appeared in the Euramerica during the late Carboniferous and then migrated to the Cathaysia, where they became more prosperous until the late Permian. • Marattialean fern Qasimia yunnanica sp. nov. is described from the Lopingian of China. • Qasimia yunnanica sp. nov. has the largest in situ spores among Paleozoic marattialeans. • An evolutionary trend of bilateral synangia is evident in late Paleozoic marattialeans. [ABSTRACT FROM AUTHOR]