Reinvestigation of the marattialean Zhutheca densata (Gu et Zhi) Liu, Li et Hilton from the Lopingian of Southwest China, and its evolutionary implications

Abstract Marattialeans were the most species-rich fern group in the Lopingian (late Permian) Cathaysian floras in the South China Block. However, most previous studies on marattialean ferns focused on their vegetative morphology; in contrast, their reproductive biology has rarely been examined. New compression materials of Zhutheca densata (Gu et Zhi) Liu, Li et Hilton from the Lopingian Xuanwei Formation in Qujing City, Yunnan Province, Southwest China, were studied and provide additional micromorphological and ultrastructural details about the reproductive organs of this marattialean plant. Reproductive organs of the Z. densata plant are characterized by radially symmetrical synangia in two or three rows on each side of the midvein, four to six exannulate sporangia laterally fused in a synangium, sporangial walls comprising three to four layers of cells, and monolete Laevigatosporites/Latosporites-type in situ spores. The spatiotemporal distribution of marattialean reproductive organs was analyzed based on a thorough comparison of 29 genera recorded in upper Paleozoic strata. Our analysis indicated that, during the Pennsylvanian and Permian periods, marattialeans with radially symmetrical synangia notably decreased in both diversification and distribution; in contrast, marattialeans with bilaterally symmetrical synangia notably increased in both diversification and distribution. After the Lopingian, marattialeans with bilaterally symmetrical synangia gradually became predominant in terms of their diversification and geographic distribution. Our study sheds new light on the evolution of marattialeans.