New Mississippian materials of the lycopsid rootstock Stigmaria Brongniart from South China: Evidence for anatomical conservatism in this fossil-genus

The fossil-genus Stigmaria Brongniart represents the underground rooting system of arborescent lycopsids, which were important elements in coal-swamp forests during the Carboniferous to Permian periods. The morphology and anatomy of Stigmaria have been extensively studied, mainly based on well-preserved materials from the Pennsylvanian of Euramerica, but knowledge about its pre-Pennsylvanian characters, taxonomy, and evolutionary history remains to be expanded. Here, new materials of Stigmaria are described from the Visean (Mississippian) Xiangbai Formation of Longli, Guizhou, southwestern China and are classified into Stigmaria cf. S. ficoides (Sternberg) Brongniart. Our plant is characterized by navel-shaped rootlet scars, which are arranged helically around rhizomorphs. Its anatomy shows a solenostele with endarch primary xylem and radially aligned secondary xylem, which is divided into wedge-shaped segments by rootlet traces. The cortex is divided into five zones, but there is no periderm. Rootlet vascular bundles are triangular monarch, protruding from the inner cortex to the rhizomorph surface. Our find represents one of the two earliest records of Stigmaria with anatomy preserved. The other Visean record (Glenarbuck, England) is represented by limited material, and thus many more anatomical features demonstrated in our plant are first known in the pre-Pennsylvanian Stigmaria. These Visean records demonstrate great similarities in anatomy with the Pennsylvanian and Permian materials of Stigmaria ficoides, indicating anatomical conservatism and long-term evolutionary stasis in this fossil-genus.