New Mississippian Materials of Lycopsid Rootstocks from South China: Implications for Anatomical Diversity in This Group and Their Wetland Adaptation

Arborescent lycopsids were an important plant group in coal-swamp forests during the Carboniferous to Permian periods, serving as key indicators of wetland environments. Herein, we report well-preserved lycopsid rooting systems from the Visean (Mississippian) of South China, providing new insights into anatomical diversity, paleogeographic range, and environmental significance of Carboniferous lycopsids. Two distinct forms are recognized: (1) Stigmaria cf. S. ficoides, representing the earliest known occurrence of Stigmaria with both anatomical and morphological preservation, showing a mixed pith, endarch primary xylem, wedge-shaped xylem segments, a five-zoned cortex, and without periderm; (2) a new unnamed type, a unique rootstock characterized by multiple vascular bundles and monarch rootlet traces, which probably occupies a basal position within the isoëtalean lineage. These fossils demonstrate the evolutionary conservatism of rootlets but also reveal unexpected plasticity in lycopsid rooting systems. The occurrence of diverse lycopsid rootstocks in the Visean of South China significantly extends their paleogeographic distribution. Moreover, the variations of the morphologies and anatomy in different rhizomorphs of early Carboniferous age may indicate different solutions to waterlogged, wetland environments. This study provides direct fossil evidence that lycopsid-dominated wetlands were already established in South China during the Mississippian, and rooting adaptations evolved earlier and more variably than previously recognized.