Climbing habit confirmed in the early Permian zygopterid fern Nemejcopteris haiwangii and its palaeoecological significance

The early Permian zygopterid fern Nemejcopteris haiwangii was originally reconstructed as a ground-cover plant with a rhizomatous stem and erect phyllophores bearing primary pinnae aligned perpendicularly to the plane of the frond. While previous studies have described prickle structures in N. haiwangii that could have facilitated climbing, direct evidence for this behavior has been lacking in the palaeontological record. In this study, we present several well-preserved fossils showing clear interactions between the fronds of N. haiwangii and the trunks of Psaronius, providing solid evidence of its climbing behavior on Psaronius tree hosts. The prickle structures on the axes of N. haiwangii suggest a hook-climbing strategy, but the small size and low flexibility of these structures imply a relatively weak climbing ability, likely insufficient for reaching the forest canopy. Given the palaeoenvironmental and palaeoecological contexts, we propose that the shift from a ground-cover to a climbing growth form in N. haiwangii was an adaptive response to periodic submergence in peat-accumulating environments, rather than a strategy for accessing sunlight in a closed-canopy forest.