Anatomical assessment of Guangdedendron micrum and new phylogenetic insights into late Palaeozoic lycopsids

Abstract Guangdedendron micrum , from the Late Devonian Xinhang forest of South China, represents a key taxon for understanding the early evolution of arborescent lycopsids. Initially reconstructed as a tree bearing the earliest known stigmarian rooting system and a simple crown, G. micrum is now further characterized by a medullated protostele, well-developed secondary xylem, and tracheids with fibrils between scalariform thickening bars. To evaluate the evolutionary relationships of Devonian–Carboniferous lycopsids, we conduct parsimony and Bayesian phylogenetic analyses, using an updated matrix of 53 characters across 35 taxa. The results show a frequent intermixing of the Late Devonian and Carboniferous taxa within rhizomorphic lycopsids. Combined with fossil evidence, these findings suggest that the evolution of arborescent lycopsids from the Middle–Late Devonian to Carboniferous was characterized by an initial burst of phenotypic innovation and a later ecological stabilization, consistent with the model of punctuated equilibria.