Reappraisal of <i>Pseudobornia ursina</i> Nathorst: implications for the evolution and systematics of Palaeozoic sphenophytes

Our reappraisal of the strobilus of Pseudobornia ursina clarifies both its morphology and its phylogenetic placement, being recovered as part of stem Equisetales, sister to Archaeocalamitaceae. This suggests an evolutionary scenario where fertile appendages of stem sphenophytes became more compact over time, with either a fusion to a bract or the development of fertile internodes, combined to the loss of the bract, leading to the two main clades of Sphenophytes (Sphenophyllales and Equisetales).