Reappraisal of Pseudobornia ursina Nathorst: implications for the evolution and systematics of Palaeozoic...

Sphenophytes, now restricted to Equisetum, were more diverse during the Palaeozoic, particularly within Carboniferous coal swamp ecosystems. Despite their significance, the origins and phylogenetic relationships of sphenophytes with stem-group monilophytes remain poorly understood. In this context, the extinct order Pseudoborniales, typified by Pseudobornia ursina (Nathorst, 1894) from the Late Devonian of Bjørnøya (Norway), plays a key role in understanding the group's origin. However, conflicting interpretations of its reproductive structures have hindered its phylogenetic placement. Here, we provide a new description and reconstruction of the reproductive structures of P. ursina to evaluate its phylogenetic relationships with other sphenophytes and closely allied groups, as well as to provide an updated perspective on the evolution of key traits among sphenopsids.

Fossils from the type locality were re-examined to clarify the morphology of the strobilus and fertile appendages. Comparative analyses were conducted with members of Sphenophyllales, Equisetales and stem-group monilophytes. Phylogenetic relationships were assessed using parsimony and Bayesian methods.

The strobilus of P. ursina displays distinctive features: (1) stalked, sporangia-bearing appendages, (2) oblique insertion of these structures in the bract axil, (3) ∼30 erect sporangia arranged on a wide-obconical receptacle and (4) deeply bisected bracts with entire margins and parallel venation. Vegetative characters suggest equisetalean affinities, while reproductive traits more closely resemble stem sphenopsids. This mosaic points to a unique combination of ancestral traits within Sphenopsida, and phylogenetic analyses place P. ursina within Equisetales.

Our reappraisal of the strobilus of P. 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 with the loss of the bract, leading to the two main clades of Sphenophytes (Sphenophyllales and Equisetales).