Reinvestigation of Stauroxylon beckii, a Possible Aneurophytalean Progymnosperm from the Mississippian of France.

Premise of research. The fossil record shows that seed plants appeared during the Devonian and started to become dominant in the Mississippian. However, the identity of their closest relatives remains uncertain with three candidates: Stenokoleales, Archeopteridalean progymnosperms, and Aneurophytalean progymnosperms. To clarify the relationships of these groups, it is necessary to document as many Devonian-Mississippian taxa as possible and increase taxon sampling in phylogenetic analyses. In this context, Stauroxylon beckii Galtier 1970 from Mississippian deposits of France is particularly interesting because of its similarities with both seed plants and some of their potential closest relatives. Here, we provide an updated description of Stauroxylon and discuss its affinities using morphoanatomical comparisons and phylogenetic analyses. Methodology. The holotype of S. beckii and a new specimen from the same formation are described using thin sections, cellulose acetate peels, and polished surfaces. The morphoanatomy of the specimens is compared with that of early seed plants and their putative relatives. Stauroxylon is also included in phylogenetic analyses of the radiatopses based on Toledo and colleague's matrix. Pivotal results. Stauroxylon beckii possess a cruciform protostele with protoxylem strands at the tips of the ribs, a central protoxylem strand, and produces second- and third-order axes in perpendicular planes. The new specimen differs by characters interpreted as developmental differences or intraspecific plasticity (organotaxis, secondary growth) and is also assigned to S. beckii. Both comparative approaches and phylogenetic analyses place Stauroxylon within the Aneurophytalean progymnosperms, a group previously only known in the Devonian. The inclusion of Stauroxylon strengthens previously established relationships within the radiatopsids. Stauroxylon also displays structural fingerprints important for leaf and pith evolution. Conclusions. Stauroxylon is interpreted as a Mississippian representative of the Aneurophytales. It provides new information on the relationships and morphoanatomical diversity of early seed plants and their closest relatives. [ABSTRACT FROM AUTHOR]