First architectural analysis of a non-arborescent cladoxlopsid identifies two possible growth habits for the Carboniferous species Cladoxylon taeniatum

Premise of the research. The cladoxylopsids s.l. are an extinct group famous for including taxa that formed the first forests during the Middle Devonian. While different growth models have been proposed for the largest cladoxylopsids, similar analyses have never been conducted on smaller representatives of the group. Here, we propose the first architectural reconstruction of a non-arborescent cladoxylopsid, based on Cladoxylon taeniatum specimens from the early Carboniferous of France. Methodology. About 200 thin-sections, wafers and peels corresponding to 53 anatomically preserved specimens of Cladoxylon from the Tournaisian Lydiennes Formation were observed to describe their anatomy and trace emission patterns. Variations in stele anatomy within the most common morphotype were also studied using principal component analysis and hierarchical clustering. Pivotal results. Four stele morphotypes are recognized based on differences in stele shape, diameter (6.5-30 mm), and secondary growth. One morphotype is assigned to Cladoxylon mirabile and the three others to C. taeniatum. C. taeniatum specimens are reconstructed as stems that bore petiole-like organs with dichotomous ultimate appendages. Variations in stele anatomy among the C. taeniatum specimens, combined with previous knowledge on cladoxylopsids growth, suggest an apoxogenetic growth and a determinate development of the secondary xylem for these stems. Two hypotheses on the likely architecture of the whole plant are presented. Conclusion. This study provides the first reconstruction of the development, organotaxis, and possible habit of Cladoxylon taeniatum, which advances our understanding of the architecture of non-arborescent cladoxylopsids.