Résumé
Wexfordia hookense, a petrified lycopsid from the Late Devonian of Ireland, provides new insights into the evolutionary history of the group. Based on anatomical traits from permineralized stem remains, Wexfordia has been interpreted as a precursor of one of the most prevalent and conspicuous groups of the Late Paleozoic flora: the arborescent Isoétales s.l. (Klavins, 2004). Unfortunately, numerous anatomical and morphological characters of Wexfordia are still unknown, preventing its assignment to a more precise phylogenetic position. In this study, permineralized axes collected at the type locality of Sandeel Bay in 2021 and 2022 were prepared as wafers and observed under the microscope, allowing a new analysis of the stelar and secondary tissues of Wexfordia’s axes. The stems display two different anatomical organizations. The first one shows a thick secondary xylem cylinder surrounding a solid to slightly medullated protostele with an exarch primary xylem. The second pattern is characterized by the total absence of secondary vascular tissues and the presence of a medullated protostele surrounded (as the previous organization) by an undefined mutizoned cortex (only a few parts of the inner and outer cortex are preserved). Further analysis of new perminceralized distal axes is expected to provide the information required to understand the dominant branching pattern = (pseudomonopodial = or iso-dichotomous) — and subsequently to test the phylogenetic hypotheses focusing on the derived vs. plesiomorphic nature of these branching patterns. Isodichotomous derived vs. plesiomorphic nature of these branching patterns. Isodichotomous (or pseudomonopodial) branching pattern, helical phyllotaxis, and a determinate growth with the appearance of a medullated protostele correlated with the lack of secondary tissues in distal parts (apoxogenic growth) allow a better description of Wexfordia’s ontogeny, as well as its habit and architecture. This in turn enables a better understanding of lycopsid evolutionary history leading to the emblematic trees that thrived and dominated the tropical coal swamp forests of the Late Carboniferous.