New information, including anatomy of the secondary xylem, on the genus Brabantophyton (Stenokoleales) from Ronquières (middle Devonian, Belgium)
Résumé
Two pyrite permineralized stems and one root are reported from the upper Middle to lowermost Late Devonian (middle Givetian to lowermost Frasnian) locality of Ronquières (Belgium) and identified as Brabantophyton runcariense Momont et al. (Stenokoleales). The stems include a three-ribbed protostele with a central protoxylem strand and other strands disposed along the midplanes of the ribs; each specimen shows secondary xylem. Tracheids of both primary and secondary xylem show circular to elliptical, multiseriate bordered pits. Rays are 1–seriate to 4–seriate; their height is highly variable, ranging from 3 to more than 100 cells. The root includes a four-ribbed protostele. Each rib shows two exarch protoxylem strands. The root also comprises a ring of secondary xylem identical to that of the stems. The characteristics of the secondary vascular anatomy of Brabantophyton appear closer to the seed plants than to any other Devonian plant lineage. However, the presence of a bifacial vascular cambium is not demonstrated, which precludes a definitive assignment of the genus to the lignophytes. A hypothesis about the ontogenetic development of Brabantophyton is proposed on the basis of the different characteristics observed in the different stem specimens of the plant.