Résumé
Despite their importance for understanding plant evolution and plant-environmental interactions through geological time, fossil roots have always received less attention than aerial parts. In the case of the lignophytes (i.e., progymnosperms and seed plants), the ability to form abundant secondary vascular tissues (secondary xylem and phloem) can, however, provide useful characters to compare isolated roots to stems from the same deposit to better understand the whole-plant structure and function of fossil plants.Methodology.We studied five permineralized lignophyte roots with well-preserved secondary xylem, and in two cases some secondary phloem, from the Early Carboniferous Lydiennes Formation of the Montagne Noire, France, using classical thin sections.Pivotal results.Analysis of the secondary vascular tissues of the roots allows us to distinguish three wood morphotypes that suggest affinities with at least three different types of stems previously reported in the Montagne Noire. One specimen is assigned to the arborescent seed plant Eristophyton; two are assigned to small, probably non-self-supporting seed plants belonging to either the Calamopityaceae or Lyginopitys; and two are assigned to the progymnosperms Protopitys or Stauroxylon.Conclusions.This study illustrates how the affinities of isolated permineralized lignophyte roots can be narrowed down by comparing their wood anatomy with that of stems from the same deposit. The diversity of stem organization seen in the Montagne Noire deposit typically illustrates the morpho-anatomical diversification of the lignophytes’ vegetative body in the Early Carboniferous. This study is a first step toward adding the underground organs to our understanding of this diversification.