Abstract : The lignophyte clade includes the free-sporing progymnosperms (Devonian-Carboniferous) and the seed plants, or spermatophytes, that dominate most of present ecosystems. In the context of our studies on the evolution of the arborescent habit in the Palaeozoic, we focussed on the first lignophytes trees. The oldest arborescent lignophytes are the archaeopteridalean progymnosperms that evolved at the end of the Middle Devonian (385 Ma) and became extinct around the Devonian-Carboniferous boundary. Following their extinction, about twenty new taxa interpreted as arborescent members of the protopitylean progymnosperms and of the spermatophytes are recorded in Europe and North America during the Early Carboniferous (Mississippian). Due to incomplete data, particularly concerning their reproduction, the taxonomic relationships of these plants remain doubtful and their importance has often been underestimated.
This thesis first documents anatomically preserved specimens of Mississippian age from Montagne Noire (France), Central Sahara (Algeria) and from four basins of Queensland (Australia). These specimens, some of which represent new taxa, illustrate the anatomical diversity of the arborescent lignophytes succeeding to the archaeopteridalean progymnosperms. They document for the first time with confidence, the occurrence of these trees in Gondwana, since the basalmost Mississippian. More generally, these results support a progressive transition between the Devonian Archaeopteridales and the new arborescent lignophytes of the Mississippian.
In the second part of the study, we compare the morpho- natomical disparity of the arborescent and non-arborescent lignophytes of the Devonian and Mississippian. An important diversification of the vegetative system is demonstrated among the Mississippian arborescent taxa; this concerns particularly the anatomy of the primary and secondary vascular system and the pattern of lateral organs emission.
Finally, a phylogenetic analysis based on a set of mostly vegetative characters is used to test the relationships of some Mississippian taxa whose mode of reproduction is unknown. Arborescent taxa a priori assigned to spermatophytes are effectively included in this clade. They constitute one para- or polyphyletic group and show affinities with some non-arborescent spermatophytes and with the Cordaitales.