Résumé
The Mississippian sees a major increase of the number of trees within the lignophytes, the clade that includes the now extinct progymnoperms and the seed plants. While they were only represented in the Devonian by the emblematic progymnosperm Archaeopteris, about 15 genera have to date been described in the Mississippian, mostly in volcanically influenced environments of the tropical belt. This informal and likely paraphyletic group of trees encompasses a great range of cortical and vascular anatomies and is a good candidate to compare systematic diversity and morpho-anatomical disparity in the plant fossil record. In this talk, we will more specifically explore some systematic, evolutive, and functional questions raised by the disparity of their wood anatomy.We will first provide an overview of the significant wood disparity among these trees. They show a significant range of variation for important characters (tracheid diameter, radial pitting, ray size, etc), and some possess unique combinations of characters. In some cases their wood fits in the “classic” morphogenera Agathoxylon or Dadoxylon, others like Dameria or Protopitys are more comparable to Mesozoic morphogenera of uncertain affinities. The wood with small diameter tracheids and large rays up to 8 cells wide and over 100 cells high of large Mississippian trees such as Pitus or Megalomyelonhas no equivalent among extant seed plants.In a second step, we will look at these diverse wood anatomies in combination with other characters that have a stronger taxonomic value, especially primary vascular anatomy, and discuss how this raises important questions about plasticity, convergence, and biases in estimating plant diversity from isolated wood fragments. Different species within a genus of Mississippian lignophyte tree can have a different type of wood, especially in terms of ray size. On the other hand, several genera that differ widely in their primary vascular anatomy display a comparable wood anatomy. For example there are several genera that have a wood comparable to that of the different species of Pitus but that differ widely in terms of primary xylem strands number, size, maturation, as well the mechanism of leaf trace production, leaf trace anatomy and phyllotaxis, and/or bark anatomy.Finally, we will discuss how this disparity compares to that of coeval non-arborescent lignophytes taxa and present future avenues of research investigating the functional properties of these woods