Terrestrialization and evolutionary radiation of basal embryophytes on the Gondwana
The conquest of land is one of the most important evolutionary steps in embryophyte evolution. It allowed plants to reach an outstanding level of diversity, conquer a variety of new ecological niches and dominate almost all terrestrial ecosystems. The early floras of Laurussia are well studied. Numerous localities have been known and studied for about two centuries; the scientific literature about those is abundant. On the contrary, the early floras from Gondwana are poorly known, even though this continent gathers the two thirds of the emerged lands in the Pridoli-Lochkovian ages. This work attempts to demonstrate the relevance of the study of Gondwanan early floras for understanding the terrestrialization processes and adaptive radiation of the embryophytes during the Pridoli-Lochkovian. Firstly, we describe two assemblages from South Africa and Brazil. These localities are located on the western margin of Gondwana during the Pridoli-Lochkovian. Six new taxa are described from the Brazilian locality. The latter assemblage demonstrates a diversity equivalent to that of the most diverse assemblage from the Lochkovian of Wales, which was part of Laurussia. Some of the morphological characters described from the latter assemblages allow inferring phylogenetical hypotheses. All these hypotheses are confirmed by a cladistic analysis. Hence the study of Gondwanan early floras also permits to decipher the affinities of some plants of rhyniophytoid architectures The Gondwanan floras also allow drawing some hypothesis about the ecology of rhyniophytoid polysporangiophytes. On the basis of architectural characters (long proximal axes and non-isotomous branchings), we postulate that the early land plants of rhyniophytoid architecture were in competition with each other for sunlight collection and spore dispersal. This implies that the rhyniophytoid communities formed rather dense vegetal mats. It is therefore likely that they were structured as a minute canopy: the long proximal axes are analogous to the tree trunk stratum and the branching crowns are analogous to the canopy. Finally, the study of the oldest South China block plant assemblage demonstrates that the flora from the equatorial belt of Gondwana is radically different from that of its western edge. Contrarily, the study of the Brazilian locality confirms the floral similarities between western Gondwanan and South Laurussian floras. This suggests…
terrestrialization, Gondwana, early land plants, Physical, chemical, mathematical & earth Sciences, Earth sciences & physical geography, Physique, chimie, mathématiques & sciences de la terre, Sciences de la terre & géographie physique
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