CLIMATIC CONSTRAINTS ON FLORISTIC EXCHANGE PATTERNS IN LATE PALEOZOIC INTERIOR AFRICA

During Middle Permian, north Africa was located in a biogeographic transition zone between the northern summerwet Euramerican realm dominated by seed plants, such as conifers and peltasperms, and the southern-temperate Gondwanan realm, dominated by glossopterids. Transitional areas near the Tethys margin show a strongly north-to-south orientated floristic exchange pattern between the tropical flora of Euramerica and the south-temperate flora of Gondwana. These floras, particularly in modern eastern Saudi Arabia, reveal the existence of mixed assemblages well into the Late Permian. New discoveries from the Tim Mersoi basin, north-central Niger, permit re-evaluation of a mixed flora that suggests also non-coastal and bi-directional migratory pathways existed in interior Africa, at least up to the Middle Permian. Recently discovered low diversity conifer-dominated megafloras from the Moradi Formation in Niger lack Gondwanan elements, suggesting that, just as along the Tethys margin, little movement occurred of Gondwanan elements into the tropics. Climate models predict increased desertification in this part of interior Africa during Late Permian, which would have affected migratory routes and composition of the floral assemblages. These results and interpretations are corroborated by morphological and chemical data from paleosol profiles in the Moradi Formation, which indicate arid conditions. However, the floras from the same stratigraphic levels, dominated by conifers, and including up to 25 meter long petrified stems, seem to contradict the interpretation that moisture was a limiting factor.