Geological factors and evolution of southwestern Gondwana Triassic plants

A synergistic model based on reciprocal influences between biotic and abiotic factors is developed for the Triassic of southwestern Gondwana. Changes in physical environment exerted a strong influence on the characteristics and evolution of plant assemblages. The Permian-Triassic extinction, and the change from palaeophytic to mesophytic floras, is one of the most striking examples of direct influence of physical environment upon plant communities. Pangea coalescence, the distribution of land masses and seas, the spreading of continental climates (megamonsoonal conditions) and the waning polar glaciation determined the expansion of xeromorphic morphotypes that became dominant during the whole Mesozoic. In southwestern Gondwana, the introduction or invasion of immigrant lineages suggests a strong asymmetrical interchange from the Euroamerican realm to the Gondwana realm. In addition, generalised extensional volcanism, development of intracratonic rifts and the palaeolatitudinal location of climatic zones during the earlyMiddle Triassic favoured extinction of the Glossopteris flora and explosive diversification of endemic groups. From the chronological viewpoint, the Barrealian, Cortaderitian and Florian stages are recognised in the Triassic of southwestern Gondwana. These stages are respectively characterised by: (a) appearance of mesophytic elements, and coexistence of Palaeozoic and Mesozoic groups, (b) maximum diversification of the Dicroidium flora, and (c) Dicroidium flora decline and replacement by morphotypes with strong Jurassic affinity. These palaeofloristic changes seem to be strongly influenced by tectonic evolution of sedimentary basins, temporal and regional distribution of sedimentary environments, and intra-Triassic palaeoclimatic change.