Aspects of Gondwana paleobotany: gymnosperms of the Paleozoic—Mesozoic transition

During the late Paleozoic and early Mesozoic the Gondwana Supercontinent underwent dramatic geographic and climatic changes. Geologic and biologic factors concurrently played an important role modelling the vegetation of that time. The gymnospermic component of plant assemblages shows significant variations in composition and in the distribution of different taxa. Analysis of the assemblages shows that some plant groups dominated the scenario, such as the pteridosperms, glossopterids, corystosperms and, to a lesser degree, cordaites and conifers. Ginkgophytes, bennettites or cycads were less important in the Paleozoic but their numbers increased in the Triassic. Paleozoic assemblages were extensively dominated by glossopterids that became extinct in the earliest Mesozoic. Pteridosperms crossed the P—M barrier and became dominant during the Triassic, at a time when corystosperms evolved and radiated. Other groups became extinct in the Paleozoic, namely dicranophylls and cordaites. Conifers were represented by different families, restricted either to the Paleozoic or the Mesozoic. They were not conspicuous in the analysed assemblages. In some areas of Gondwana, taxa of the Euramerican alliance are present through a migrational mechanism that occurred during continental displacements which produced global climatic changes. Recent studies have shown that there are far more common elements between Euramerica and Gondwana than suspected up to now. These elements find their distribution especially in the western part of Gondwana (Africa—South America).