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Equisetales represents a diverse and abundant group of articulate plants with a wide geographical distribution that had their greatest development in late Palaeozoic and early Mesozoic times. The order is represented by 12 families, composed mostly by extinct families of late Palaeozoic age, with only a unique living family, the Equisetaceae. Here we describe the new equisetalean fossil-genus and species Iberisetum wegeneri gen. nov., sp. nov. from the lower Stephanian C (Gzhelian, Upper Pennsylvanian) of intramontane Douro Basin, in northwestern Portugal. This new fossil taxon represents another endemic species for the Douro Basin. Iberisetum wegeneri consists of a leafy articulated stem that shows a unique combination of macro-morphological features that are common in various fossil-genera of different equisetalean families, and that includes exclusive (autapomorphic) characters. The singular morphology of foliar sheaths of Iberisetum wegeneri represents an evolutionary novelty into the Equisetales and suggests a heliotropic organisation (heliotropism) of the plant during its lifetime. Such functional morphology is the result of an evolutionary adaptation to the climatic and ecological conditions restricted to intramontane environments of Douro Basin. [ABSTRACT FROM AUTHOR]