Results of research on permineralized wood samples from the Irati Formation (middle to upper Permian) in the State of São Paulo, Brazil add to our understanding of several questionable problems concerning samples showing affinities with the vertebrarian types of axes. A new xylic structural plan including even roots and stems would have been characterized throughout Gondwanaland from post-glacial time through the Late Permian. Until now a number of samples had been described as a single taxonomic unit under the designation of Vertebraria indica Royle 1939. Presumably this taxon does not include only a single generic group, but rather it represents a complex of forms distributed throughout Gondwanaland that are grouped together on the basis of important and similar features, such as: 1) wedge-shaped stelic configuration resembling that of the vertebrarian stelic plan; 2) wavy grouwth rings and asymmetric growth of the vascular cilynder resulting from the cambial activity of ray cells and parenchyma cells in the rings; 3) a frequent and peculiar pitting arrangement on the radial walls of the tracheids which now appears to be also a common feature among glossopteridean shoots and vertebrarian axes. It was possible to identify some samples as independent and definite genera on the basis of their well-preserved leaf and root traces, their nodal structures and also their very characteristic manner of issuing adventitious branches. Eventhough they are not classified in the genus Vertebraria, they are closely comparable in many aspects to axes of this genus. A discussion is presented here of the genus Vertebraria and it is concluded that the related genus Tordoxylon Krãusel 1956 is a valid taxon that can be distinguished within the vertebrarian complex on the basis of its characteristic xylic plan. Some problems concerning taxonomic names are also discussed. A preliminary account of the paleoecological significance of these anatomical structures is presented in view of their rapid rise and extinction during geologic time.
