Enigmatic, structurally preserved stems from the Triassic of Central Europe: a fern or not a fern?Enigmatic, structurally preserved stems from the Triassic of Central Europe: a fern or not a fern?

Fern fossils from the Triassic are predominantly impressions and compressions; structurally preserved specimens are comparatively rare and primarily known from the Southern Hemisphere. Several permineralized stem portions from the Triassic of Upper Silesia are assigned to the poorly known genus Knorripteris (Knorripteridaceae), an enigmatic taxon initially attributed to the lycopsids but later reinterpreted as belonging to the ferns. Because the new stems differ from the known species of Knorripteris, they are assigned to a new species, for which the name Knorripteris taylorii nov. sp. is proposed. K. taylorii is characterized by a discontinuous phloem cylinder, leaf trace xylem strands that continue down to and converge at the center of the stem, cells with spiral thickenings (i.e., transfusion tissue) accumulating within the central zone of the stem, and xylem strands with abaxial protoxylem. Knorripteris exhibits several major features not typically seen in ferns, including a stelar organization that does not conform to the protostele/siphonostele/dictyostele types traditionally recognized in fossil and living ferns, the abaxial versus adaxial orientation of the protoxylem in leaf traces, as well as the acropetal decrease versus increase of the xylem in the leaf trace. The vascular system of Knorripteris is unique and challenges current concepts with regard to stelar evolution in ferns. The discontinuous phloem cylinder shows a cauline dictyostelic aspect, while the xylem appears to be foliar in origin. Knorripteris lacks a central protostelic column of vascular tissue; rather, the small foliar xylem strands come into close contact, but there is no evidence of fusion of tracheid bundles. Despite the wealth of new information on the internal organization of the Knorripteris stem provided by the specimens described in this study, the position of Knorripteris and the Knorripteridaceae within the ferns remains elusive.