Neoarthropitys gondwanaensis gen. nov. et sp. nov. from the Middle Triassic of Gondwana: an intermediate stage in the anatomical trend of Equisetalean stems

A new petrified stem from the Middle Triassic Quebrada de los Fósiles Formation (Puesto Viejo Group, San Rafael Basin) in Mendoza Province, central-western Argentina, is described. The specimen shows affinities with fossil and extant Sphenophyta, such as the presence of a medullar cavity and primary xylem with carinal canal. A new genus and species - Neoarthropitys gondwanaensis - is established based on the presence of a secondary xylem with interfascicular rays and fascicular wedges, and the development of a mesarch-endarch primary xylem. These anatomical features allow differentiating it from all previously known Equisetales taxa, showing an intermediate stage between Calamitaceae which is characterized by mesarch primary xylem and by presence of the secondary xylem, and the Equisetaceae having endarch primary xylem and lacking of secondary xylem. The presence of a mesarch-endarch primary xylem in the Triassic Argentinian specimen has phylogenetic value, appearing as an intermediate stage in the anatomical trends of Equisetales stems. Neoarthropitys gondwanaensis gen. nov. et sp. nov. has a set of equisetalean features not present in any other clade, supporting the idea that they were part of a distinct group. The presence of transfusion tracheid piths supports previous hypothesis that the plants recorded in the Quebrada de los Fósiles Formation grew in an ecologically disturbed environment by intermittent volcanic ash falls, in the floodplains of a high-sinuosity fluvial system under seasonally sub-humid climatic conditions.