Internal anatomy of the Late Triassic Equisetocaulis gen. nov., and the evolution of modern horsetails

The occurrence of permineralized stem fragments with diagnostic equisetophyte anatomy in Petrified Forest National Park, east-central Arizona provides an opportunity to characterize the internal structure of a Late Triassic (Norian Stage) equisetophyte. Features of Equisetocaulis muirii gen. et sp. nov. constitute the first evidence for internal anatomy of a Triassic equisetophyte from the Northern Hemisphere. Anatomically preserved stem fragments occur in pyrite nodules and are revealed on etched surfaces of nodule wafers. Stems range 7-16 mm in diameter, with a large hollow pith and 24-40 cauline bundles, each with a distinct carinal canal and several rows of radially aligned secondary tracheids. Cauline bundles converge at nodes to produce a ring of tracheids within the nodal diaphragms. Leaf traces and branch bases occur at nodes on alternating radii. These features confirm a close relationship between at least one of the equisetophytes from the Chinle Formation and modern species, amplify our knowledge of equisetophyte organization during a crucial period in the evolution of the clade, and further refine our understanding of the sequence of structural changes leading to crown group Equisetum L.