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.
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