"Lycostrobus" chinleana, an equisetalean cone from the Upper Triassic of the southwestern United States and its phylogenetic implications

Detailed study of the cone Lycostrobus chinleana Daugherty shows that the f ossil was incorrectly attributed to the Lycopodiales by the author and to t he quillworts by Retallack and that it actually should be assigned to the E quisetales. The cone, which occurs in the Upper Triassic Chinle Formation a t several localities in the southwestern United States, is similar to 2.5 c m wide and nearly 6 cm long and consists of a stout axis bearing whorls of peltate sporangiophores. Each sporangiophore is composed of a slender stalk and a hexagonal disk, which typically bears a single, generally long, lanc eolate, forward-directed leaf-like umbo tip on the outer surface and severa l recurrent sporangia on the inner surface. Small round to oval trilete spo res occur in the sporangia. Since the leaf-like umbo tip is similar to the sterile leaves found in certain calamite cones and the recurrent sporangia are equisetalean-like, it appears that the cone may represent a intermediat e stage between Calamites and modern Equisetum. According to this hypothesi s, the nonbracteate Equisetum cone could have developed from a bracteate ca lamite cone, through reduction and fusion of the bracts and the sporangioph ores, rather than by the loss of whorls of bracts of the Calamites cone as suggested earlier by others. As a result of this study the cone is assigned to the new Equisetalean genus Equicalastrobus and redescribed under the na me E. chinleana (Daugherty) Grauvogel-Stamm and Ash, n. comb.