Hamatophyton from the Late Devonian of Anhui Province, South China and Evolution of Sphenophyllales

Well-preserved specimens of Hamatophyton verticillatum colleered from the Upper Devonian (Famennian)Wutong Formation of Chaohu district,Anhui Province,South China,display more complete fertile axes in three orders and multiple divisions.Comparisons indicate that Hamatophyton possibly does not have palmate planate sterile leaves but hook-like Iinear ones with rare divisions.We propose seven definitive characters of Sphenophyllales:(1)completely whorled lateral organs;(2)sterile leaves;(3)strobili;(4)"sporangiophores"or stalks with reflexed tips bearing sporangia;(5)three-or four-ribbed primary xylem;(6)exarch maturation of primary xylem;and(7)secondary xylem.The Sphenophyilales probably originated from the lridopteridales based on similarities in whorled lateral organs,ribbed primary xylem and peripheral protoxylem strands.In transition from Iridopteridales to Sphenophyilales,morphological changes involve partially whorled to completely whorled lateral organs,sterile ultimate appendages to leaves,and fertile ultimate appendages to "sporangiophores"/stalks with bracts;anatomical modifications include configuration and maturation of primary xylem,and presence of secondary xylem.