Further Study of the Late Devonian Sphenopsid Hamatophyton verticillatum from China

Hamatophyton verticillatum is one of the most widespread plants in the Late Devonian (Famennian) of South China. Using fossils from the coeval Yuelushan Formation, Yiyang District, northern Hunan Province, we reinvestigated this important plant. Its vegetative axes pseudomonopodially branch and possess ribbed internodes and nodes bearing undivided leaves or leaves with one division. Linear leaves are in whorls. Their tips taper or slightly expand and sometimes curve adaxially to form a hooklike shape. Although H. verticillatum has been reported by many workers, knowledge of its fertile characters is still problematic. This article shows that sporangial stalks at nodes of the fertile axis lack bracts. Each stalk is undivided, curves adaxially in the distal part, and terminates in a single spiny and elliptical sporangium. Stalks may occur with a fertile lateral axis at the same node of the fertile main axis. This plant may represent the most primitive type in the Sphenopsida and, further, may prove the greater reproductive diversity of the Sphenophyllales. Detailed anatomical study shows a three-ribbed primary xylem of exarch maturation in protostele and a secondary xylem lacking parenchyma cells. Anatomical comparison with other members indicates that the Sphenophyllales are consistent in primary-xylem architecture and are moderately variable in secondary-xylem structure.