Study On A New Fern Sensu Lato From The Middle Devonian Tiaomachian Formation Of Liuyang,Hunan

Devonian is a critical period of the evolution and radiation of land vascular plants,when the diversity of plants and the number of species increased a lot.Devonian flora in China,including dominant worldwide species and abundant local plant groups,is well-developed,and is of great significance to the study on origin and evolution of early land plants.Moreover,Devonian strata in China which widely distributed,possesses different sedimentary types and provides a reliable geological basis for the detailed study.Devonian strata in Hunan is well-developed and is one of the important districts to carry out stratigraphic and paleontological study of Devonian strata in South China.Specially,there is a set of fragmental rock deposits developing in Hunan after the Kwangsian Orogeny,whose geological age is Early-Middle Devonian,such as the Middle Devonian Tiaomachian Formation.Study on the clastic rock deposits,which yields a mass of well-preserved plant fossils,is of great significance to the cognition of Devonian flora,and provides evidence for the division and comparison of biostratigaphy.We selected a new fern sensu lato-Liuyangia formosa gen.et sp.nov.from the Tiaomachian Formation of Liuyang,Hunan to carry out morphological and systematic paleobotanical studies.The main characteristic of Liuyangia formosa gen.et sp.nov.is:The plant has three orders of axes,and all the axes are covered with longitudinal groves and ridges.First-order axis is vertical and shows no obvious tapering or bifurcation.First-order axis is equal in width,up to 230 mm long,and 4.7-11.1 mm wide.Second-order axes are inserted in first-order axis alternately in pairs which called quadriseriate branching patterns.The internode length of first-order axis is 25.4 mm.Second-order axis can be up to 100.5 mm long and 2.0-6.2 mm wide.Third-order axes diverge from second-order axis alternately and respectively,the internode length of second-order axis is 19.3 mm.Third-order axis is up to 60.1 mm long and 0.7-2.7 mm wide.Ultimate units,including vegetative appendages and fertile organs,attach to third-order axis alternately and respectively.The internode length of third-order axis is 9.6 mm.Vegetative appendages are three-dimensional,and dichotomize up to five times terminated with recurved tips.The length from attachment to ultimate tip is 8.6-10.1 mm,and the length between the dichotomies is 1.5-2.5 mm.The width of vegetative appendages decreases from the basal to the distal part,the basal portion of vegetative appendages is 0.4-0.6 mm wide,and the distal portion of vegetative appendages is 0.2 mm wide.Fertile organs are three-dimensional,and comprise fertile appendages dichotomized up to five times and paired sporangia.The length from attachment to ultimate tip is 7.4-9.1 mm,and the length between the dichotomies is 1.4-2.5 mm.The width of fertile appendages decreases from the basal to the distal part,the basal portion of fertile appendages is 0.4-0.5 mm wide,and the distal part of fertile appendages is 0.2 mm wide.Sporangia are elongate and fusiform,without sporangiophore or dehiscent lines.Sporangia are 1.2-1.4 mm long and 0.4-0.5 mm wide,and the length-to-width ratio is 2.8.Study on this plant enriches the features of the Middle Devonian flora,meanwhile,it also provides evidence for the division and comparison of the regional stratigraphic sequences,especially the earliest deposits after the Kwangsian Orogeny.