Cretaceous Ginkgo Head And The Cikang Head Plants Reproductive Organs

Xiu-Qun Liu (Botany) Directed by Professor Cheng-Sen Li and Professor Yu-Fei Wang Three reproductive organs of Ginkgoales and Czekanowskiales are found from Yixian Formation (the Early Cretaceous) and Yangcangou Formation (the Late Triassic) of West Liaoning Province, China. Comparing G. liaoningensis Liu, Crane, Li et Wang sp. nov. and Sorosaccus gracilis Harris 1935 emend. Liu, Hueber, Li et Wang belonging to pollen cones of Ginkgoales with those of extant Ginkgo biloba L., we discuss the possible evolutionary trend of male reproductive organs of Ginkgoales from the Late Triassic. At the same time, we emend Leptostrobus cancer Harris 1951 (the female reproductive organs of Czekanowskiales), and discuss all species of the genus Leptostrobus Heer based on the new materials from West Liaoning. The results are summarized as follows: 1. Pollen cones of Ginkgo——G. liaoningensis Ginkgo liaoningensis, the well-preserved pollen cones of genus Ginkgo, was found from Yixian Formation (Early Cretaceous) of Liaoning Province, China. The pollen cone, catkin-like, consists of main axes bearing scales on its base and spirally arranged sporophylls bearing 3 to 4 (rarely 2) oval or elliptical pollen sacs. Sporangia are pendulous and dehiscent by a longitudinal slit. The mature pollen grain is monocolopate and elongate-elliptical, while the juvenile pollen grains are found in the tetrad stage. The characteristics of G. liaoningensis are very similar to those of the living G. biloba. The main difference between them is that sporophylls of G. liaoningensis bear 3 to 4 (2 rarely) pollen sacs while those of G. biloba bear mostly 2 pollen sacs. G. liaoningensis differs from fossil cones of G. huttoni from Jurassic of Yorkshire, UK, and of a unnamed specimen of Ginkgo from Late Cretaceous of Alberta, Canada in the size and the number of pollen sacs. The comparison between G. liaoningensis (fossil) and G. biloba (living) supports the reduction hypothesis of ovule organs in Ginkgo that the number of pollen sacs is experienced the process of reducing from 3-4 to 2 since the Early Cretaceous. 2. Pollen cones of Ginkgoales——Sorosaccus gracilis Study of gymnospermous pollen cones, identified with Sorosaccus gracilis Harris 1935, from the Yangcaogou Formation, Late Triassic of China, has led to the identification of new and significant characteristics of the species. The new specimens show distinct variations in the morphology of the distal laminar portions of the sporophlls. These well-preserved fossils are helpful in elucidating, reconstructing and revising the diagnostic features of Sorosaccus gracilis. After comparing the characteristics of the specimens from China with those of Sorosaccus sibiricus Prynada 1962, and of the pollen cones which were respectively assigned to Baiera longifolia (Pom.) Heer 1876 from Siberia, Russia, Sorosaccus minor Harris 1935 from Greenland, and S. umaltensis Krassilov 1972 from Bureya River of Russia, we consider that these four names should be reduced to the synonymy of S. gracilis. Thus we revise the diagnosis of Sorosaccus. The significance of Sorosaccus in evolution of pollen cones is discussed. We consider that that Sorosaccus is possibly basic to the evolution of Ginkgo by the reduction of the number of pollen sacs and of sporophylls. Ginkgo liaoningensis Liu, Crane, Li et Wang sp. nov. from represents likely one of morphologically intermediate steps between Sorosaccus and G. biloba in evolution. 3. The female reproductive organ of Czekanowskiales——Leptostrobus cancer The study of a Mesozoic seed plant Leptostrobus cancer Harris 1951, from theYangcangou Formation of Late Triassic and the Yixian Formation of Early Cretaceous, Liaoning Province, China, provides new insight into its general morphology and geographical distribution. The materials of L. cancer from the Yangcaogou Formation are earlier than all of last findings, and add the record of L. cancer in the age of Late Triassic, while the collections from the Yixian Formation elucidate the long history of L. cancer from Late Triassic to Early Cretaceous. Some specimens from Liaoning Province possess intact tops of axes make it possible perfect the reconstruction of L. cancer. Based on the new information given by new materials, three species L. marginatus, L. sinensis and L. sphaericus are mergered to L. cancer. Thus the diagnosis of L. cancer is modified slightly. Incidentally, we review the studying history of the genus Leptostrobus since its establishment in 1876, and discuss the main characteristics of every species.