A Study Of The Ediacaran Biota And Paleo-environmental Changes In Northeaststern Guizhou And Its Adjacent Areas

The Ediacaran is a key period for early life evolution and development, and has constituted the most important turning phase for biospheric devolvement in the earth history. The Doushantuo formation was deposited during a significant transgression after the Neoproterozoic Nantuo ice age in the Yangtze region, accompanied with the background of global climate warming up and sea level rising. This formation has a wide distribution in South China, and is rich in various fossils which provided the key evidence for metazoan life rapid evolution after the snowball earth. In recent years, complicated acanthomorphic acritarchs, multicellular alga, animal embryos and metazoan were discovered from this formation, providing an excellent window into the early life evolution and the mutual interactions of earth surface system environments and the early life from the view pointof earth system science.This paper deals mainly with the Sinian stratigraphy, sedimentary facies and the biotas in the Jiangkou area of northeastern Guizhou and the Weng’an area of western Hunan, with some data also supplemented from other areas, such as those from Tongren and Sandu of Guizhou, Shimen of Hunan etc. A quite number of important fossils have been collected from the deep-water facies of the Sinian in the above areas. Based on these materials and the data of previous researches, the present thesis has made a comprehensive study of those fossils from the angles of systematic paleontology, petrology, sedimentology and geochemistry. According to the present study, the biota in the Doushantuo formation are prolific, including several groups of microbes and arcritaches, as well as multicellular algae and possible radial-symetrical macro-metazoan. In vertical succession, these fossils show several punctuated occurrence stages, which may provide clues for early biotic evolution incorporated with environmental changes. On this basis some of the relationship between early biotic evolution and the environmental changes in the Ediacaran period is discussed. In addition, some tentative explanations for the mechanism of biotic evolution affected by palaeoenvironmental changes are suggested.The study deals with a series of new fossils, including animal embryos, micro- and macro-algae,“string of beads”and Eoandromeda octobrachiata which may belong to cnidarians. These prolific fossils partly reflect the origin and radiation of metazoan after the Nantuo ice age （Marinoan）. Abundant fossils of“string of beads”have been found from the Liuchapo formation. After a relatively detailed study, the author thought that the string of beads fossil may have flexible transitional shell instead of crust, probably belong to primitive forams. The study shows that the metazoan life actually had experienced a long time of evolution prior to the Cambrian rather than a sudden appearance just at the beginning of Cambrian. It seems in the Ediacaran, life has evolved through a process of acanthomorphic acritarch→animal embryos→macroscopic algae→flexible shell metazoan→week calcified metazoan. In the Doushantuo formation, abundant authigenic minerals, such as pyrite and aragonite have been recognized, which can be used as indicators for environmental explanation. The study shows that the marine environment remained anoxic and reductive in early Ediacaran period until late when it become oxidative and less euxinic. These might provide premises for the development of metazoan. Isotopic changes in late Neoproterozoic have been used to discuss the interaction between early life evolution and environmental changes. In general, the rapid excursion ofδ¹³C often corresponds rise and decline of life.