| Summary: | 碩士 === 國立清華大學 === 分子與細胞生物研究所 === 94 === An extraordinary fossil record of multicellular life were found in phosphorites from the Precambrian Doushantuo Formation（550~600 Mya）, southern China. These fossils, which three-dimensional cellular structure details were perfectly preserved, have been classified as animal embryo, larva, and adult forms, as well as multicellular thallophytes, giant acritarch, and spheroidal chlorophyta. The well-preserved structural details indicates a rapid burial process, thus it is possible to simulate fossilization process in laboratory time. Ulva fasciata, a green algae, was selected as the major experimental material, and simplified control factors were apply to explore the most probable factors that influence fossilization and determine the possible roles they play during fossilization process.
In previous experimental approach, the replicated morphology of soft-bodied fossils has been attributed to adsorption of pre-existing mineral clays and correlated to bacteria activity. But the interior of the subjects was not mineralized in early experiments. We attempted to develop several phosphorite deposition systems based on conditions of ancient environment and fossils records. We induced the formation of calcium phosphate deposits by increasing the temperature and the following rise of pH value. The results showed that the interior of algal cells was mineralized and detail morphology was preliminarily replicated. The results also indicated that magnesium played an important role in the phoosphorite deposition systems. No deposition occurred within the cells in the absence of magnesium. We suggest that the formation of fossils in Doushantuo Formation may be the consequence of a catastrophic rapid burial and phosphorites deposition at an elevated temperature on the results mentioned above.
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