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- Agić, Heda, 1989-
(author)
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Palaeobiology and diversification of Proterozoic-Cambrian photosynthetic eukaryotes
- 2015
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Doctoral thesis (other academic/artistic)abstract
- One of the most important events in the history of life is the evolution of the complex, eukaryotic cell. The eukaryotes are complex organisms with membrane-bound intracellular structures, and they include a variety of both single-celled and multicellular organisms: plants, animals, fungi and various protists. The evolutionary origin of this group may be studied by direct evidence of past life: fossils. The oldest traces of eukaryotes have appeared by 2.4 billion years ago (Ga), and have additionally diversified in the period around 1.8 Ga. The Mesoproterozoic Era (1.6-1 Ga) is characterised by the first evidence of the appearance complex unicellular microfossils, as well as innovative morphologies, and the evolution of sexual reproduction and multicellularity. For a better understanding of the early eukaryotic evolution and diversification patterns, a part of this thesis has focused on the microfossil records from various time periods and geographic locations. Examination of microfossil morphology, cell wall microstructure and biochemical properties, reflect their intracellular complexity and function, and allow reconstructions of their life cycle, as well as observing the evolutionary pattern of change from Mesoproterozoic, to Cambrian-Ordovician transition. Several case studies included assemblages deriving from Mesoproterozoic, Neoproterozoic and early Paleozoic time intervals that show disparate morphotypes and innovative features indicative of algal clades. The Mesoproterozoic Ruyang Group in northern China has yielded a diverse microfossil assemblage that provides important clues about the diversification of different eukaryotic groups. Furthermore these microfossils contributed an additional evidence for the emergence of the crown group Eukarya by 1.7-1.4 Ga. In another part of this thesis, examination of wall microstructure and chemical properties via Raman spectroscopy has been used to assess the biological affinities of various Neoproterozoic problematic carbonaceous compression fossils. Studies on the early Phanerozoic (c. 545-485 Ma) assemblages from Estonia reconstructed patterns of the early radiations of phytoplankton and its evolutionary innovations. A continuing theme in this thesis has been using a combination of evidence of microfossils’ fine-scale morphology, ecology and chemical properties to determine their function in life, in addition to their systematic position.
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| 2. |
- Willman, Sebastian, Docent
(author)
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The Ediacaran Diversification of Organic-walled Microbiota : Ocean Life 600 Million Years Ago
- 2008
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Doctoral thesis (other academic/artistic)abstract
- The only direct evidence of past life is provided by fossils. Fossils tell us about the evolution of life on Earth and they give us clues concerning ancient environments. The Ediacaran Period (roughly 635-542 million years ago) is characterised by the appearance and diversification of various microbiota and also the diversification of metazoans. Well-preserved organic-walled microfossils referred to as acritarchs occur abundantly in Ediacaran sedimentary successions in the Officer Basin in South Australia. Acritarch assemblages from the Giles 1 and Murnaroo 1 drillcores show a wide morphological disparity and are taxonomically diverse. Assemblages change over short stratigraphic intervals which enables the recognition of different biozones. The presence of taxa common between Australia, Siberia, Baltica and China provides a means for global correlation of the Ediacaran System. Examination of the wall ultrastructure of several acritarch specimens by use of transmission electron microscopy reveals a complexity in the cell wall that is not seen in prokaryotes but is indicative in some cases of particular clades of microalgae. Wall ultrastructures range from single-layered to three- and four-layered and from homogeneous to porous. The wall ultrastructure can be used to assess biological affinities and the affinities of the studied taxa in relation to green algae, dinoflagellates and metazoans are discussed. However, before taxonomic interpretations can be made with confidence, an understanding of taphonomic degradation of microorganisms is required. With focus on illustrated specimens, one part of this thesis explains what happens to an acritarch as it undergoes various types of degradation and why an understanding of these processes is important for taxonomic identification. A meteorite impact in South Australia spread an ejecta layer over a 550 km radius area. This ejecta layer is recognised in subsurface drillcores and provides an independent stratigraphic marker horizon that supports an acritarch-based correlation.
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