To summarize, we recognize several different phosphatic textures, probably indicating different phosphatization processes responsible for the preservation of Doushantuo microfossils. Phosphatic encrustation by micrometric, perpendicularly oriented crystals is most pervasive in the Doushantuo Formation. It occurs as bot-ryoidal and isopachous cements on a variety of substrates, including animal egg/ embryo cell surfaces, egg/embryonic envelopes, algal cell walls, organic filaments, as well as secondary coatings on pre-existing phosphatic surfaces. Phosphatic infilling of intracellular space by sub-micrometric, tangentially oriented crystals was probably driven by abundant random nucleation within algal cells or acritarch vesicles. These random crystals probably became tangentially oriented when pushed against the relatively recalcitrant algal cell walls and acritarch vesicle walls. In essence, the cells and acritarchs were molded by crystals growing inside. Tangentially aligned crystals do not occur in animal cells perhaps because animal cell membranes were more labile and would be deflated and degraded before cell lumens were completely phosphatized. Phosphatic impregnation by sub-micrometric, randomly oriented crystals occurred when nucleation was initiated within organic substrates (such as more recalcitrant tube walls and cell walls) after only minimal degradation. Finally, there is tentative evidence for bacterial (or nanobacterial) activities preserved in the Doushantuo microfossils. At the present, the exact taphonomic roles of these bacterial activities are still uncertain.

Our analysis shows that the preservational quality and taphonomic resolution of phosphatic impregnation and intracellular infilling is much better than phos-phatic encrustation. This is because the former processes tend to preserve more recalcitrant structures, during earlier diagenesis, after less degradation, by smaller crystals. Thus, analysis of Doushantuo microfossils suggests that the recalcitrance, degree of degradation, and crystal size all play a significant, controlling role in phosphatization of soft-bodied microorganisms (Briggs, 2003).

4. Relevance to Astrobiology

As discussed in the opening section, any extraterrestrial ecosystems likely started with a biosphere dominated by soft-bodied microorganisms. Additionally, from a practical point of view, the amount of extraterrestrial samples available for astropal-eobiological investigation is likely small (even if a Mars sample return mission is conceivable in the near future). Thus, the astropaleobiological focus has been and must continue to be on microscopic fossils, and the Precambrian biosphere is the most suitable analog for such investigation. Among all of the taphonomic pathways discussed in the introduction section, Bitter Spring-type (silicification) and Doushantuo/Orsten-type preservation (phosphatization) hold the greatest potential in astropaleobiological investigation. Beecher's trilobite-type preservation (pyritization) has poor resolution because of the large size of pyrite crystals and its intrinsic dependence on organic degradation and destruction as a source of sulfide. Burgess Shale-type preservation and Ediacara-type preservation tend to preserve macroscopic organisms. In addition, the two-dimensional compression in Burgess Shale-type preservation also decreases its taphonomic fidelity, and instability of organic carbon in many strongly oxidative extraterrestrial environments also makes Burgess Shale-type preservation less relevant. Thus, in the search for ancient extraterrestrial ecosystems, we need to follow the silica and phosphate. Detailed investigation of silicified and phosphatized biotas preserved in ancient rocks on the Earth, together with an experimental approach to better understand of the molecular and geochemical processes of silicification and phosphatization (Martin et al., 2003; Raff et al., 2006), will certainly help us to more effectively choose astrobiological landing/sampling sites.

5. Acknowledgments

We would like to acknowledge the NASA Exobiology Program, NSF Sedimentary Geology and Paleobiology Program, Petroleum Research Fund, National Natural Science Foundation of China, and Chinese Ministry of Science and Technology for support. We thank Stefan Bengtson, Phil Donoghue, James W. Hagadorn, John W. Huntley, Michal Kowalewski, Richard A. Krause, Xunlai Yuan, and Chuanming Zhou for discussion. Steve Dornbos and an anonymous reviewer provided constructive comments on an earlier version of this contribution.

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Biodata of Professor Vinod C. Tewari author of "Proterozoic Unicellular and Multicellular Fossils from India and Their Implications"

Professor Vinod C. Tewari is currently the Head of the Sedimentology Group at Wadia Institute of Himalayan Geology, Dehradun and a Senior Associate of International Centre for Theoretical Physics, Trieste, Italy. He obtained his Ph.D. from the University of Lucknow in Geology in 1986 and continued his research in Wadia Institute. Dr. Tewari taught Geology at Kumaon Univerisity, Nainital, Uttarakhand, India as Professor of Geology. Professor Tewari's scientific interests are in the areas of Precambrian stromatolites, sedimentation, carbon isotope chemostratigraphy, genesis, early evolution and diversification of life and its astrobiological significance. He is associated with the International Geological Correlation Program (I.G.C.P.) Project 493 on The Rise and Fall of Vendian Biota. He has 75 research papers published to his credit, and edited several volumes of Himalayan Geology, India and Journal of Nepal Geological Society, Kathmandu, Nepal. Professor Tewari has organized first Indo-Soviet Symposium on Stromatolites and Stromatolitic Deposits and other IGCP meetings in India. He is one of the organizers of the World Summit on Ancient Microscopic Fossils to be held in University of California, Los Angeles, USA in 2008.

E-mail: [email protected]

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