Summary

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.

6. References

Allison, C.W., and Hilgert, J.W. (1986) Scale microfossils from the early Cambrian of northwest

Canada, J. Paleontol. 60(5), 973-1015. Allison, P.A., and Briggs, D.E.G. (1993) Exceptional fossils record: Distribution of soft-tissue preservation through the Phanerozoic, Geology 21, 527-530.

Bailey, J.V., Joye, S.B., Kalanetra, K.M., Flood, B.E., and Corsetti, F.A. (2007) Evidence of giant sulphur bacteria in Neoproterozoic phosphorites, Nature 445, 198-201.

Bengtson, S. (1976) The structure of some Middle Cambrian conodonts, and the early evolution of conodont structure and function, Lethaia 9, 185-206.

Bengtson, S. (1994) The advent of animal skeletons, In: S. Bengtson (ed.) Early Life on Earth. Columbia, New York, pp. 412-425.

Bengtson, S., and Budd, G. (2004) Comment on "Small Bilaterian Fossils from 40 to 55 Million Years Before the Cambrian", Science 306, 1290a-1291a.

Bengtson, S., and Yue, Z. (1997) Fossilized metazoan embryos from the earliest Cambrian, Science 277, 1645-1648.

Brasier, M., McLoughlin, N., Green, O., and Wacey, D. (2006) A fresh look at the fossil evidence for early Archaean cellular life, Phil. Trans. Royal Soc. London B: Biol. Sci. 361, 887-902.

Briggs, D.E.G. (2003) The role of decay and mineralization in the preservation of soft-bodied fossils, Annu. Rev. Earth Planet. Sci. 31, 275-301 (doi: 10.1146/annurev.earth.31.100901.144746).

Briggs, D.E.G., Bottrell, S.H., and Raiswell, R. (1991) Pyritization of soft-bodied fossils: Beecher's trilobite Bed, Upper Ordovician, New York State, Geology 19(12), 1221-1224.

Butterfield, N.J. (1995) Secular distribution of Burgess Shale-type preservation, Lethaia 28, 1-13.

Butterfield, N.J. (2003) Exceptional fossil preservation and the Cambrian Explosion, Integr. Comp. Biol. 43, 166-177.

Cai, Y., and Hua, H. (2007) Pyritization in the Gaojiashan biota, Chinese Sci. Bull. 52, 645-650.

Chen, J., Oliveri, P., Li, C.-w., Zhou, G.-q., Gao, F., Hagadorn, J.W., Peterson, K.J., and Davidson, E.H. (2000) Precambrian animal diversity: Putative phosphatized embryos from the Doushantuo Formation of China, Proc. Nat. Acad. Sci. USA 97(9), 4457-4462.

Chen, J.-Y., Bottjer, D.J., Oliveri, P., Dornbos, S.Q., Gao, F., Ruffins, S., Chi, H., Li, C.-W., and Davidson, E.H. (2004) Small bilaterian fossils from 40 to 55 million years before the Cambrian, Science 305, 218-222.

Conway Morris, S., and Chen, M. (1992) Carinachitiids, hexangulaconulariids, and Punctatus: Problematic metazoans from the early Cambrian of South China, J. Paleontol. 66(3), 384-406.

Ding, L., Zhang, L., Li, Y., and Dong, J. (1992) The Study of the Late Sinian - Early Cambrian Biotas from the Northern Margin of the Yangtze Platform. Scientific and Technical Documents Publishing House, Beijing.

Dong, X.-P., Donoghue, P.C.J., Cheng, H., and Liu, J.-B. (2004) Fossil embryos from the Middle and Late Cambrian period of Hunan, south China, Nature 427, 237-240.

Donoghue, P.C.J., Kouchinsky, A., Waloszek, D., Bengtson, S., Dong, X.-p., Val'kov, A.K., Cunningham, J.A., and Repetski, J.E. (2006) Fossilized embryos are widespread but the record is temporally and taxonomically biased, Evol. Dev. 8, 232-238.

Dornbos, S.Q., Bottjer, D.J., Chen, J.-Y., Oliveri, P., Gao, F., and Li, C.-W (2005) Precambrian animal life: Taphonomy of phosphatized metazoan embryos from southwest China, Lethaia 38, 101-109.

Dornbos, S.Q., Bottjer, D.J., Chen, J.Y., Gao, F., Oliveri, P., and Li, C.W. (2006) Environmental controls on the taphonomy of phosphatized animals and animal embryos from the Neoproterozoic Doushantuo Formation, southwest China, PALAIOS 21, 3-14.

Duncan, I.J., and Briggs, D.E.G. (1996) Three-dimensionally preserved insects, Nature 381, 30-31.

Duncan, I.J., Briggs, D.E.G., and Archer, M. (1998) Three-dimensionally mineralized insects and millipedes from the Tertiary of Riversleigh, Queensland, Australia, Palaeontology 41(5), 835-851.

Efron, B. (1981) Nonparametric standard errors and confidence intervals, Can. J. Statistics 9, 139-172.

Fedo, C.M., Whitehouse, M.J., and Kamber, B.S. (2006) Geological constraints on detecting the earliest life on Earth: A perspective from the Early Archaean (older than 3.7 Gyr) of southwest Greenland, Phil. Trans. Roy. Soc. Lond. B 361, 851-867.

Fedonkin, M.A., and Yochelson, E.L. (2002) Middle Proterozoic (1.5 Ga) Horodyskia moniliformis Yochelson and Fedonkin, the oldest known tissue-grade colonial eucaryote, Smithsonian Contrib. Paleobiol. 94, 1-29.

Folk, R.L. (1999) Nannobacteria and the precipitation of carbonate in unusual environments, Sediment. Geol. 126, 47-55.

Folk, R.L., and Rasbury, E.T. (2002) Nanometre-scale spheroids on sands, Vulcano, Sicily: Possible nannobacterial alteration, Terra Nova 14, 469-475.

Fralick, P., Davis, D.W, and Kissin, S.A. (2002) The age of the Gunflint Formation, Ontario, Canada: Single zircon U-Pb age determinations from reworked volcanic ash, Can. J. Earth Sci. 39, 1085-1091.

Gabbott, S.E., Hou, X.G., Norry, M.J., and Siveter, D.J. (2004) Preservation of early Cambrian animals of the Chengjiang biota, Geology 32, 901-904.

Gaines, R.R., Kennedy, M.J., and Droser, M.L. (2005) A new hypothesis for organic preservation of Burgess Shale taxa in the middle Cambrian Wheeler Formation, House Range, Utah, Palaeogeogr. Palaeoclimatol. Palaeoecol. 220(1-2), 193-205.

Gehling, J.G. (1999) Microbial mats in terminal Proterozoic siliciclastics: Ediacaran death masks, PALAIOS 14, 40-57.

Grant, S.W.F. (1990) Shell structure and distribution of Cloudina, a potential index fossil for the terminal Proterozoic, Am. J. Sci. 290-A, 261-294.

Grimes, S.T., Davies, K.L., Butler, I.B., Brock, F., Edwards, D., Rickard, D., Briggs, D.E.G., and Parkes, R.J. (2002) Fossil plants from the Eocene London clay: The use of pyrite textures to determine the mechanism of pyritization, J. Geol. Soc. Lond. 159, 493-501.

Grotzinger, J.P., Watters, W.A., and Knoll, A.H. (2000) Calcified metazoans in thrombolite-stromatolite reefs of the terminal Proterozoic Nama Group, Namibia, Paleobiology 26(3), 334-359.

Hagadorn, J.W., Fedo, C.M., and Waggoner, B.M. (2000) Early Cambrian Ediacaran-type fossils from California, J. Paleontol. 74(4), 731-740.

Hagadorn, J.W., Xiao, S., Donoghue, P.C.J., Bengtson, S., Gostling, N.J., Pawlowska, M., Raff, E.C., Raff, R.A., Turner, F.R., Yin, C., Zhou, C., Yuan, X., McFeely, M.B., Stampanoni, M., and Nealson, K.H. (2006) Cellular and subcellular structure of Neoproterozoic embryos, Science 314, 291-294.

Han, T.-M., and Runnegar, B. (1992) Megascopic eukaryotic algae from the 2.1 billion-year-old Negaunee Iron-Formation, Michigan, Science 257, 232-235.

Hua, H., Chen, Z., Yuan, X., Zhang, L., and Xiao, S. (2005) Skeletogenesis and asexual reproduction in the earliest biomineralizing animal Cloudina, Geology 33(4), 277-280.

Jensen, S., Gehling, J.G., and Droser, M.L. (1998) Ediacara-type fossils in Cambrian sediments, Nature 393, 567-569.

Jones, B., Renaut, R.W., and Rosen, M.R. (1997) Biogenicity of silica precipitation around geysers and hot-spring vents, North Island, New Zealand, J. Sediment. Res., A: Sediment. Petrol. Process. 67(1), 88-104.

Jones, B., Konhauser, K.O., Renaut, R., and Wheeler, R.S. (2004) Microbial silicification in Iodine Pool, Waimangu geothermal area, North Island, New Zealand: Implications for recognition and identification of ancient silicified microbes, J. Geol. Soc. Lond. 161, 983-993.

Jones, B., Renaut, R.W., and Rosen, M.R. (1997) Biogenicity of silica precipitation around geysers and hot-spring vents, North Island, New Zealand, Journal of Sedimentary Research, Section A: Sedimentary Petrology and Processes 67(1), 88-104.

Knoll, A.H. (1985) Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats, Phil. Trans. Royal Soc. Lond. B 311, 111-122.

Knoll, A.H. (2003) Biomineralization and evolutionary history, Rev. Mineral. Geochem. 54, 329-356.

Kowalewski, M., Goodfriend, G.A., and Flessa, K.W. (1998) High resolution estimates of temporal mixing within shell beds: The evils and virtues of time-averaging, Paleobiology 24, 287-304.

Liu, P., Xiao, S., Yin, C., Zhou, C., Gao, L., and Tang, F. (2008) Systematic description and phyloge-netic affinity of tubular microfossils from the Ediacaran Doushantuo Formation at Weng'an, South China, Palaeontology 51, 339-366.

Maliva, R.G., Knoll, A.H., and Siever, R. (1989) Secular change in chert distribution: A reflection of evolving biological participation in the silica cycle, PALAIOS 4, 519-532.

Maliva, R.G., Knoll, A.H., and Simonson, B.M. (2005) Secular change in the Precambrian silica cycle: Insights from chert petrology, GSA Bull. 117(7), 835-845.

Martill, D.M., and Wilby, P.R. (1994) Lithified prokaryotes associated with fossil soft tissues from the Santana Formation (Cretaceous) of Brazil, Kaupia, Darmstaedter Beitraeger zur Naturgeschichte 4, 71-77.

Martin, D., Briggs, D.E.G., and Parkes, R.J. (2003) Experimental mineralization of invertebrate eggs and the preservation of Neoproterozoic embryos, Geology 31(1), 39-42.

Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P., and Friend, C.R.L. (1996) Evidence for life on Earth by 3800 million years ago, Nature 384, 55-59.

Müller, K.J. (1985) Exceptional preservation in calcareous nodules, Phil. Trans. Roy. Soc. Lond. B 311, 67-73.

Müller, K.J., and Hinz-Schallreuter, I. (1993) Palaeoscolecid worms from the Middle Cambrian of Australia, Palaeontology 36(3), 549-592.

Narbonne, G.M. (2005) The Ediacara Biota: Neoproterozoic origin of animals and their ecosystems, Annu. Rev. Earth Planet. Sci. 33, 421-442.

Nealson, K.H. (1997) Nannobacteria: Size limits and evidence, Science 276, 1776.

Nisbet, E.G., and Sleep, N.H. (2001) The habitat and nature of early life, Nature 409, 1083-1091.

Orr, P.J., Briggs, D.E.G., and Kearns, S.L. (1998) Cambrian Burgess Shale animals replicated in clay minerals, Science 281, 1173-1175.

Orr, P.J., Benton, M.J., and Briggs, D.E.G. (2003) Post-Cambrian closure of the deep-water slope-basin taphonomic window, Geology 31, 769-772.

Perri, E., and Tucker, M. (2007) Bacterial fossils and microbial dolomite in Triassic stromatolites, Geology 35, 207-210.

Porter, S.M., and Knoll, A.H. (2000) Testate amoebae in the Neoproterozoic era: Evidence from vase-shaped microfossils in the Chuar Group, Grand Canyon, Paleobiology 26(3), 360-385.

Raff, E.C., Vilinski, J.T., Turner, F.R., Donoghue, P.C.J., and Raff, R.A. (2006) Experimental tapho-nomy shows the feasibility of fossil embryos, Proc. Natl. Acad. Sci. USA 103, 5846-5851.

Renaut, R.W., Jones, B., and Tiercelin, J.J. (1998) Rapid in situ silicification of microbes at Loburu hot springs, Lake Bogoria, Kenya Rift Valley, Sedimentology 45, 1083-1103.

Rickard, D., Grimes, S., Butler, I., Oldroyd, A., and Davies, K.L. (2007) Botanical constraints on pyrite formation, Chem. Geol. 236, 228-246.

Schneider, D.A., Bickford, M.E., Cannon, W.F., Schulz, K.J., and Hamilton, M.A. (2002) Age of volcanic rocks and syndepositional iron formations, Marquette Range Supergroup: Implications for the tectonic setting of Paleoproterozoic iron formations of the Lake Superior region, Can. J. Earth Sci. 39(6), 999-1012.

Schopf, J.W. (1968) Microflora of the Bitter Springs Formation, Late Precambrian, central Australia, J. Paleontol. 42, 651-688.

Schopf, J.W. (2006) Fossil evidence of Archaean life, Phil. Trans. Roy. Soc. Lond. B 361, 869-885.

Southam, G., and Donald, R. (1999) A structural comparison of bacterial microfossils vs. "nanobac-teria" and nanofossils, Earth Sci. Rev. 48, 251-264.

van Zuilen, M.A., Lepland, A., and Arrhenius, G. (2002) Reassessing the evidence for the earliest traces of life, Nature 418, 627-630.

Walossek, D. (2003) The "Orsten" window - a three-dimensionally preserved upper Cambrian meiofauna and its contribution to our understanding of the evolution of Arthropoda, Paleontol. Res. 7, 71-88.

Wood, R.A., Grotzinger, J.P., and Dickson, J.A.D. (2002) Proterozoic modular biomineralized meta-zoan from the Nama Group, Namibia, Science 296, 2383-2386.

Xiao, S. (2004) New multicellular algal fossils and acritarchs in Doushantuo chert nodules (Neoproterozoic, Yangtze Gorges, South China), J. Paleontol. 78(2), 393-401.

Xiao, S., and Knoll, A.H. (1999) Fossil preservation in the Neoproterozoic Doushantuo phosphorite Lagerstätte, South China, Lethaia 32, 219-240.

Xiao, S., and Knoll, A.H. (2000) Phosphatized animal embryos from the Neoproterozoic Doushantuo Formation at Weng'an, Guizhou, South China, J. Paleontol. 74(5), 767-788.

Xiao, S., Zhang, Y., and Knoll, A.H. (1998) Three-dimensional preservation of algae and animal embryos in a Neoproterozoic phosphorite, Nature 391, 553-558.

Xiao, S., Yuan, X., and Knoll, A.H. (2000) Eumetazoan fossils in terminal Proterozoic phosphorites?, Proc. Nat. Acad. Sci. USA 97(25), 13684-13689.

Xiao, S., Knoll, A.H., Yuan, X., and Pueschel, C.M. (2004) Phosphatized multicellular algae in the Neoproterozoic Doushantuo Formation, China, and the early evolution of florideophyte red algae, Am. J. Bot. 91, 214-227.

Xiao, S., Shen, B., Zhou, C., Xie, G., and Yuan, X. (2005) A uniquely preserved Ediacaran fossil with direct evidence for a quilted bodyplan, Proc. Nat. Acad. Sci. USA 102, 10227-10232.

Xiao, S., Yuan, X., Steiner, M., and Knoll, A.H. (2002) Macroscopic carbonaceous compressions in a terminal Proterozoic shale: A systematic reassessment of the Miaohe biota, South China, J. Paleontol. 76(2), 345-374.

Xiao, S., Zhou, C., and Yuan, X. (2007) Undressing and redressing Ediacaran embryos, Nature 446, E9-10.

Yuan, X., Xiao, S., Li, J., Yin, L., and Cao, R. (2001) Pyritized chuarids with excystment structures from the late Neoproterozoic Lantian Formation in Anhui, South China, Precambr. Res. 107(3-4), 251-261.

Yue, Z., and Bengtson, S. (1999) Embryonic and post-embryonic development of the Early Cambrian cnidarian Olivooides, Lethaia 32, 181-195.

Zhang, W., and Babcock, L.E. (2001) New extraordinarily preserved enigmatic fossils, possibly with Ediacaran affinities, from the Lower Cambrian of Yunnan, China, Acta Palaeontol. Sinica 40(supplement), 210-213.

Zhang, X., and Pratt, B.R. (1994) Middle Cambrian arthropod embryos with blastomeres, Science 266, 637-639.

Zhang, Y., Yin, L., Xiao, S., and Knoll, A.H. (1998) Permineralized fossils from the terminal Proterozoic Doushantuo Formation, South China, J. Paleontol. 72(4), 1-52(supplement).

Zhu, M., Babcock, L.E., and Steiner, M. (2005) Fossilization modes in the Chengjiang Lagerstätte (Cambrian of China): Testing the roles of organic preservation and diagenetic alteration in exceptional preservation, Palaeogeogr., Palaeoclimatol., Palaeoecol. 220(1-2), 31-46.

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]

Was this article helpful?

0 0

Post a comment