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| Stromatolite - Jurassic Russian fossil limizite |
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Stromatolite en.wikipedia.org Stromatolites were much more abundant on the planet in Precambrian (Pre-Cambrian) times. While older, Archean (a.k.a. Archeozoic) fossil remains are presumed to be single-celled colonies of blue-green bacteria, younger (that is, Proterozoic) fossils may be primoidial forms of the eukaryote chlorophytes (that is, green algae). One genus of stromatolite very common in the geologic record is Collenia. Prior to 2.4 billion years ago, the earth's atmosphere was rich in carbon dioxide. However, the Precambrian air lacked the oxygen that sustains the complex multicellular life that has evolved since the "Cambrian explosion" 540 million years ago. Stromatolites in the fossil record decline sharply in both diversity and number during the late Proterozoic eon, although they are present, but not common, in Paleozoic era strata. Today, stromatolites are quite uncommon in marine environments. As a result, they have become valuable "living fossils." Their former abundance may be because there were no burrowing or grazing animals back during the Precambrian to destabilize sediments and consume growing microbial mats, thereby favoring the preservation of these microbialites. Also, changing chemical conditions in the ocean during this time could be responsible for the precipitation of non-biological stromatolites through the growth of tiny crystals.... http://www.fossilmall.com/Science/About_Stromatolite.htm Stromatolites are most often described as biogenically-produced structures formed by colonies of photosynthesizing cyanobacteria. However, this is an enormous oversimplification. Science now knows that all domains of life (the Archaeans, Eubacteria, and Eukaryotes) all appeared in the Archaean Era. Which of the prokaryotes came first, the Archaeans or the Eubacteria remains a mystery, but a consensus is emerging that these primitive microorganisms laterally exchanged genes; if so, the concept of the single common ancestor for all life becomes a bit fuzzy (see Woese, 1998). While formation by colonies of cyanobacteria is probably the primary mechanism for formation of stromatolite in the deep time of the Archaean and half way through the Proterozoic, it is unlikely to have been the only mechanism. Recent research indicates the other prokaryotic and the most genetically diverse domain of life, the Archaeans, evolved alongside and possibly swapped genes with the Eubacteria. All prokaryotes (both Eubacteria and the Archaeans reproduce by cell division (binary or multiple fission) and, lacking sex, are essentially clones and among the slowest evolving organisms. Moreover, molecular fossils indicate that primitive Eukaryotic microorganisms appeared more than 3.5 Ba. Thus, before the end of the Archaean time some 2.5 Ba, all three domains of life (Eubacteria, Archaea, and Eukaryotes) existed and were likely already quite diverse. Some were autotrophs, some chemotrophs and some heterotrophs, and collectively they had a multiplicity of metabolic processes from which to derive their energy. Just as microorganisms were extremely diverse in deep time, so is there a corresponding extreme diversity of biogenic and chemical mechanisms that are plausible for the formation of laminar carbonate and other structures that we call stromatolite. Ascribing all stromatolite formation in the Archaean and Proeterozoic to cyanobacteria, seems an unreasonable assumption.... http://www.ucmp.berkeley.edu/bacteria/cyanofr.html At right is a layered stromatolite, produced by the activity of ancient cyanobacteria. The layers were produced as calcium carbonate precipitated over the growing mat of bacterial filaments; photosynthesis in the bacteria depleted carbon dioxide in the surrounding water, initiating the precipitation. The minerals, along with grains of sediment precipitating from the water, were then trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which then continued to grow upwards through the sediment to form a new layer. As this process occured over and over again, the layers of sediment were created. This process still occurs today; Shark Bay in western Australia is well known for the stromatolite "turfs" rising along its beaches..... |
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