RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES6003, doi:10.2205/2005ES000183, 2005

Peculiarities of Microfossil Morphological Differences

[61]  Microfossils from both the SBFS sediments and localities in other regions [Horodyski, 1993; Mikhailova and Podkovyrov, 1987; Moorman, 1974; Jankauskas, 1982] are characterized by a property of retaining the envelope volume and internal characters during the diagenesis and initial stage of greenschist facies of metamorphism. This property and lesser size differentiate them from a lot of acritarchs that become flattened to a plate at the very beginning of the deposit lithification [Burzin, 1997] and that are mainly known to most of microphytologists. The peculiarities of volumetric forms and their relationships with other acritarchs and with the deposit were reported previously [Stanevich, 1997, 2003]. A suggestion on occurrence of the two types of microfossil forms with the envelopes differently responding to lithostatic pressure appeared from the study of regional materials and their comparison with the Precambrian "microbiotas" of other regions. This resulted in the distinction of a new subgroup Implethomorphitae Jank. et Mikh. published in the generalized summary on microfossils [Jankauskas et al., 1989]. This innovation carried the discussed issue to a formal, classification channel without touching on the prospects of its solution. At the same time the available records indicate that the difference between the two microfossil types mostly results from their original properties. Secondly, this problem has many aspects and is in a realization stage. Thirdly, though indirectly, the indicated difference permits the search and specification of criteria for the biological separation of certain forms referred to acritarchs.

[62]  Lack of attention to possible genetic and biochemical differences between the forms united in terms Precambrian "microfossils" or "acritarchs", leads to a notion about similar preservation properties in ancient organisms. There is a widespread opinion that the organic-walled fossils can possess volume only on formation of mineral pseudomorphs. At the same time numerous known localities yield 3-25- m m to 50- m m-sized forms that retain their three-dimensional structure and diverse features, including the finest ones, in conditions of initial stage of the metamorphic greenschist facies [Nemerov and Stanevich, 2001; Stanevich and Faizulina, 1992]. The experimental data on determination of original resistance of modern bacteria to pressure [Kuznetsov et al., 1962] are a good illustration for the discussed problem. Certain bacteria in a growth phase died at 500-600 atm and their reproduction was retarded at 300 atm. Some species are well propagated even at 600 atm. Endospores and resting cells of bacteria proved to be the most resistant to pressure. It may be therefore inferred that volume and morphological features of a lot of bacteria can be retained at the pressure values corresponding to conditions of initial metamorphism.

[63]  Most of microfossils from the Neoproterozoic sediments of the SBFS including those described above, possess a surprising capacity to withstand lithostatic pressure. Compared to other regions, the microfossil assemblages from the SBFS yield from 70% to 90% of forms retaining volume and features in conditions of regional metamorphism [Stanevich, 1997]. Most of rocks bearing like microfossils are quartz-micaceous, rarely chloritic silty pelites (shales). They are marked by the presence of layers, lenses, and clots of carbonaceous matter associated with microfossil findings. The mineral composition of rocks corresponds to the lower-middle stages of greenschist facies or to PT-conditions at 1-1.5 kbar and 150-300o C [Vinkler, 1979]. Studies of alteration of sporopollenin and like polymers composing the envelopes of spores and examined microfossils [Bruck and Show, 1973; Ruchnov, 1981] indicated that the forms are yellowish or light brown in color at up to 120-150o C. Within 150-250oC (as in our case) they become dark brown or grey and at 250-400o C the forms are coalificated and are represented by black bodies with distorted morphology. These petrographic criteria can serve as indicators of in situ position and show the possibilities of microfossil taxonomic identification in metamorphic rocks. Studies of the slides derived from the chlorite-quartz-sericite shales showed that carbonaceous matter limits the growth of minerals and is a conserver for microfossil preservation. Thus the described microfossil forms retain volume and features in a wide range of PT-conditions. This differs them from the more well-known microfossils that become crumpled at the very beginning of diagenesis. The flattened and volumetric forms commonly occur in the same samples. Most of volumetric microfossils are lesser in size and possess characters unknown in the flattened forms. A combination of characters of volumetric forms from the SBFS sediments determines a taxonomic position of the species and genera. The presence in certain forms of features of two genera resulted in the use of the so-called intergeneric taxa proposed previously [Stanevich and Faizulina, 1992] and not contradictory to the Code.

[64]  Therefore, the Neoproterozoic acritarchs from the SBFS compared in this paper with bacteria, green algae, and dinoflagellates, have a morphologically pronounced property that likely reflects a genetic resistance of the bacteria and algae reproductive organs to unfavorable environment and that can be used as a non-formal character in the acritarch classification. The distribution of acritarch taxa according to the following scheme of modified classification corresponds well to the conducted ecobiological interpretation of microfossils (Table 1).


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Citation: Stanevich, A. M., V. K. Nemerov, Yu. K. Sovetov, E. N. Chatta, A. M. Mazukabzov, V. I.  Perelyaev, and T. A. Kornilova (2005), Precambrian microfossil-characterized biotopes from the southern margin of the Siberian craton, Russ. J. Earth Sci., 7, ES6003, doi:10.2205/2005ES000183.

Copyright 2005 by the Russian Journal of Earth Sciences

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